Designing Service Entrance Panelboard Equipment

Engineering Encyclopedia Saudi Aramco DeskTop Standards

DESIGNING SERVICE ENTRANCE PANELBOARD EQUIPMENT

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX-103.02

For additional information on this subject, contact PEDD Coordinator on 874-6556

Engineering Encyclopedia

Electrical Power Systems III Designing Service Entrance Panelboard Equipment

Section

Page

INTRODUCTION............................................................................................................ 6 MAJOR COMPONENTS OF A SERVICE ENTRANCE ................................................. 7 Introduction ............................................................................................................... 7 Elements of a Grounding System ............................................................................. 8 Grounding Electrode............................................................................................ 9 Grounded Service Conductor (Neutral) ............................................................. 10 Grounding Electrode Conductor ........................................................................ 10 Neutral Bus........................................................................................................ 10 Equipment Grounding Bus................................................................................. 10 Equipment Grounding Conductors .................................................................... 11 Bonding Jumper and Main Bonding Jumper...................................................... 11 Service Entrance Conductor Components.............................................................. 11 Phase Conductors ............................................................................................. 11 Grounded Conductor (Neutral) .......................................................................... 12 Types of Branch Circuits......................................................................................... 12 Lighting Branch Circuit ...................................................................................... 12 Receptacle Branch Circuit ................................................................................. 12 Power Branch Circuit......................................................................................... 13 Types of Panelboards ............................................................................................. 13 Power Distribution Panelboard .......................................................................... 13 Lighting and Appliance Branch Circuit Panelboard............................................ 13 Main Lugs Only (MLO) Panelboard ................................................................... 14 Main Breaker Panelboard .................................................................................. 14 SIZING LIGHTING, RECEPTACLE, AND POWER BRANCH CIRCUITS.................... 18 Branch Circuit Definitions........................................................................................ 18 General.............................................................................................................. 18 Lighting Branch Circuits..................................................................................... 18 Receptacle Branch Circuits ............................................................................... 20 Power Branch Circuits ....................................................................................... 20 Lighting Branch Circuits .......................................................................................... 20 Color Coding of Conductors .............................................................................. 20 Voltage Limitations ............................................................................................ 21 Receptacle Branch Circuits..................................................................................... 23 Types of Receptacles and Plugs ....................................................................... 23 General Purpose Ratings .................................................................................. 27 Specific Equipment Ratings............................................................................... 28 Color Coding...................................................................................................... 28 Voltage Limitations ............................................................................................ 29 Ground Fault Circuit Interrupters (GFCI) ........................................................... 31 Common Neutrals ................................................................................................ 32 Power Branch Circuits ............................................................................................ 33

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Receptacle and Other Types ............................................................................. 33 Color Coding and Voltage Limitations .............................................................. 33 NEC Requirements for Branch Circuits Over 50 Amperes ................................ 33 Branch Circuit Sizing Selection Factors .................................................................. 34 Loads................................................................................................................. 34 Conductors ........................................................................................................ 35 Protection .......................................................................................................... 38 Conduit Sizing (NEC Tables)............................................................................. 39 SIZING LIGHTING AND POWER DISTRIBUTION PANELBOARDS .......................... 41 Introduction ............................................................................................................. 41 Types of Loads ....................................................................................................... 41 Lighting Branch Circuits..................................................................................... 41 Receptacle Branch Circuits ............................................................................... 42 Power Branch Circuits ....................................................................................... 42 Feeder Circuits .................................................................................................. 42 NEC Panelboard Requirements.............................................................................. 42 Used as a Service Entrance .............................................................................. 42 Phase Arrangements......................................................................................... 43 Lighting Panelboard........................................................................................... 43 Number of Overcurrent Devices ........................................................................ 44 Ratings .............................................................................................................. 44 Circuit Directory ................................................................................................. 45 Panelboard Selection Factors ................................................................................. 45 Load .................................................................................................................. 46 Protection .......................................................................................................... 47 Miscellaneous Factors ....................................................................................... 50 SIZING SERVICE ENTRANCE CONDUCTORS ......................................................... 54 National Electric Code (NEC) Requirements .......................................................... 54 Number of Services Permitted........................................................................... 54 Insulation ........................................................................................................... 54 Physical Protection ............................................................................................ 54 Termination at Service Equipment..................................................................... 55 Disconnecting Means ........................................................................................ 55 Overcurrent Protection ...................................................................................... 55 Sizing Selection Factors ......................................................................................... 56 Load Data and Phase Conductors..................................................................... 56 Grounded Conductors ....................................................................................... 57 SIZING THE SERVICE ENTRANCE GROUNDING SYSTEM ..................................... 60 Introduction ............................................................................................................. 60 Types of Grounding Electrodes............................................................................... 60 Water Piping ...................................................................................................... 61 Structural Steel Building Frame ......................................................................... 61 Concrete-Encased Electrodes .......................................................................... 62

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Ground Ring (Peripheral) .................................................................................. 63 Ground Rod and Pipe Electrodes ...................................................................... 64 Buried Plate Electrodes ..................................................................................... 69 Grounded Service Conductor (Neutral) Selection Factors ...................................... 70 Routing .............................................................................................................. 70 Sizing................................................................................................................. 70 Grounding Electrode Conductor Selection Factors ................................................. 71 Outside Premises Location................................................................................ 71 Panelboard Connection ..................................................................................... 71 Requirements Based on Size of Service Entrance Conductors......................... 72 Neutral and Equipment Grounding Bus Selection Factors...................................... 73 Neutral Bus........................................................................................................ 73 Ground Bus ....................................................................................................... 73 Bonding Jumper Sizes....................................................................................... 73 WORK AID 1: RESOURCES USED TO SIZE LIGHTING, RECEPTACLE, AND POWER BRANCH CIRCUITS ........................... 74 Work Aid 1A: SAES-P-100 (11 MAR 96)................................................................ 74 Work Aid 1B: SAES-P-104 (7 DEC 94) .................................................................. 75 Work Aid 1C: SAES-P-111 (24 FEB 96) ................................................................ 76 Work Aid 1D: National Electric Code (NEC) Handbook Articles 210 and 220 ....................................................... 78 Work Aid 1E: Applicable Procedures for Sizing Lighting, Receptacle, and Power Branch Circuits...................................................................... 79 WORK AID 2: RESOURCES USED TO SIZE LIGHTING AND POWER DISTRIBUTION PANELBOARDS ........................................................ 85 Work Aid 2A: SAES-P-100 (11 MAR 96 ) .............................................................. 85 Work Aid 2B: SAES-P-104 (7 DEC 94 ) ................................................................. 85 Work Aid 2C: SAES-P-111 (24 FEB 96 ) ............................................................... 85 Work Aid 2D: 1996 NEC Handbook Article 384 ..................................................... 85 Work Aid 2E: Applicable Procedures ..................................................................... 85 WORK AID 3: RESOURCES USED TO SIZE SERVICE ENTRANCE CONDUCTORS ................................................................................... 87 Work Aid 3A: SAES-P-100 (11 MAR 96 ) .............................................................. 87 Work Aid 3B: SAES-P-104 (7 DEC 94 ) ................................................................. 87 Work Aid 3C: SAES-P-111 (24 FEB 96 ) ............................................................... 87 Work Aid 3D: 1996 NEC Handbook Articles 230 and 240...................................... 87 Work Aid 3E: Applicable Procedures ..................................................................... 87 WORK AID 4: RESOURCES USED TO SIZE A SERVICE ENTRANCE GROUNDING SYSTEM ....................................................................... 89 Work Aid 4A: SAES-P-100 (11 MAR 96 ) .............................................................. 89 Work Aid 4B: SAES-P-104 (7 DEC 94 ) ................................................................. 89 Work Aid 4C: SAES-P-111 (24 FEB 96 ) ............................................................... 89 Work Aid 4D: 1996 NEC Handbook Article 250 ..................................................... 89

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Word Aid 4E: ANSI/IEEE Standard 142-1991 (IEEE Green Book) ........................ 89 Work Aid 4F: Applicable Procedures ..................................................................... 90 GLOSSARY ................................................................................................................. 92

LIST OF FIGURES Figure 1. Service Entrance Components....................................................................... 7 Figure 2. Elements of a Grounding System................................................................... 8 Figure 3. Typical Grounding Electrodes ........................................................................ 9 Figure 4. Service Entrance Lighting and Appliance Branch Circuit Panelboard ............................................................................................. 15 Figure 5. MLO Panelboard Protected by a Feeder Breaker ........................................ 16 Figure 6. Panelboard Supplied Through a Transformer .............................................. 16 Figure 7. Panelboards Tapped Off a Main Feeder ...................................................... 17 Figure 8. Color Coding of Branch Circuits ................................................................... 19 Figure 9. NEMA Configurations for Receptacles and Plugs ........................................ 24 Figure 10. Types of Single Receptacles...................................................................... 25 Figure 11. Duplex Receptacle ..................................................................................... 26 Figure 12. Triplex Receptacle ..................................................................................... 27 Figure 13. Non-Interchangeability of Receptacles and Plugs...................................... 30 Figure 14. GFCI Circuit ............................................................................................... 32 Figure 15. Conduit Fill Rate Example.......................................................................... 40 Figure 16. UL-Approved Panelboard Protection.......................................................... 48 Figure 17. Feeder Device Protection of a Panelboard................................................. 49 Figure 18. Typical Lighting Panelboard Schedule ....................................................... 51 Figure 19. Circuit Wiring Layout Diagram.................................................................... 52 Figure 20. Ground Fault Detection .............................................................................. 56 Figure 21. Balanced 60 Hz Currents - Neutral Current Equals Zero (0) ...................... 58 Figure 22. Third Harmonic (180 Hz) Currents - Neutral Current Equals 3.0 p.u........................................................................................................... 59 Figure 23. Structural Steel Building Frame.................................................................. 61

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Figure 24. Concrete-Encased Electrodes................................................................... 62 Figure 25. Ground Ring (Peripheral) ........................................................................... 63 Figure 26. Ground Rod Electrode ............................................................................... 64 Figure 27. Ground Rod Length and Spacing............................................................... 65 Figure 28. Ground Rod Rock Bottom Installation ........................................................ 65 Figure 29. Effects of Deep Driven Ground Rods ......................................................... 66 Figure 30. Effects of Ground Rod Diameter ................................................................ 67 Figure 31. Effects of Multiple Ground Rods................................................................. 68 Figure 32. Buried Plate................................................................................................ 69 Figure 33. Grounding Electrode Conductor Connection to the Panelboard................. 72 Figure 34. SAES-P-100 Ambient Temperatures ......................................................... 74 Figure 35. Standard Saudi Aramco Wire Sizes ........................................................... 82 Figure 36. Typical Main Breaker Panelboard Ratings ................................................. 86 Figure 37. Ground Fault Detection Methods ............................................................... 91

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INTRODUCTION The service entrance equipment is the main control and means of cutoff for the supply or source of electrical power to the building. The service entrance equipment consists of conductors, a disconnecting means, a grounding system (electrodes and conductors), switchboards and panelboards, and the different overcurrent devices that protect the various groupings of electrical loads throughout the building. The purpose of this Module is to teach the Participant to: •

Size lighting, general purpose receptacle, and small power branch circuits.

•

Size the lighting and power distribution panelboards.

•

Size the service entrance conductors, which includes the phase conductors, the grounded conductor (neutral), and the equipment grounding conductor.

•

Size the service entrance grounding system, which includes the grounding electrode and the grounding electrode conductor.

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MAJOR COMPONENTS OF A SERVICE ENTRANCE Introduction The major components of a service entrance are the grounding system, service entrance conductors, branch circuits, and panelboard. Figure 1 illustrates the major components of a service entrance system.

Figure 1. Service Entrance Components

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Elements of a Grounding System Figure 2 is a typical circuit diagram of the elements of a grounding system.

Figure 2. Elements of a Grounding System

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Grounding Electrode As specified in NEC Articles 250-81 and 250-83, the grounding electrode is the nearest available effectively grounded structural member of the building, metal water pipe, or other “made” electrodes. Figure 3 illustrates several types of grounding electrodes that are approved by the NEC .

Figure 3. Typical Grounding Electrodes

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Grounded Service Conductor (Neutral) The NEC defines the grounded conductor as the conductor that is intentionally grounded. For purposes of this Module, the grounded conductor is assumed to be solidly grounded. Note: The grounded conductor will also be called (labeled) the neutral conductor and the service entrance conductor; all three terms are used interchangeably throughout the Module. Grounding Electrode Conductor The grounding electrode conductor is the conductor that is used to connect the grounded conductor (neutral) to the grounding electrode. Outside the premises, the grounding electrode conductor connects the middle wire or neutral point of the transformer to the grounding electrode. Neutral Bus The neutral bus is the common point (connection) inside a switchboard or panelboard where the grounded conductor (neutral) is connected to the grounding electrode conductor. The neutral bus is the terminal origination point for all feeder or branch circuit neutral conductors (where applicable) are connected, and it is also the point in a grounded system where the equipment grounding bus is bonded, by means of the main bonding jumper, to the grounded conductor at the service disconnecting location for grounded power systems. Equipment Grounding Bus The equipment grounding bus is the common point (connection) inside a switchboard or panelboard where all feeder or branch circuit equipment grounding conductors are connected. It is also the point where the neutral bus, by means of the main bonding jumper, is bonded to the equipment grounding bus (see the above paragraph).

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Equipment Grounding Conductors The equipment grounding conductors are the conductors that are used to connect the non-current-carrying metal parts of equipment, raceway, panelboards, etc., to the equipment grounding bus (see the above paragraph) . Bonding Jumper and Main Bonding Jumper A bonding jumper (or equipment bonding jumper) is a conductor that serves to permanently join metal parts to form an electrically conductive path that will ensure electrical continuity. In addition, the bonding jumper has the capacity to conduct safely any current that is likely to be imposed, and it will maintain an equipotential condition on the equipment enclosure or housing to which it is bonded. The main bonding jumper is a conductor that serves to connect the grounded circuit conductor and the equipment grounding conductor at the service entrance. The main bonding jumper is one of the most critical elements in the grounding system, because it provides the bonding conductor connection link between the grounded service conductor, the equipment grounding conductor, and the grounding electrode conductor. The connection carries the fault current for the service enclosure as well as from the equipment system.

Service Entrance Conductor Components The service entrance conductor components consist of phase conductors and the grounded conductor (neutral). Phase Conductors Service entrance phase conductors supply the power to the facilities, and they are sized to carry the loads, as computed according to NEC Article 220.

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Grounded Conductor (Neutral) The service entrance grounded service conductor (neutral), as previously described, is the conductor that is intentionally grounded, and it is physically routed with the service entrance phase conductors. The grounded conductor is sized in accordance with NEC Article 250-23b.

Types of Branch Circuits The NEC defines a branch circuit as the circuit conductors that are between the final overcurrent device protecting the circuit and the outlet(s). This Module will describe three types of branch circuits: lighting, receptacle, and power. Other types of branch circuits, for example, motor branch circuits, are beyond the scope of this Module. Lighting Branch Circuit For purposes of this Module, a lighting branch circuit is a branch circuit that supplies power to lighting fixtures. The load for a lighting branch circuit is computed based on the voltampere (VA) rating of the lamps, and ballasts if arc discharge type lamps such as fluorescent lamps are being used. Receptacle Branch Circuit For purposes of this Module, a receptacle branch circuit is a branch circuit that supplies power to general purpose receptacles. NEC Article 220-3 requires that the load for receptacle branch circuits be computed based on 180 VA per outlet.

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Power Branch Circuit For purposes of the Module, a power branch circuit is a branch circuit that supplies power to a specific appliance or other load, except for lighting, receptacle, motor, or other special purpose loads. For example, a branch circuit supplying dedicated power to an office copier machine would be considered a power branch circuit.

Types of Panelboards Power Distribution Panelboard A power distribution panelboard, which is often incorrectly called a switchboard, is defined by the NEC as a single panel or a group of panel units that are designed for assembly in the form of a single panel. A power distribution panelboard includes buses and automatic overcurrent devices for the control of light, heat, or power circuits, and it is designed to be placed in a cabinet or cutout box placed in or against a wall or partition and accessible only from the front. All panelboards, not separately defined as lighting and appliance branch circuit panelboards, are classified as power distribution panelboards. A switchboard, although similar to a panelboard, is generally accessible from the rear as well as from the front, and it is not intended to be installed in cabinets. Lighting and Appliance Branch Circuit Panelboard NEC Article 384-14 defines a lighting and appliance branchcircuit panelboard as a panelboard that has more than 10 percent of its overcurrent devices rated 30 amperes or less, for which neutral connections are provided. Figure 4 illustrates a typical lighting and appliance panelboard. Note: Lighting and appliance panelboards are often just referred to as lighting panelboards.

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Main Lugs Only (MLO) Panelboard Each lighting and appliance panelboard is required by NEC Article 384-16 to be individually protected on the supply side of the panelboard; however, if a feeder supplying the panelboard has overcurrent protection that does not exceed the rating of the panelboard, individual protection at the panelboard is not required. This type of panelboard, without overcurrent protection at its supply side, is called a main lugs only (MLO) panelboard (Figure 5). Note: Saudi Aramco design standards do not permit MLO panelboards. Main Breaker Panelboard Main breaker panelboards, such as the panelboard that is shown in Figure 4, are protected by means of main breakers installed in the same cabinet enclosure as the phase, neutral, and equipment grounding buses, and the branch circuit overcurrent devices. Panelboards supplied through a stepdown transformer (Figure 6) or tapped off a feeder conductor (Figure 7) are required by the NEC to include main breakers for the protection of the panelboards.

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Figure 4. Service Entrance Lighting and Appliance Branch Circuit Panelboard

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Figure 5. MLO Panelboard Protected by a Feeder Breaker

Figure 6. Panelboard Supplied Through a Transformer

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Figure 7. Panelboards Tapped Off a Main Feeder

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SIZING LIGHTING, RECEPTACLE, AND POWER BRANCH CIRCUITS

Branch Circuit Definitions The NEC defines 26 types of branch circuits. To limit the scope of this Information Sheet, only lighting, receptacle, and power branch circuits will be covered. NEC Articles 210 and 220 describe the general procedures for sizing branch circuits that are covered in this Information Sheet. General NEC Article 100 defines a branch circuit as “the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s)”. An outlet is defined by the same article as “a point on the wiring system at which current is taken to supply utilization equipment.” Lighting Branch Circuits A lighting branch circuit consists of conductors supplying power intended for the direct connection of a lighting fixture (luminaire). Branch circuits for lighting shall have a maximum rating of 20 amperes unless the lighting units have heavy-duty lampholders. Therefore, branch circuits for fluorescent lighting and for the smaller wattage, medium base incandescent lamps (up to 300 watts), are restricted to 15 or 20 amperes. Fixed lighting units with heavy duty lampholders, for example, the larger wattage, mogul-base incandescent and high intensity discharge (HID) lamps, such as mercury vapor (MV), metal halide (MH), and high pressure sodium (HPS) can be connected to circuits rated up to 50 amperes when installed in commercial or industrial facilities.

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Phase Conductors, which are defined by the NEC as being ungrounded conductors, are required to be colored (insulation) or marked (Figure 8), such that they are clearly distinguishable from grounded (e.g., the neutral conductor) and grounding conductors (e.g., the equipment grounding conductors). These phase (ungrounded) conductors shall be distinguished by colors other than white, natural gray, or green, or by a combination of color plus distinguishing markings. The distinguishing markings shall also be in a color other than white, natural gray, or green, and shall consist of a stripe or stripes or a regularly spaced series of identical marks.

Figure 8. Color Coding of Branch Circuits

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Receptacle Branch Circuits NEC Article 100 defines a receptacle as “a contact device installed at the outlet for the connection of a single attachment plug.” A single receptacle installed on an individual branch circuit must have an ampere rating not less than that of the branch circuit rating. For example, a single receptacle on a 20-ampere circuit must be rated at 20 amperes. As the only exception to this rule, 2 or more 15-ampere receptacles are permitted on a 20-ampere receptacle branch circuit. Receptacles installed on 15 and 20ampere branch circuits must be of the grounding type, which means that all 120-volt general purpose plug outlets must be of the three-pole type. The grounding terminal must be properly bonded to the equipment grounding conductor and the outlet box (if metal). Power Branch Circuits For purposes of this Information Sheet, power branch circuits are those branch circuits supplying power to non-generalpurpose receptacle or non-lighting loads. For example, a 30ampere branch circuit supplying power to a computer aided drafting (CAD) machine would be considered as a power branch circuit if it is hard-wired or plug-connected.

Lighting Branch Circuits Color Coding of Conductors Note: The color coding of conductors applies to all types of branch circuit conductors, whether they are lighting, receptacle, or power branch circuits. Grounded Conductors, or the neutral conductor of a branch circuit, are required by the NEC to be identified by a continuous white or natural gray conductor (Figure 8).

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Equipment Grounding Conductors for branch circuits are required by the NEC to be identified by a continuous green color or a continuous green color with one or more yellow stripes (Figure 8). Voltage Limitations NEC Article 210-6 describes the voltage limitations of lighting branch circuits. 120/240 V and 208Y/120 V Circuits - All lighting units that use medium base, screw shell, incandescent type lamps, regardless of their location, are restricted by the NEC to branch circuits that do not exceed 120 volts. The ballasts for arc discharge lighting (fluorescent and HID) are permitted to be connected to circuits exceeding 120 volts, but they are not permitted to be connected to circuits that exceed 277 volts nominal-to-ground. Ballasts from HID lamps may be operated from line-to-ground or line-toline voltages; however, if ballasts are operated line-to-line, two winding ballasts should be used to permit grounding of the mogul-base shell. General illumination of small, commercial and industrial facilities is usually supplied at 120 volts line-to-neutral, either from 120/240-volt single-phase, three-wire systems, or from 208Y/120-volt three-phase, four-wire systems. One of the major disadvantages of 120-volt lighting branch circuits is that every 1 volt drop on the 120-volt circuit results in a 0.833 percent voltage drop (100 x 1/120). Restricting the voltage drop to approximately 3.6 volts (3 percent) or less, as required by the NEC and Saudi Aramco design standards, would limit the conductor runs to approximately 117 feet, using AWG No. 12 wire for a 16- ampere fluorescent lighting load. See Example A. Example A: What is the maximum length of AWG No. 12 copper wire that is permitted by the NEC for a 16ampere, 0.95 p.f. fluorescent lighting load on a 120-volt circuit? Assume that ZL = (2 + j 0.068) Ω per 1000 ft (NEC Table 9) and VD max = 3.6 V.

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Answer: •

VD = I (R cos θ + X sin θ) = IZ = 3.6 V (max)

•

I = 16 A, R = 2 Ω/1000 ft, X = 0.068 Ω/1000 ft

•

cos θ = 0.95, sin θ = sin (cos-1 0.95) = .312

•

3.6 = 16[(2/1000)(.95)(# ft) + (.068/1000)(.312)(# ft)]

•

3.6 = (.0304)(# ft) + (.00034)(# ft)

•

# ft ≈ (3.6)/(.0304 + .00034) ≈ (117 ft total length)

•

# ft ≈ 117/2 = 58.5 ft (one-way length)

The other major disadvantage of 120-volt lighting branch circuits is that 120-volt branch circuits would require additional circuits including circuit breakers, wiring, conduits, etc., versus higher 277-volt lighting circuits. See Example B. Example B: How many branch circuits are required to supply 20 kVA of lighting at 120 volts line-to-neutral? at 277 volts line-to-neutral? Assume that there are no more than 16 amperes per circuit. Answer: 1.

2.

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120 V: •

I = VA/V = 20000/120 = 166.7 A

•

# circuits = 166.7/16 = 10.4 circuits = 11 circuits

277 V: •

I = 20000/277 = 72.2 A

•

# circuits = 72.2/16 = 4.5 circuits = 5 circuits

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3.

Using 120-volt circuits versus 277-volt circuits would require 6 additional circuits (protective devices, conduit, wiring, etc.).

480Y/277 V Circuits - The general illumination for large office and industrial areas uses fluorescent and HID lamps that are supplied at 277 volts line-to-neutral from 480Y/277-volt, threephase, 4-wire systems. The major advantages of this system voltage (480Y/277 V) are lower voltage drops of approximately 0.36 percent (100 x 1/277) for every 1-volt drop and a lesser number of circuits (see Example B). Other advantages are that (1) larger areas of a facility can be served from a single lighting panelboard and (2) three-phase motors can be supplied from the same source. A disadvantage is that 120-volt general purpose receptacle and lighting loads must be supplied by separate dry-type transformers that are strategically located throughout the building.

Receptacle Branch Circuits NEC Article 210-7 describes the requirements for receptacles and plug and cord-connected equipment. Types of Receptacles and Plugs Figure 9a illustrates the NEMA configuration for 125 V, 250 V, and 277 V single-phase, 2-pole, 3-wire grounded receptacles and plugs; Figure 9b illustrates the NEMA configuration for 125/250 V and three-phase, 250 V, 3-pole, 4-wire grounded plugs.

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Figure 9. NEMA Configurations for Receptacles and Plugs

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Single Receptacles installed on an individual branch circuit must have an ampere rating that is not less than that of the branch circuit. For example, a single receptacle installed on a 30-ampere branch circuit must be rated at 30 amperes. See NEMA receptacle types 5-30R, 6-30R, 7-30R of Figure 9a or types 14-30R and 15-30R of Figure 9b. The one exception to this ampere rating requirement is that two or more 15-ampere receptacles (e.g., NEMA type 5-15R of Figure 9a) are permitted on a 20-ampere circuit. Figure 10a is an illustration of a single, 2-pole (2P), 3-wire (3W) grounded receptacle. Figure 10b is an illustration of a twist-lock single receptacle and plug.

Figure 10. Types of Single Receptacles

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Duplex Receptacles are the most common type of general purpose receptacles that are used in residential, commercial, and industrial facilities. Figure 11 is an illustration of a general purpose 125-volt, 15-ampere duplex receptacle.

Figure 11. Duplex Receptacle

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Triplex Receptacles are the least common types of receptacles that are found in commercial and industrial use. Many engineers are reluctant to specify triplex receptacles because of their concerns that the branch circuit might end up being overloaded. Figure 12 is an illustration of a triplex receptacle.

Figure 12. Triplex Receptacle General Purpose Ratings The majority of receptacles are installed for general purpose and the exact loads are more than likely unknown to the design engineer. In this general purpose case, the NEC specifies a minimum loading of 180 voltamperes (VA) for each general purpose receptacle. The 180 VA rating applies to each outlet regardless of whether it is a single, duplex, triplex, or any combination of receptacles installed on the individual branch circuit. Example C: What are the maximum number of outlets, for example, duplex receptacles, permitted by the NEC on a 120-volt, 15-ampere branch circuit? a 20-ampere branch circuit? Answer: 1.

No. of outlets = (V x A)/180 VA

2.

15 A branch circuit: •

3.

20 A branch circuit: •

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No. of outlets = (120 x 15)/180 = 10 outlets No. of outlets = (120 x 20)/180 = 13 outlets

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Specific Equipment Ratings In general, the minimum load for an outlet installed for a specific piece of equipment, appliance, or load is determined by the ampere rating of the piece of equipment, appliance, or load. As previously discussed, a single receptacle installed on an individual branch circuit shall have an ampere rating that is not less than the rating of the branch circuit. In most cases, the NEC restricts the continuous load served by the branch circuit to 80 percent of the branch circuit’s rating, unless the overcurrent protective device is listed for continuous operation at 100 percent of its rating. Color Coding As mentioned previously under the section “Color Coding of Conductors”, the branch circuit phase (ungrounded) conductors may be any color except white, natural gray, or green. The grounded conductor (neutral) colors are white or natural gray and the equipment grounding conductor colors are green or green with yellow stripes.

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Voltage Limitations Receptacles are commercially available that range from 125 to 277 volts in general purpose-nonlocking types, and that range from 125 to 600 volts in specific purpose-locking types. Although receptacles are readily available at all voltage levels, NEC Article 210-6 is very specific at defining what types of equipment (e.g., light fixtures, receptacles, cord and plug connected equipment, etc.) may be supplied at different voltage levels. For example, all 120-volt, 15 and 20-ampere outlets must be the grounded, three-pole type. As another example, NEC Article 210-6 requires that receptacles having different voltages, amperes, etc., and located on the same premises, shall be of such design that the attachment plugs used on these circuits are not, as illustrated in Figure 13, interchangeable. The 20ampere receptacles illustrated in Figures 13a and 13c will not accept the 30-ampere plugs that are illustrated in Figures 13b and 13d, nor will the 250-volt receptacles illustrated in Figures 13c and 13d accept the 125-volt plugs that are illustrated in Figures 13a and 13b. Figure 9 also illustrates that none of the plugs and receptacles at different voltage and ampere ratings are interchangeable.

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Figure 13. Non-Interchangeability of Receptacles and Plugs

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Ground Fault Circuit Interrupters (GFCI) NEC Articles 210-8(b)(1) and 210-8(b)(2) require GFCI protection for personnel, for all 125-volt, 15 and 20-ampere receptacles that are installed in bathrooms and rooftops. A variety of GFCI devices are available. For example, GFCI can be installed on the branch molded case circuit breaker or as part of the receptacle. Figure 14 illustrates GFCI for personnel protection. The operating characteristic of GFCI is very simple. As long as the current flowing out on the phase conductor (Iφ) equals the return current flowing on the neutral conductor (IN), the device (e.g., receptacle or circuit breaker) remains in a closed position. If either of the conductors come in contact with a grounded object, either directly or through a person’s body, some of the current, called the unbalanced current (Iu), will return by an alternative path. The unbalanced current is sensed by the toroidal coil, and a secondary current (Is) flows on the secondary side of the toroidal coil, which will shunt-trip open the circuit. The GFCI typically operates on an unbalanced current of approximately 5 mA (0.005 A), with a range of 4 to 6 mA being the values that are required by standard. The GFCI devices also have test circuits (e.g., a pushbutton), so that the GFCI can be periodically checked to ensure proper operation.

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Figure 14. GFCI Circuit Common Neutrals One important advantage over the years of using 3-phase, 4wire wye power system configurations is that it permits up to three circuits (one on phase A, one on phase B, and one on phase C) to be connected to one common neutral. Under balanced conditions, the neutral current equals zero, and under unbalanced conditions, the neutral conductor only carries the unbalance. Even if one of the circuits is totally disconnected, the worst case of current flowing in the neutral conductor would be the phasor sum of the two remaining currents, which still only equals the magnitude of one of the phase currents; however, in recent years, harmonic currents (non-sinusoidal currents) routinely flow in building branch circuits. The most prominent harmonic current is the third harmonic (f = 180 Hz), which is caused by non-linear loads, such as arc-discharge light source electronic ballasts and the electronic circuitry of office copiers, personal computers, etc. Third harmonic currents, unlike their 60 Hz counterpart currents, do not cancel in the neutral; instead, they add together. Under many conditions, the current that flows in the neutral conductor can actually be greater than the currents that flow in the phase conductors. Therefore, the NEC, in many different articles, cautions designers to account for harmonic currents if the electrical equipment is harmonic producing. As a result of these harmonic currents, many private industries no longer permit local use of common neutral circuits. Industry engineers also oversize the neutral conductors to compensate for third harmonic currents that flow in the power system.

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Power Branch Circuits For the most part, NEC power branch circuit requirements are the same as for lighting and receptacle branch circuits. Receptacle and Other Types NEC Articles 210-23(a) and (b) cover the requirements for 30, 40, and 50- ampere branch circuits. A 30-ampere branch circuit is permitted for fixed lighting units, with heavy duty lampholders for non-dwelling units, or for any utilization equipment. A 40 or 50-ampere branch circuit has the same requirements as the 30ampere branch circuits, and they also are permitted to supply infrared heating units. Color Coding and Voltage Limitations Color coding and voltage limitations are identical to the color coding and voltage limitations of lighting and receptacle branch circuits. NEC Requirements for Branch Circuits Over 50 Amperes NEC Article 210-23(d) specifies that branch circuits over 50 amperes only are permitted to supply nonlighting outlet loads.

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Branch Circuit Sizing Selection Factors Loads NEC Article 220 provides the following requirements for determining the number of branch circuits as well as for computing the branch circuit loads: •

The branch circuit rating shall not be less than the noncontinuous load plus 125 percent of the continuous load. If the assembly, including the overcurrent devices, is listed for continuous operation at 100 percent of its rating, the branch circuit rating shall not be less than the total load.

•

Outlets for specific equipment loads shall equal the ampere rating of the equipment.

•

Outlets supplying light fixtures shall be the maximum VA rating of the equipment (e.g., ballasts) and lamps.

•

Outlets for heavy duty lampholders shall not be less than 600 VA.

•

General purpose outlets shall not be less than 180 VA per outlet.

•

The minimum number of branch circuits shall be determined from the total connected load and the size or rating of the circuits that are used.

Example D: Using Work Aid 1E, Step 1a, and the following information, calculate the minimum number of branch circuits to illuminate a large office area on a 208Y/120-volt system. Information: 1.

60 2-tube luminaires, each lamp 113 watts

2.

luminaire watts, including the ballast - 252 watts

3.

ballast p.f. - 95%

4.

20 A branch circuits - 80% rated

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Answer: •

IL = P/(V x p.f.) = 252/(120 x 0.95) = 2.21 A/luminaire

•

NL80 = 16 A/IL = 16/2.21 = 7.23, select 7 (maximum)

•

NBC = TNL/NL80 = 60/7 = 8.57, select 9 (minimum)

•

Recommend 6 branch circuits with 7 luminaires and 3 branch circuits with 6 luminaires. The luminaire placement (layout) will dictate the final number of branch circuits; however, 9 branch circuits are the NEC minimum.

Example E: Repeat Example D if the system voltage is 480Y/277 volts . Answer:

IL = 252/(277 x 0.95) = 0.96 A NL80 = 16/0.96 = 16.7, select 16 (maximum) NBC = 60/16 = 3.75, select 4 (minimum)

Conductors Operating Temperature - The maximum continuous current carrying capability of a conductor is determined by the temperature at which it is allowed to operate over its lifetime. The type of insulation surrounding the material ultimately determines the operating temperature. The NEC temperature rating classifications are 60ºC, 75ºC, and 90ºC, but SAES-P-104 does not permit use of 60ºC insulation. If the operating temperature is exceeded for any long period of time, the insulation ages prematurely, it becomes hard and brittle, which eventually leads to early failure.

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Ambient Temperature - The ambient temperature is defined as the temperature of the medium (air or earth) surrounding the conductor. When the ambient temperature increases, there is less of a temperature differential surrounding the conductor, which means that the heat dissipating rate of the conductor is less. As the ambient temperature increases, the current carrying capability of the conductor must be decreased to prevent exceeding the operating temperature which is, once again, based on the insulation material. The NEC ampacity ratings for building wire are based on an ambient temperature of 30ºC. SAES-P-100 specifies a design ambient temperature of o the indoor air-conditioning system or 30 C, whichever is greater, for indoor air-conditioned spaces. NEC Derating Factors - If the ambient temperature exceeds 30ºC, the NEC requires derating of the conductor in accordance with the correction factors that are listed in NEC Table 310-16. Current-Carrying Conductors - For determining the number of conductors in a raceway, only conductors that normally carry current are considered as current-carrying conductors. For example, the equipment grounding conductor (green wire) is not considered a current-carrying conductor. On the other hand, in a 3-wire circuit consisting of two phase wires and the neutral of a 3-phase, 4-wire, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors, and it is considered to be a current-carrying conductor. (This is the case in Saudi Aramco for sizing the branch circuits where use of common neutral is prohibited.) See Note 10 to NEC Article 310-16. Terminal Limitations - Although the NEC specifies the ampacity ratings of conductors, the Underwriters Laboratories, Inc. (UL) approves the use and conditions of the electrical equipment. The termination provisions of the UL-approved equipment are based on 60ºC and 75ºC operating temperature terminations (terminals, lugs, etc.). 100 Amperes or Less: NEC Article 110-14(c)(1) specifies that termination provisions of equipment for circuits that are rated 100 A or less, or marked for sizes AWG No. 14 through AWG No. 1 conductors, shall be used with conductors rated at 60ºC operating temperatures. The NEC permits the following two exceptions:

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•

High-temperature-rated conductors may be used, provided the ampacity of the conductors is based on the 60ºC operating temperature ampacity ratings.

•

The equipment is listed for the higher temperature-rated conductors.

Greater Than 100 Amperes: NEC Article 110-14(c)(2) specifies that termination provisions of equipment for circuits that are rated over 100 A, or marked for conductors larger than AWG No. 1, shall be used with conductors rated at 75ºC operating temperatures. The same two exceptions apply for circuits rated more than 100 A, except that the ampacity of the conductors is based on 75ºC operating temperature ampacity ratings. Phase and Neutral conductors are sized based on the load with conditions, as stated previously in this Information Sheet under the section “Loads”. Equipment Grounding Conductors are sized in accordance with NEC Table 250-95, which specifies that the minimum size equipment grounding conductor’s ampere rating (size) is based on the rating of the branch circuit protective device’s ampere rating. Voltage Drop (VD) is the difference between the voltage at the source end (Vs) of the branch circuit, which is assumed to be a fixed value, and the voltage at the load end (VL), which varies as a function of the load or branch circuit current (IL). The voltage drop calculated is a line-to-neutral drop (one-way). The line-to-line voltage drop for a single-phase system is 2VD, and the line-to-line drop for a three-phase system is 3 VD. The current (IL) flowing in the circuit is assumed to be the load or branch circuit current. The voltage drop is then equal to the load current times both the resistance (R) and the reactance (X) of the circuit. The power factor (p.f.) of the load also directly affects the voltage drop. As the power factor decreases, the voltage drop will increase. Note: For purposes of this Module, assume that all power factors are lagging. And finally, the voltage drop (ILZ) is directly proportional to the branch circuit lengths, because the line impedance Z depends on the length. As the length of the branch circuit increases, the impedance of the line increases. Note: NEC Table 9 lists the impedance of branch circuit conductors.

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Protection For purposes of this Information Sheet, all protection (overload and short circuit) for the branch circuit conductors will be provided by thermal-magnetic molded case circuit breakers (MCCBs) and, where required by the NEC for personnel protection, by GFCI type MCCBs. Overload and Fault protection of branch circuits are required by both the NEC and Saudi Aramco design standard SAES-P104. MCCBs are a class of breaker that are rated at 600 volts and below, and they consist of a switching device and an automatic protective device assembled in an integral housing of insulated material. Solid-state trip units incorporated into some styles of MCCBs provide for their coordination with power breakers. MCCBs are generally sealed to prevent tampering, which in turn precludes any inspection of the contacts. MCCBs are generally not designed to be maintained in the field, and manufacturers recommend total replacement if a defect appears. MCCBs are available in several different types. The thermalmagnetic type, which is the most widely used type, employs thermal tripping for overloads and magnetic tripping for short circuits. For cases where only short circuit interruption is required, the magnetic type of MCCB employs only instantaneous magnetic tripping. The integrally-fused type of MCCB combines regular thermal- magnetic protection with current limiting fuses to respond to applications where higher short circuit currents are available. In addition, the integrallyfused current limiting type of MCCB offers high interrupting capacity protection, while at the same time limiting the letthrough current to a significantly lower value than is usual for conventional MCCBs. GFCI Criteria requires, once again, that personnel be protected where 15 and 20-ampere branch receptacle circuits are installed in bathrooms or on rooftops. NEC Article 210-8(b) defines a bathroom as an area including a basin with one or more of the following: a toilet, a tub, or a shower.

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Conduit Sizing (NEC Tables) The number of conductors permitted in a raceway is restricted by the NEC. The total cross-sectional area of the conductors, which includes the insulation, must not exceed a specified percentage of the wireway or conduit cross-sectional area. The NEC refers to this restriction as “percentage fill”. Exceeding the percentage fill can cause physical damage to the conductors as they are being installed (pulled) through the raceway. Additionally, the heat buildup in the raceway could be excessive, resulting in damage to the insulation. Conductor Insulation - Because each type of conductor has different insulation thicknesses, the percentage fill for each type of conductor (insulation) is different. Table 5, Chapter 9, of the NEC lists the dimensions (diameter and area) for each size and type of rubber-covered and thermoplastic-covered conductors. Number of Conductors - Determining the permissible number of the same type of conductors (size and types of insulation) 40% fill in different types of conduit can be selected from Appendix C, Tables C1 through C12 of the NEC. Determining the permissible number of conductors in a wireway, or determining the number of conductors of mixed sizes and types of insulations, must be calculated based on the fill rates that are permitted by the NEC. Tables - Tables C1 through C12 are based on a 40-percent fill rate of conductors in a given type and trade size of conduit (Figure 15). If conductor insulation types are mixed, the tables cannot be used and the fill rate for a particular mix of conductors in a given trade size of conduit must be calculated as previously explained in Module EEX103.01. The number of conductors, for fill rate purposes, includes all conductors, regardless of whether they are considered as current-carrying conductors or not. For example, the “green” equipment grounding conductor is considered for percentage fill restrictions, even though it does not carry current, except under line-to-ground fault conditions.

12/01/97

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Figure 15. Conduit Fill Rate Example

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SIZING LIGHTING AND POWER DISTRIBUTION PANELBOARDS

Introduction Lighting and appliance branch circuit panelboards (referred to as simply lighting panelboards) are defined in NEC Article 384 as “one having more than 10 per-cent of its overcurrent devices rated 30 amperes or less for which neutral connections are provided.” Article 384 also limits the number of overcurrent devices (branch circuit poles) to a maximum of 42 devices in any one cabinet. When the 42 poles are exceeded, two or more separate panels are required. Power distribution panelboards, which consist of all other panelboards not defined as lighting and appliance branch circuit panelboards, are restricted only to practical physical limitations, such as standard box heights and widths. Additional boxes and fronts are required when the components required for one panelboard exceed the standard box dimensions.

Types of Loads This Information Sheet will briefly describe the following types of loads: •

Lighting and receptacle (general purpose) branch circuits

•

Power branch circuits

•

Feeder circuits

Lighting Branch Circuits The load for lighting branch circuits will be computed based on the voltampere (VA) ratings of the lamps and ballasts (if applicable).

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Receptacle Branch Circuits The load for general purpose receptacles will be computed based on 180 VA per receptacle outlet. Although the NEC minimum rating is 180 VA per outlet, many design engineers use a more conservative rating of 240 VA per outlet. Room layout may dictate another rating for the general purpose receptacles. For example, dividing the receptacle circuits of three different rooms into three branch circuits may be a designers preferred choice, rather than a minimum of only two branch circuits, which was determined by computation. Power Branch Circuits For purposes of the Module, if the branch circuit supplies a load other than for lighting or general purpose receptacles, it will be called a power branch circuit. Feeder Circuits Feeder circuit conductors are the conductors that supply electrical power from the service equipment location (e.g., a panelboard) to the enclosure (e.g., a sub-panelboard) containing the final branch circuit overcurrent protective devices. See Figure 7.

NEC Panelboard Requirements Used as a Service Entrance When panelboards are used as service entrance equipment (see Figure 4), NEC Articles 230-F and G and Underwriter’s Laboratories (UL), Incorporated require the following: •

Panelboards used as service entrance equipment must be located near the point where the supply conductors enter the building.

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A panelboard having main lugs only (MLO) shall have a maximum of six service disconnects to de-energize the entire panelboard from the supply conductors. Where more than six disconnects are required, a main service disconnect must be provided.

•

Panelboards must include connections for bonding and grounding the neutral conductor.

•

A service entrance type UL label must be factory-installed.

•

Ground fault protection (GFP) of equipment, as required by NEC Article 230-95, shall be provided for solidly grounded wye electrical services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnecting means that is rated 1000 amperes or more. Note: GFP of equipment should not be confused with GFCI protection of personnel, as previously discussed.

Phase Arrangements The phase arrangements on three-phase buses, is required by NEC Article 384-3(f) to be A, B, C from front to back, top to bottom, or left to right as viewed from the front of the panelboard. Figure 4 illustrates the correct phase arrangement of a panelboard. Lighting Panelboard NEC Article 384-14 describes a lighting panelboard as a panelboard that has more than 10 percent of its overcurrent devices (e.g., MCCBs) that are rated less than or equal to 30 amperes and for which neutral connections are provided. See Figure 4.

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Number of Overcurrent Devices NEC Article 384-15 restricts the number of overcurrent devices in a panelboard to 42 devices. The same article also requires that the panelboard be provided with “physical means” to prevent installation of more overcurrent devices than the number of devices for which the panelboard was designed, rated, and approved. For purposes of the NEC, two-pole (2P) breakers are considered as two (2) overcurrent devices and three-pole (3P) breakers are considered as three (3) overcurrent devices. Ratings NEC Article 384-13 requires that all panelboards shall have a rating not less than the minimum feeder capacity required for the load that is computed in accordance with Article 220. Panelboards shall be durably marked by the manufacturer with the voltage and the current rating, the number of phases for which they are designed, and the manufacturer’s name or trademark in such a manner as to be visible after installation, without disturbing the interior parts or wiring. The following list is a sample set of panelboard ratings available from a particular vendor of pre-assembled panelboards: •

Voltage:

240 vac Max. 480Y/277 vac Max. 250 vdc Max.

•

Main Lugs: 100 thru 600 amperes

•

Main Breakers: 100 thru 600 amperes

•

Branches: 15 through 100 amperes

•

Interrupting Capacity (Sym.): 240 vac: 65 kA Fully-Rated 240 vac: 100 kA thru 200 kA Series-Rated 480Y/277 vac: 14 kA Fully-Rated

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480Y/277 vac: 22 kA thru 150 kA Series-Rated 250 vdc: 10 kA and 14 kA Fully-Rated •

Service: 3-Phase, 4-Wire, 208Y/120 V, 120/240 V Delta, 480Y/277 V 1-Phase, 3-Wire, 120/240 V 1-Phase, 2-Wire, 120 V 3-Phase, 3-Wire 120, 208, and 240 V 1-Phase, 2-Wire, 125 vdc 2-Phase, 2-Wire, 250 vdc

Circuit Directory NEC Article 384-13 further requires that all panelboard circuits and circuit modifications shall be legibly identified as to purpose or use and that this identification be located on a circuit directory on the face or inside of the panel doors.

Panelboard Selection Factors The two major factors plus the miscellaneous factors to consider when selecting or sizing a panelboard are the following: •

Loads (continuous or non-continuous).

•

Protection (main, feeder, branch circuit breakers).

•

Miscellaneous factors (phase balance, number of spaces, lighting panelboard schedule, ambient temperatures, special conditions).

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Load Panelboard continuous current ampere ratings are based on the load that is computed in accordance with NEC Article 220. The first consideration in computing the load is to determine whether the loads are continuous or non-continuous loads. Continuous Loads - The NEC defines a continuous load as a load that , in normal operation, will continue (to operate) for three hours or more. For example, lighting loads are considered to be continuous loads. The continuous current rating of the panelboard cannot be less than the service entrance or feeder conductors that supply power to the panelboard. These service entrance or feeder conductors are sized based on the sum of the non-continuous loads plus 125 percent of the continuous loads. The NEC does permit, as an exception, the ampacity ratings of the conductors to be sized based on the sum of the continuous and noncontinuous loads, if the overcurrent device protecting the conductors is listed (e.g., UL) for operation at 100 percent of its rating. Non-Continuous Loads are loads that, under normal conditions,

do not operate for three hours or more. For example, a general purpose receptacle branch circuit load is considered to be a non-continuous load.

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Protection Protection for, or in a panelboard, must consider the following factors: •

The main circuit breaker that is enclosed in the panelboard.

•

A feeder protective device that protects the feeder as well as a main lugs only (MLO) panelboard.

•

The branch circuit breakers.

Main Breaker on a Panelboard - The main breaker, although it is not necessarily the most economical method of protecting a panelboard, it is the best or preferred method of protection. The main breakers continuous current rating, as mentioned previously, is based on the sum of the non-continuous load plus 125 percent of the continuous load, unless the main breaker is a 100-percent-rated device. UL permits panelboards to be labeled with a short circuit rating of up to 200 kA (symmetrical) where UL-listed combinations of main and branch circuits are used. These combinations consist of main breakers or fusible devices connected ahead of and in series with approved conventional breakers used as branch devices. Note: Saudi Aramco standards do not permit seriesrated combinations. Two arrangements are acceptable, and both arrangements comply with UL standards for panelboards. The main circuit breaker may be installed in the panel as a main device (Figure 16a), or it may be mounted remote (Figure 16b) from the panel. In either case, the approved main and branch combinations must be followed. These arrangements are acceptable and are UL-listed because they have been tested in accordance with UL 67 standards.

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Figure 16. UL-Approved Panelboard Protection

If the main breaker and the branch circuit breakers have not been listed and approved as a series combination, the short circuit rating of the panelboard is that of the lowest interrupting rating of the main or branch circuit breakers. For example, if in Figure 16a, CB1, CB2, ..., CB16 have not been series-tested with MB1, panelboard LPA’s short circuit rating is 22 kA. On the other hand, if in Figure 16b, CB1, CB2, …, CB16 have been series-tested, approved, and listed, panelboard LPB’s short circuit rating is 65 kA.

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Feeder Protective Device - Many designers, primarily for economical purposes, choose MLO type panelboards (Figure 17a) versus main breaker panelboards (Figure 17b). Either method is permitted by the NEC. Saudi Aramco design standards only permit main breaker panelboards (Figure 17b).

Figure 17. Feeder Device Protection of a Panelboard

Branch Circuit Breakers are selected based on their continuous current and short circuit ratings. Neither the setting nor the rating of the branch circuit breakers is permitted to exceed the ampacity ratings of the conductor, although NEC Article 240-3 does permit “rounding up” of the breaker to the next standard device size, as listed in NEC Article 240-6. If the branch circuit breakers are 100-percent-rated, the NEC permits 100-percent loading for continuous loads; if the branch circuit breakers are not 100-percent-rated, NEC Article 384-16 restricts loading of the branch circuit breaker to 80 percent of its ampere rating for continuous loads.

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Miscellaneous Factors Phase Balance - The total load on the panelboard should be balanced, as much as possible, between all three phases (A, B, C); many design engineers try to keep the unbalance within + 10 percent. The neutral conductor will carry any unbalanced current and maintain the line-to-neutral voltage magnitude across each phase; however, if the neutral conductor is disconnected or broken, the loads would have voltages significantly different than the nominal line-to-neutral voltages. Number of Spaces - Panelboards are typically available with any number of spaces up to the NEC-maximum limit of 42 spaces. When the requirement dictates more than 42 spaces, two or more panelboards are required. Although there are no criteria available for panelboard layouts, it is usual practice to locate the lighting branch circuits at the top, followed by the general purpose receptacle branch circuits. The lighting or receptacle branch circuits are usually grouped in sets of three (A1, A3, A5 or A2, A4, A6) so that one common neutral may be used, where local design standards permit, and no third harmonic currents are present. It is also a designer’s usual practice to include several spare breakers and spare spaces for future loads. Lighting Panelboard Schedule - The purpose of a lighting panelboard is to show, in tabular, graphical, or chart form, the following items: •

circuit breaker number - #10

•

description - lighting (Room 5)

•

number of poles - 1

•

breaker frame size - 50 A

•

breaker trip rating - 20 A

•

phase connection - phase A

Figure 18 shows a typical LP schedule, in chart form, for the circuit wiring layout diagram that is shown in Figure 19.

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Figure 18. Typical Lighting Panelboard Schedule

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Figure 19. Circuit Wiring Layout Diagram

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Ambient Temperatures - The primary function of an overcurrent device is to protect the conductor and its insulation against overheating. In selecting the sizes of the devices and conductors, the engineer should consider the ambient temperature that surrounds the conductors within and external to the panelboard. Cumulative heating within the panelboard may cause premature operation of the overcurrent protective devices. Note: The average temperature inside of a panelboard enclosure is assumed to be 40ºC. Special Conditions - Standard panelboards, assembled with standard components, are adequate for most applications. However, special consideration should be given to those required for application under special conditions, such as the following: •

Excessive vibration or shock

•

Frequencies above 60 Hz

•

Altitudes above 6600 feet

•

Possible fungus growth

•

Compliance with codes and local and national standards

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SIZING SERVICE ENTRANCE CONDUCTORS Note: Work Aid 3 has been developed to teach the Participant procedures for sizing service entrance conductors.

National Electric Code (NEC) Requirements The NEC defines the service entrance conductors as the supply conductors that extend from the street main or from transformers to the service equipment of the premises being supplied. Note: NEC Article 230 covers service entrance conductors and equipment. Number of Services Permitted NEC Article 230-2 permits only 1 set of service conductors per building or structure. The same article lists seven exceptions to the general rule, but, if multiple services are installed, a permanent plaque or directory must be installed at each of the different services denoting all of the other services in the building. Insulation Service entrance conductors entering or on the exterior of buildings are required by NEC Article 230-41 to be insulated conductors. Physical Protection Service entrance conductors are required to be physically protected from damage. There are many different approved methods for protecting service entrance conductors. One approved method, which is the method that is required at Saudi Aramco, is to use rigid steel conduit.

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Termination at Service Equipment NEC Article 230-55 requires that any service raceway terminate at the inner end in a box, cabinet, or equivalent fitting that effectively encloses all energized metal parts. Saudi Aramco standards require that the service conductors terminate at approved and listed (e.g., UL) panelboards. Disconnecting Means NEC Article 230-70 requires that means shall be provided to disconnect all conductors in a building or structure from the service entrance conductors. The same article further requires that the disconnect shall be readily accessible, and that it shall be permanently marked to identify it as a service disconnecting means. Overcurrent Protection Overloads and Phase Faults - NEC Article 230-90 requires that each ungrounded conductor (the phase conductors) be protected by an overcurrent device (e.g., MCCB) having a rating or setting not higher than the allowable ampacity of the conductor. Ground Fault Protection (GFP) is required by NEC Article 230-95 for the protection of equipment. Note: Do not confuse GFCI protection, which is protection of personnel, for GFP. Equipment ground fault protection (GFP) is required whenever the service disconnecting means is rated or can be set at 1000 amperes or higher, and the voltage exceeds 150 volts line-toground for 3-phase, 4-wire solidly grounded systems. The Saudi Aramco service that falls within this requirement is the 480Y/277 V, 3-phase, 4-wire solidly grounded service. Figure 20 illustrates several methods of providing GFP, with the zerosequence CT being the preferred Saudi Aramco GFP method.

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Figure 20. Ground Fault Detection

Sizing Selection Factors This Information Sheet will explain the following factors for sizing service entrance conductors: •

Load data and phase conductors

•

Grounded conductor including harmonic currents

Load Data and Phase Conductors The service entrance phase conductors must be sized to carry the non-continuous loads plus 125 percent of the continuous loads. The only exception to this rule is that, if the protective device is 100-percent-rated, the conductors need only be ampacity-rated to carry the total loads (continuous plus noncontinuous).

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Grounded Conductors The grounded conductor (neutral) is sized based on the size of the grounding electrode conductor in accordance with Article 250-23(b) and Table 250-94 of the NEC. Table 250-94 specifies that the size of the grounding electrode conductor and, by inference, the grounded conductor, is based on the size of the largest service entrance conductors. The grounded conductors are also required to be routed with the ungrounded service entrance phase conductors and, if the service entrance phase conductors are paralleled, the size of the grounded conductor shall also be based on the equivalent area for parallel conductors. Of course, if the paralleled service entrance phase conductors are routed in separate conduit runs, the grounded conductors must be paralleled and routed in the same conduits as the phase conductors. Harmonic Currents - Neutral current in three-phase power systems is often thought to be only the result of the imbalance of the phase currents. With computer systems, arc-discharge lighting systems, office copier machines, and other similar nonlinear, electronic type of loads, very high neutral currents have been observed, even when the phase currents are balanced. On three-phase wye power systems, the neutral current is the vector sum of the three line-to-neutral currents. With balanced, three-phase, linear currents, which consist of sine waves spaced 120 electrical degrees apart, the sum at any instant in time is zero, and there is no neutral current flowing in the circuit (Figure 21).

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In most three-phase power systems supplying single-phase loads, there will be some phase current unbalance and some neutral current. Small neutral currents resulting from slightly unbalanced loads do not cause problems for typical building power distribution systems; however, there are conditions where even perfectly balanced single-phase loads can result in significant neutral currents. Nonlinear loads, such as rectifiers and power supplies, have phase currents that are not sinusoidal. The vector sum of balanced, non-sinusoidal threephase currents does not necessarily equal zero. In three-phase circuits, the triplex harmonic neutral currents (third, sixth, ninth, etc.) add, instead of cancelling. Being three times the fundamental power frequency and spaced in time by 120 electrical degrees based on the fundamental power frequency, the triplex harmonic currents are in phase with each other and they add in the neutral circuit (Figure 22). If third harmonic neutral currents are available in the system, the design engineer must full-size the grounded (neutral) conductor the same size as the phase conductors.

Figure 21. Balanced 60 Hz Currents - Neutral Current Equals Zero (0)

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Figure 22. Third Harmonic (180 Hz) Currents - Neutral Current Equals 3.0 p.u.

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SIZING THE SERVICE ENTRANCE GROUNDING SYSTEM Note: Work Aid 4 has been developed to teach the Participant procedures to size the service entrance grounding system.

Introduction The National Electric Code (Article 250, Section H) requires all elements of a grounding system to be bonded together to form the grounding electrode system. Any one of the elements presented in this Information Sheet, except for the metal underground water pipe, can be used as the “single” grounding electrode for a facility; however, if more than one of the elements are available on the premise, they must all be bonded together to form the grounding system. The first four electrodes, described below, are listed in the NEC as “available” electrodes (Article 250-81); the remainder are “made” electrodes (Article 250-83).

Types of Grounding Electrodes This Information Sheet will describe the following listed types of grounding electrodes commonly used in industrial power systems. •

Water piping

•

Structural steel building frame

•

Concrete-encased bars)

•

Ground ring (peripheral)

•

Rod and pipe (e.g., ground rods)

•

Buried plate electrodes

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Water Piping NEC Article 250-81(a) requires that a metal underground water pipe that is in direct contact with the earth for 10 feet (3.05 m) or more, and that is electrically continuous, shall be used as part of the building grounding electrode system; however, if the water piping system is used as a grounding electrode, it must be supplemented by at least one other type of an approved electrode. Note: SAES-P-111 does not permit the water piping system to be used as a grounding electrode. Structural Steel Building Frame NEC Article 250-81(b) requires that the structural steel (metal) frame of the building (Figure 23), if effectively grounded, be used as part of the building’s grounding electrode system. Effectively grounded means intentionally connected to earth through a ground connection or connections of sufficiently low impedance, and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazard to connected equipment or to persons. Note: SAES-P-111 (Section 7.1.1) permits building steel to be used as a grounding electrode, provided that it is continuous and it is effectively grounded.

Figure 23. Structural Steel Building Frame

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Concrete-Encased Electrodes Rod or wire electrodes, encased in concrete, usually results in lower resistance grounding electrodes than when similar electrodes are placed directly in the earth. NEC Article 25081(c) permits the use of concrete-encased electrodes. One acceptable concrete-encased electrode method, which is widely used in industry, is the use of steel reinforcing bars (rebar) in concrete footings and foundations (Figure 24). It is only necessary to bring out an electrical connection from the rebar of each footing for attachment to the building ground bus or structural steel. Note: SAES-P-111 (Section 7.1.2) specifies that if a concrete-encased electrode is used, the conductor must be bare copper.

Figure 24. Concrete-Encased Electrodes

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Ground Ring (Peripheral) A peripheral ground system (Figure 25) generally consists of a bare, tinned copper conductor, that connects a series of ground rods buried around a structure, is another NEC-approved (Article 250-81(d)) “made” grounding electrode system. The peripheral ground conductor buried around the structure must be at least 30 inches deep, and it must consist of at least 20 feet of bare copper conductor not less than AWG No. 2. The ground rods should be bonded to the conductor by thermal welding, and a pigtail should be extended into the building for connection to the main ground bus. Note: SAES-P-111 (Section 7.1.3) permits ground ring electrodes.

Figure 25. Ground Ring (Peripheral)

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Ground Rod and Pipe Electrodes The most common method of establishing a grounding electrode system is the use of single ground rods (Figure 26), which is considered by NEC Article 250-83 as a “made” electrode. The most common type of ground rod material is copper clad steel.

Figure 26. Ground Rod Electrode Rod and pipe electrodes must be at least 8 feet long. If more than one electrode is required to meet the NEC-minimum resistance level of 25 Ω per a single ground rod, the minimum spacing is 6 feet (Figure 27). Note: There is no NEC requirement for two or more ground rods to be less than or equal to 25 Ω; only a single ground rod must be less than or equal to 25 Ω. Pipe electrodes must be at least 3/4-inch diameter and, if they are made of steel, they must have the outer surface galvanized to prevent corrosion. Rods of iron or steel must be at least 5/8-inch diameter, and nonferrous or stainless steel rods must be “listed” and at least 1/2-inch diameter. If rock is encountered, at least 8 feet of the rod must still be in contact with the earth, as shown in Figure 28. Note:

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SAES-P-111 (Section 7.1.3) permits use of ground rods as primary grounding electrodes.

Figure 27. Ground Rod Length and Spacing

Figure 28. Ground Rod Rock Bottom Installation

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Deep Driven Ground Rods represent an economical method for securing better ground connections. This type of grounding electrode is particularly applicable in areas where it is difficult to obtain low resistance earth connections by means of single eight or ten-foot rods. This method (deep driven) is also usually more convenient and effective than multiple rods or soil treatment. As shown in Figure 29, doubling the length of a ground rod reduces resistance (R) by approximately 40 percent, which is an excellent means of reducing R.

Figure 29. Effects of Deep Driven Ground Rods

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Ground Rod Diameter - As shown in Figure 30, doubling the diameter has little effect on ground rod resistance, and, in fact, it only decreases resistance by about 10 percent. Based on this fact, the only valid good reason to increase ground rod diameter is for mechanical strength. For example, it is usually easier to hammer a 1-inch diameter ground rod into hard soil than it is to hammer a 1/2-inch diameter ground rod into hard soil.

Figure 30. Effects of Ground Rod Diameter

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Multiple Ground Rods - The effect of multiple rods in parallel is shown in Figure 31. Adding a second ground rod does not halve the resistance, but it does help to reduce resistance and it is a generally accepted means of reducing R. Increasing the spacing between the multiple ground rods, as also shown in Figure 31, will also reduce R.

Figure 31. Effects of Multiple Ground Rods

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Buried Plate Electrodes In locations where the soil is extremely shallow, a horizontal plate (Figure 32) may be used as a grounding electrode. A horizontal plate represents a greater contact area to the soil in a given volume than does a rod. Each plate is required by NEC Article 250-83(d) to expose 2 ft2 or more of surface area to the soil. By comparison, a 3/4-inch, 10-foot ground rod has only 2 2 1.96 ft of lateral surface area compared to the area (2.83 ft ) of a 1/4-inch, 1-foot-square iron or steel plate. Note: Saudi Aramco uses buried plate electrodes as primary ground sources.

Figure 32. Buried Plate Example F: What is the surface area of a 3/4-inch diameter, 10-feet long ground rod? a 1/4-inch thick, 1-foot square plate (Figure 32)? Answer: 1.

A (cylinder) = 2πrh = 2π (0.75/2)(10 x 12) 2 2 = 282.74 in = 1.96 ft

2.

Area (plate) = (2 x l x w) + (4 x l x w) = (2 x 1 x 1) + (4 x 1 x 0.25/12) = 2.083 ft2

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Grounded Service Conductor (Neutral) Selection Factors Routing The grounded service conductor (neutral) is required by NEC Article 250-23(b) to be routed with the phase conductors to the service disconnecting means. The same article also requires that the neutral be bonded to the disconnecting means enclosure. In practice, this bonding is typically accomplished by routing the neutral conductor to the neutral bus and connecting the neutral bus to the equipment grounding bus by either a main bonding jumper (Figure 2) or a screw. Sizing The grounded conductor (neutral) is sized in exactly the same way as the grounding electrode conductor, which is sized based on the size of the service entrance phase conductors, as specified in NEC Table 250-94. If the size of the service entrance phase conductors is larger than 1100 kcmil, the grounded (neutral) conductor must be sized at least 12.5% of the size of the largest service entrance conductor. When the service entrance phase conductors are paralleled, the size of the grounded (neutral) conductor shall be based on the equivalent area for phase conductors. Example E: The size of the copper service entrance conductors for a particular installation is 500 kcmil. What is the minimum size neutral conductor permitted by the NEC? Answer:

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Per NEC Table 250-94, the neutral conductor must be an AWG No. 1/0 or larger copper conductor.

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Grounding Electrode Conductor Selection Factors Outside Premises Location The grounding electrode conductor must be connected to the grounded service conductor at any accessible point, from the load end of the service drop or service lateral, to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means. See Figures 1 and 2. Where the transformer supplying the service is located outside the building, as shown in Figure 1, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means. Panelboard Connection The grounding electrode conductor is normally connected (terminated) directly at the neutral bus (Figure 33a); however, if the main bonding jumper that is specified in NEC Articles 25053(b) and 250-79 is a wire or busbar, and it is installed from the neutral bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor is permitted to be connected to the equipment grounding terminal bar or bus to which the main bonding jumper is connected (Figure 33b) .

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Figure 33. Grounding Electrode Conductor Connection to the Panelboard

Requirements Based on Size of Service Entrance Conductors The size of the grounding electrode conductor (GEC), as specified by NEC Article 250-94 and Table 250-94, is based on the size of the service entrance conductors. The same article permits the following exceptions to the basic rule. •

If using “made” grounding electrodes, the GEC is not required to be larger than AWG No. 6 copper.

•

If using concrete-encased electrodes, the GEC is not required to be larger than AWG No. 4 copper.

•

If using a ground ring electrode, the GEC is not required to be any larger than the conductors used for the ground ring.

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Neutral and Equipment Grounding Bus Selection Factors Neutral Bus Unless otherwise specified, panelboard vendors full-size the insulated neutral bus to be the same size (ampacity rating) as the phase bus. Ground Bus An equipment grounding bus is included as a normal component of all panelboards. Bonding Jumper Sizes As stated previously, and as shown in Figures 1 and 2, the neutral bus must be bonded to the equipment grounding bus. Where a main bonding jumper is used in lieu of a screw, the main bonding jumper, per NEC Table 250-94, must be sized (ampacity rating) the same as the grounding electrode conductor.

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WORK AID 1:

RESOURCES USED TO SIZE LIGHTING, RECEPTACLE, AND POWER BRANCH CIRCUITS

Work Aid 1A:

SAES-P-100 (11 MAR 96)

1.

2.

Section 5.3 specifies that steady state voltage drops shall be as follows : a.

Summation of voltage drops in feeders from main to distribution center, panelboard, or transformer that supply lighting, instrumentation, or other low voltage requirements shall be a maximum of 2 percent.

b.

Voltage drops for branch circuits that supply lighting, instrumentation, or other low voltage requirements shall be an average of 2 percent, with a maximum of 4 percent to the most distant outlet, providing that the maximum voltage drop for the main, feeder, and branch circuit does not exceed 5 percent.

Section 7.2 specifies that the following criteria (Figure 35) shall be used to establish equipment derating when specific requirements are not covered in an SAES or SAMSS. Location

Ambient Temperature Average Monthly Maximum Normal Maximum Daily Peak ºC ºC

Outdoors (Air)

45

50

Earth (Soil)

40

40

Ocean (Water)

30

30

Indoors Well Ventilated Buildings

40

50

Indoors Air-Conditioned Buildings

Note 1

Note 1

Note 1. Per the design temperature of the air conditioning system (see o SAES-K-001) or 30 C, whichever is greater.

Figure 34. SAES-P-100 Ambient Temperatures

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Work Aid 1B:

SAES-P-104 (7 DEC 94)

1.

Section 4.1 specifies that design and installation of wiring and cable systems shall be in accordance with NFPA 70 (National Electric Code NEC), as supplemented by this Standard.

2.

Section 4.2.1 specifies that wire and cable shall have copper conductors.

3.

Section 4.2.2 specifies that low voltage wire and cable (600 V or 600/1000 V and below) shall have a minimum rating of 75ºC.

4.

Section 4.2.5 specifies that power conductors shall be stranded copper 2 except that solid copper conductors 6 mm (No. 10 AWG) and smaller may be used in non-industrial locations and for specialty applications.

5.

Section 4.2.10 specifies that for 600 V and below power conductors, the minimum size conductor permitted is 2.5 mm2 (No. 14 AWG).

6.

Section 4.3.1 specifies that direct buried conduit shall be threaded, rigid steel, hot-dip galvanized, and PVC-coated, or type DB PVC conduit.

7.

Section 4.3.2 specifies that conduit above ground in outdoor industrial facilities shall be threaded, rigid steel, and be hot-dip galvanized.

8.

Section 4.3.4 specifies that the minimum conduit size shall be 3/4-inch, except on instrument panels, inside buildings, and on prefabricated skids, where the minimum size conduit shall be 1/2-inch.

9.

Section 4.3.6 specifies that electric metallic tubing (EMT) is acceptable only in nonhazardous indoor locations.

10.

Section 8.2.2 specifies that a feeder cable serving a load bus shall be sized in accordance with the NEC plus a 20% growth factor, but not to exceed the maximum rating of the bus.

11.

Section 8.2.5 specifies that a derating factor of 15% shall be applied to cables that require fireproofing.

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Work Aid 1C:

SAES-P-111 (24 FEB 96)

1.

Section 4.1 specifies that, except as noted in 4.2, grounding and ground system installation shall be designed in accordance with ANSI/IEEE 142 and meet the requirements of ANSI/NFPA 70 (NEC), and ANSI C2, as supplemented or amended by this Standard. Section 4.1.1 specifies that all grounding (system and equipment) shall be in accordance with NFPA 70 (NEC), as supplemented by this Standard.

2.

Section 4.2 specifies that, except as specifically noted, electrical installations in residential facilities, recreational facilities, schools and office buildings (including office buildings associated with plants and industrial facilities) shall be grounded in accordance with the industry standards referenced in 4.1 and are not required to meet the additional requirements contained in this standard.

3.

Section 4.3 specifies that, unless otherwise approved by the Coordinator, Systems Division, Consulting Services Department all grounding electrodes used for system grounding in plants, bulk distribution facilities, or other industrial areas shall be interconnected to form a single ground system. The grounding electrode used for system grounding (including separately derived systems) for each area in the facility or plant shall have a minimum of two connections to the overall grounding system. This requirement can be met by connections to the grounding electrode of the substation(s) which supply the area that needs to be interconnected to the overall plant system

4.

Section 5.3 specifies that ground rods or pipe electrodes shall be copper or copper-jacketed steel ("Copperweld" or equivalent). Copper jacketed steel rods shall meet the requirements of U.L. 467.

5.

Section 5.5 specifies that grounding connections to grounding grids or grounding electrodes shall be minimum 25 mm2 (No.4 AWG).

6.

Section 5.5 specifies that connections to grounding grids or between conductors and/or ground rods in grounding grids shall be made by thermite welding, brazing, or approved compression grounding connectors (Burndy "Hyground" system or equivalent).

7.

Section 7.1.1 specifies that reinforcing bar of buildings shall not be used as a grounding electrode. Structural steel of a building may be used as a grounding electrode in accordance with the NEC provided it is continuous and effectively grounded by connecting at least every other structural steel

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column on the perimeter of the building to a concrete-encased electrode or a ground ring installed per the NEC and this standard. 8.

Section 7.1.2 specifies that, if a concrete-encased electrode is used, the conductor must be bare copper.

9.

Section 7.1.3 specifies that a ground grid and/or ground rods used as the ground electrode for system grounding shall consist of either (1) rod or pipe electrode(s), or (2) a grid or loop of bare copper conductors buried a minimum of 460 mm. Suitable combinations of (1) and (2) are permitted. Multiple rod or pipe electrodes shall be interconnected by bare or insulated copper conductors using thermite welding or approved connectors per 5.6. Conductors used to interconnect rod or pipe electrodes shall be buried a minimum of 460 mm.

10.

Section 7.2 specifies that, in addition to the equipment grounding conductors run with the power conductors as required by the NEC, supplementary grounding per NEC 250-91© shall be provided in outdoor industrial areas, process plant areas, and in substations not covered by 6.1 above. In areas where no electrical equipment is installed, this supplementary grounding is not required unless otherwise specified. Supplementary electrodes shall onsist of ground rods, bare or insulated ground conductors, or combinations. Resistance to ground of each supplementary grounding electrode system shall meet the minimum requirements of NEC Article 250-84 for made electrodes. Where multiple items of equipment are connected, the supplementary grounding electrodes shall be interconnected to form grids or loops. The grids or loops shall be buried a minimum of 460 mm. This grounding electrode shall be bonded to the equipment grounding system in the area and may constitute a made electrode required to meet NEC requirements. The following equipment shall be connected to the supplementary grounding electrodes. (1) Structural steel supports for process equipment and piping and structural steel columns for buildings. Connections shall be made at least every 25 m (e.g., No part of the base of the structure shall be more than 25 m from a grounded support or column.) with a minimum of two connections at opposite corners of each structure or building. (2) Frames of equipment (motors, generators and transformers) operating at 1000 V or greater shall have two connections to a supplementary electrode. (3) Motors, transformers, and generators operating at a nominal voltage of 480 volts shall have a minimum of one connection to a supplementary grounding electrode. (4) The following equipment when not bolted to grounded structural steel shall be connected to a supplementary grounding electrode: Metallic enclosures for panelboards, circuit breakers, switches, fuses, motor controllers, switchgear, switchracks, motor control centers, and motors

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and transformers not covered above. Metal vessels, stacks, exchangers and similar equipment. Loading and unloading facilities. (5) If an above ground bus or loop is used for extending the supplementary electrode, this bus or loop shall have two connections to the supplementary electrode. 11.

Section 7.2 specifies that system grounding connections shall be made directly to the grounding electrode and be separate from equipment grounding connections.

12.

Section 9.1 specifies that, except as otherwise noted below, conduit, cable tray, or cable armor, shall not be relied on as the equipment grounding conductor and a bare or insulated copper conductor shall be installed in the same conduit, cable tray, cable, or cord or shall otherwise accompany the power conductors. In hazardous locations equipment grounding conductors run in conduit or cable tray shall be insulated or enclosed within the jacket of a multi-conductor cable. Exception: Conduit or cable armor may be used in accordance with the NEC for grounding electronic instrumentation operating at 24 V DC nominal or below. Note to Section 11: In accordance with the NEC an equipment grounding conductor is not required between the neutral point of a transformer and a service disconnecting means. The grounded circuit conductor (neutral) required by the NEC is sufficient.

13.

Section 9.1 specifies that metallic conduit shall be grounded at both end points. Conduit grounding may be accomplished (1) externally with approved grounding clamps and conductors or (2) through a grounded enclosure having integral threaded bushings or using a conduitt hub (such as a Myers hub) which is approved for grounding purposes or (3) through an approved grounding bushing. Grounding with locknuts is not acceptable.

14.

Section 9.1 specifies that metallic cable trays shall be bonded at both end points and a minimum of every 25 meters to the local ground grid or ground electrode or to structural steel which is bonded to the local ground grid or ground electrode.

Work Aid 1D:

National Electric Code (NEC) Handbook Articles 210 and 220

For the content of Work Aid 1D, refer to Handout 1.

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Work Aid 1E:

Applicable Procedures for Sizing Lighting, Receptacle, and Power Branch Circuits

Step 1.

Determine the load current. Note: Step 1a applies to lighting branch circuits, Step 1b applies to general purpose receptacle branch circuits, and Step 1c applies to power branch circuits. Steps 2 through 8 apply to all 3 types of branch circuits.

Step 1a.

Lighting branch circuits: (1)

IL = S/V or P/(V x p.f.) •

where:

IL = load current of each luminaire in amperes (A) S = load apparent power in voltamperes (VA) P = load real power in watts (W) V = line-to-neutral voltage in volts (V) p.f. = load power factor expressed as a decimal

• (2)

NL80 = 16 A/IL •

(3)

where NL100 equals the number of luminaires per 20-ampere branch circuit where the overcurrent device is 100% rated.

NBC = TNL/NL80 or NBC = TNL/NL100 •

(5)

where NL80 equals the number of luminaires per 20-ampere branch circuit where the overcurrent device is not 100% rated.

NL100 = 20 A/IL •

(4)

For arc-discharge lighting loads (e.g., fluorescent), include the ballast load as well as the lamp loads.

where NBC equals the total number of branch circuits and TNL equals the total number of luminaires.

Ic = 1.25 (NL80)(IL) or Ic = 1.0 (NL100) (IL)

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•

Step 1b.

where Ic equals the initial branch circuit conductor ampacity rating before applying any derating factors such as temperature, fireproofing, etc.

General purpose receptacle branch circuits: (1)

IR = S/V = 180 VA/V or IL = S/V = 240 VA/V •

(2)

where S equals an assumed 180 VA rating per receptacle outlet (NEC minimum) or S equals an assumed 240 VA rating per receptacle outlet (more conservative rating).

NR = IBC/IR = 20/IR or 15/IR •

where

IBC = 20 for 20-ampere-rated branch circuits = 15 for 15-ampere-rated branch circuits NR = the number of receptacles per branch circuit. Note: Only 20-ampere-rated branch circuits will be designed in this course.

(3)

NBC = TNR/NR •

(4)

Ic = IR NR •

Step 1c.

where NBC equals the total number of branch circuits and TNR equals the total number of receptacles.

where Ic equals the initial branch circuit conductor ampacity rating before applying any derating factors such as temperature, fire-proofing, etc.

Power branch circuits: (1)

Iφ = Sφ/Vφ or Iφ = Pφ/(Vφ x p.f.) IL = S3φ/( 3 x VL ) or IL = P3φ/( 3 x VL x p.f. ) •

where Iφ = phase current in amperes (A) for single-phase loads Sφ = single-phase apparent power in voltamperes (VA)

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Pφ = single-phase real power in watts (W) Vφ = phase voltage in volts for single-phase loads. Note: Vφ can equal a line-to-neutral voltage or a line-toline voltage depending on the voltage rating of the load. p.f. = load power factor expressed as a decimal IL = line current in amperes (A) for three-phase loads S3φ = three-phase apparent power in voltamperes (VA) P3φ = three-phase real power in watts (W) VL (2)

Ic = Iφ or IL •

(3)

= line-to-line voltage in volts (V) for three-phase loads

where Ic equals the initial branch circuit conductor ampacity, before any derating factors are applied, and for non-continuous loads or continuous loads where the branch circuit protective device is 100% rated.

Ic = 1.25 Iφ or 1.25 IL •

where Ic equals the initial conductor ampacity, before any derating factors are applied, and for continuous loads where the branch circuit protective device is not 100% rated.

Step 2.

Initially select a 75ºC or 90ºC conductor from NEC Table 310-16 (Handout 1, page 230), based on the initial conductor ampacity (Ic) that is calculated in Step 1 (1A, 1B, or 1C).

Step 3.

Apply derating correction factors (if applicable) as follows: (a)

Fireproofing (where required): 15%

(b)

Ambient temperature: Select the correction factor from NEC Table 310-16 (Handout 1, page 230).

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(c)

Step 4.

More than three current-carrying conductors in a raceway: Select the correction factor from Note 8 to NEC Table 310-16 (Handout 1, page 235) .

Specify the selected conductor as follows: •

Size - AWG or kcmil Note: See Figure 36 for the nearest metric equivalent size conductor.

•

Material - Copper (Cu)

•

Insulation Type - e.g., THWN, THHN, etc.

•

Number of Conductors - e.g., 2/c, 3/c, etc.

CONDUCTOR SIZES AWG or kcmil*

mm2

AWG or kcmil*

mm2

14

2.5

2/0

70

12

4

4/0

120

10

6

250*

120

8

10

350*

185

6

16

500*

240

4

25

750*

400

2

35

1000*

500

1/0

50

Figure 35. Standard Saudi Aramco Wire Sizes

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Step 5.

Calculate the conductor voltage drop. Note: For purposes of this Module, assume that the total load current is at the farthest end of the branch circuit. a.

-1 Calculate the load reactive factor (sin θ). sin θ = sin (cos p.f.)

b.

Determine feeder impedance (ZΩ) per 1000 feet from NEC Table 9 (Handout 1, page 886) .

c.

Calculate the feeder impedance. •

d.

Calculate VD line-to-neutral (one-way drop). •

e.

f.

ZΩ = [(R + jX) Ω per 1000 ft] (number of feet )

VD = I(R cos θ + X sin θ)

Calculate VD line-to-line. •

VD =

3 VD (3φ branch circuit)

•

VD = 2VD (1φ branch circuit)

Calculate VD(%) •

VL = VS - VD

•

VD% = 100 [(VS - VL)/VS]

•

where

VL = load voltage in volts VS = source voltage in volts (e.g., 120 V for a lighting or receptacle branch circuit) VD% = percent voltage drop

g.

If VD% exceeds 3 percent, increase the conductor size to the next standard size and repeat steps 5b through 5f.

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Step 6.

Select the branch circuit protective device. a.

Select the ampacity rating of an MCCB based on the ampacity rating of the conductor as determined in Step 4 or as modified (increased) in Step 5. Notes: (1) NEC Article 240-6 (Handout 1, page 130) lists the standard ampere ratings of circuit breakers, (2) NEC Article 240-3 (Handout 1, page 128) requires low voltage conductors to be protected in accordance with their ampacities as listed in NEC Tables 310-16 through 310-19 (Handout 1, pages 230 through 233) and their accompanying notes (Handout 1, pages 229 through 237), and (3) overcurrent protection for AWG No. 12 copper conductors is not permitted to exceed 20 amperes, and for AWG No. 10 copper it is not permitted to exceed 30 amperes, after any correction factors for ambient temperature and number of conductors have been applied.

b.

If the conductor ampacity does not correspond with a standard ampere rating of a circuit breaker, select the next standard ampere rated device, but do not exceed 800 amperes (round-down rule). Note: See NEC Article 240-3(b) (Handout 1, page 129).

Step 7.

Select the size of the equipment grounding conductor from NEC Table 250-95 (Handout 1, page 184) based on the size of the selected protective device from Step 6.

Step 8.

Select the conduit size. a.

For type and number of conductors all of the same size, select the conduit (raceway) size from Appendix C, Tables C1 through C12 of the NEC (Handout 1, pages 930 through 965). Note: The term “conductors” includes the phase conductors, the neutral conductor, and the equipment grounding conductor.

b.

For type and number of conductors of different sizes, perform the following: (1) Using Table 5 of the NEC (Handout 1, pages 880 through 884), compute the total cross-sectional area of the individual conductors. (2) Select a conduit from Table 4 of the NEC (Handout 1, pages 877 through 879), where the 40% (or 31%) fill rate area of the standard conduit size is greater than the conductor crosssectional area that is computed in Step 8b(1).

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WORK AID 2:

RESOURCES USED TO SIZE LIGHTING AND POWER DISTRIBUTION PANELBOARDS

Work Aid 2A:

SAES-P-100 (11 MAR 96 )

For the content of Work Aid 2A, refer to Work Aid 1A.

Work Aid 2B:

SAES-P-104 (7 DEC 94 )

For the content of Work Aid 2B, refer to Work Aid 1B.

Work Aid 2C:

SAES-P-111 (24 FEB 96 )

For the content of Work Aid 2C, refer to Work Aid 1C.

Work Aid 2D:

1996 NEC Handbook Article 384

For the content of Work Aid 2D, refer to Handout 1.

Work Aid 2E: Step 1.

Step 2.

Step 3.

Calculate the continuous load current. •

ILC = 1.25 x kVA/( 3 x kV) or

•

ILC = 1.25 x kW/( 3 x kV x p.f.)

Calculate the non-continuous load current. •

ILN = kVA/( 3 x kV) or

•

ILN = kW/( 3 x kV x p.f.)

Sum the continuous plus non-continuous loads. •

Step 4.

Applicable Procedures

ILT = ILC + ILN

Apply a 20 percent growth factor per SAES-P-104. •

IL = 1.20 ILT

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Step 5.

Select the next standard fully-rated panelboard, with main circuit breaker protection and full-size neutral from Figure 37, based on the total load current (IL), service voltage (208Y/120 or 480Y/277), and short circuit symmetrical current available. Note: Selection of the total number of panelboard spaces, and the individual single-pole (1P), two-pole (2P), and three-pole (3P) branch circuit breakers is beyond the scope (time limits) of this Module.

Panel Catalog Number

Ampere Rating

Interrupting Rating (kA Symmetrical) 240 vac 480 vac

MBPD1 MBPD2 MBPD3 MBPD4 MBPD5 MBPD6

100 100 100 100 100 100

18 65 100 200 200 200

14 25 65 100 150 200

MBPD7 MBPD8 MBPD9 MBPD10

150 150 150 150

18 65 100 200

14 25 65 100

MBPD11 MBPD12 MBPD13 MBPD14 MBPD15 MBPD16

225 225 225 225 225 225

10 22 42 65 100 200

... ... ... 25 65 100

MBPD17 MBPD18 MBPD19 MBPD20 MBPD21

400 400 400 400 400

65 65 100 200 200

... 35 65 100 200

MBPD22 MBPD23 MBPD24 MBPD25

600 600 600 600

42 65 100 200

30 35 65 100

Figure 36. Typical Main Breaker Panelboard Ratings

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WORK AID 3:

RESOURCES USED TO SIZE SERVICE ENTRANCE CONDUCTORS

Work Aid 3A:

SAES-P-100 (11 MAR 96 )

For the content of Work Aid 3A, refer to Work Aid 1A.

Work Aid 3B:

SAES-P-104 (7 DEC 94 )

For the content of Work Aid 3B, refer to Work Aid 1B.

Work Aid 3C:

SAES-P-111 (24 FEB 96 )

For the content of Work Aid 3C, refer to Work Aid 1C.

Work Aid 3D:

1996 NEC Handbook Articles 230 and 240

For the content of Work Aid 3D, refer to Handout 1.

Work Aid 3E: Step 1.

Applicable Procedures

Assume that the ampacity of the conductors (IL) is equal to the rating of the panelboard selected in Step 5 of Work Aid 2. •

IL = _________ amperes

Step 2.

Initially select a 75ºC or 90ºC conductor from NEC Table 310-16 (Handout 1, page 230). Note: Consider use of parallel conductors for required conductor sizes 500 kcmil and larger.

Step 3.

Apply derating correction factors (if applicable) as follows: a)

Fireproofing (where required): 15%

b)

Ambient temperature: Select the correction factor from NEC Table 310-16 (Handout 1, page 230) .

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c)

Step 4.

If there are more than three current-carrying conductors in a raceway, select the correction factor from Note 8 to NEC Table 310-16 (Handout 1, page 235) .

Specify the selected conductor (or parallel conductors) as follows: •

Size - AWG or kcmil Note: See Figure 36 of Work Aid 1E for the nearest metric equivalent size conductor.

•

Material - Copper (Cu)

•

Insulation Type - e.g., 75 ºC THWN, 90 ºC THHN, etc.

•

Number of Conductors - e.g., 4/c, 8/c, etc. Note: Assume that there is a full-sized neutral conductor.

Step 5.

Calculate the load reactive factor (sin θ). sin θ = sin (cos-1 p.f.)

Step 6.

Determine service entrance conductor impedance (ZΩ) per 1000 feet from NEC Table 9 (Handout 1, page 886).

Step 7.

Calculate the feeder impedance. •

Step 8.

Calculate VD line-to-neutral •

Step 9.

Step 10.

•

VD = 2VD (1φ system)

•

VD =

3 VD (3φ system)

Calculate the load voltage (VL) VL = VS - VD

Calculate VD as a percentage (VD%). •

Step 12.

VD = I(R cos θ + x sin θ)

Calculate VD line-to-line.

• Step 11.

ZΩ = [(R + jX) Ω per 1000 ft] (number of feet )

VD% = 100 [(VS - VL)/VS]

If VD% exceeds 2 percent, increase the conductor to the next standard size and repeat Steps 6 through 12.

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WORK AID 4:

RESOURCES USED TO SIZE A SERVICE ENTRANCE GROUNDING SYSTEM

Work Aid 4A:

SAES-P-100 (11 MAR 96 )

For the content of Work Aid 4A, refer to Work Aid 1A.

Work Aid 4B:

SAES-P-104 (7 DEC 94 )

For the content of Work Aid 4B, refer to Work Aid 1B.

Work Aid 4C:

SAES-P-111 (24 FEB 96 )

For the content of Work Aid 4C, refer to Work Aid 1C.

Work Aid 4D:

1996 NEC Handbook Article 250

For the content of Work Aid 4D, refer to Handout 1.

Word Aid 4E:

ANSI/IEEE Standard 142-1991 (IEEE Green Book)

Paragraph 2.7.4.5 of the IEEE states that use of phase overcurrent devices to detect and clear ground faults is not ideal. The same paragraph recommends that ground faults be detected in one of three methods: (1) ground return [Figure 38(a)], (2) zero sequence [Figure 38(b)], or (3) differential current [Figure 38(c)]. Note: SAES-P-114 specifies ground fault detection that uses the zero- sequence method [(Figure 38(b)].

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Work Aid 4F: Step 1.

Step 2.

Determine the size of the ungrounded (phase) and grounded (neutral) service entrance conductors from Step 4 of Work Aid 3E. •

Phase conductor size -

•

Neutral conductor size -

Size the grounding electrode conductor in accordance with NEC Article 250-94 (Handout 1, page 180) and NEC Table 250-94 (Handout 1, page 181), which is based on the size of the service entrance conductors as listed in Step 1 above. •

Step 3.

Applicable Procedures

Grounding electrode conductor size -

Size the required bonding jumpers in accordance with NEC Article 250-79 (Handout 1, page 173) and Table 250-94 (Handout 1, page 181), which is based on the size of the service entrance conductors that are selected in Step 1 above. •

Bonding jumper size -

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Figure 37. Ground Fault Detection Methods

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GLOSSARY ampacity

The current-carrying capacity of electric conductors that is expressed in amperes.

ANSI

American National Standards Institute

approved

Acceptable to the authority having jurisdiction, which for the purpose of this Module is Saudi Aramco.

bonding jumper

A bonding jumper (or equipment bonding jumper) is a conductor that serves to permanently join metal parts to form an electrically conductive path that will ensure electrical continuity. In addition, the bonding jumper has the capacity to conduct safely any current that is likely to be imposed, and it will maintain an equipotential condition on the equipment enclosure or housing to which it is bonded.

branch circuit

The circuit conductors that are between the final overcurrent device protecting the circuit and the outlet(s).

bus

A conductor, or group of conductors, that serves as a common connection for two or more circuits.

busbar

A solid copper bar conductor, usually used for large current carrying capacity, that can be used as a common connection for two or more circuits.

busway

Bus conductors that are totally enclosed and supported in a housing. Usually formed in sections of uniform length that may be bolted together to form long runs of large current-carrying capacity.

cable, single-conductor

A conductor stranded with or without insulation or other coverings.

cable jacket

A covering where the principal function is to protect the insulation from mechanical damage. A jacket can also be used to protect the insulation from chemicals, sunlight, fluids, weathering, and flame.

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cable, multipleconductor

A combination of two or more conductors that are insulated from one another. Commonly designated as three conductor (3/c), four conductor (4/c), etc.

conductor

A metallic substance or body that is specifically designed to allow electrical current to pass continuously along it from one point to another with minimal resistance.

conduit

A metallic or non-metallic tube that is used to protect electric wires and cables. See raceway .

continuous load

A load where the maximum current is expected to continue for three hours or more.

controller

A device or group of devices that serves to govern, in some predetermined manner, the electric power delivered to the apparatus to which it is connected.

damp location

Partially protected locations that are subject to moderate degrees of moisture.

dead front

A means of power termination whereby devices are so designed, constructed, and installed such that no current-carrying parts are normally exposed at the front of the assembly, or whereby a protective barrier is interposed between all live parts and the operator, or whereby exposed live parts are insulated and grounded.

disconnecting means

A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.

dry location

A location that is not normally subject to dampness or wetness.

duct

A single enclosed underground raceway that is used to enclose electric wires and cables.

duct bank

An arrangement of underground conduits that provide, for conductors and/or cables, one or more

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continuous ducts between two points. feeder

All circuit conductors that are located between the supply point or entrance to a building, structure, or otherwise defined area and the final branch-circuit overcurrent device.

ground

A conducting connection, whether intentional or accidental, that is between an electric circuit or equipment and the earth, or to some conducting body that serves in place of the earth.

grounded conductor

A system or circuit conductor that is intentionally grounded, for example, a grounded system neutral.

grounding conductor

A conductor that is used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

grounding conductor (equipment)

The conductor that is used to connect the noncurrentcarrying metal parts of equipment, raceways, and enclosures to the system’s grounded conductor, and/or the grounding electrode conductor, at the service equipment or at the source of a separately derived system.

grounding electrode

A conductor that is used to establish a ground and to connect electrical systems to the earth.

grounding electrode conductor

The conductor that is used to connect the grounding electrode to the equipment grounding conductor, and/or to the grounded conductor, of the circuit at the service equipment or at the source of a separately derived system.

ground fault circuit interrupter (GFCI)

A device that is intended for the protection of personnel and that functions to deenergize a circuit within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device.

ground fault protection (GFP)

A system that is intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the

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faulted circuit. GFP is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device. hazardous (classified) locations

An area where ignitable vapors or dust may cause a fire or explosion created by energy emitted from lightning, electrical equipment, or electrostatic generation. Hazardous (classified) locations are further defined by Article 500 of the NEC.

IEEE

Institute of Electrical and Electronics Engineers

insulation

A dielectric substance that permanently offers a high resistance to the passage of current and to disruptive discharges through the substance.

interrupting rating

The highest current at rated voltage that a device is intended to interrupt under standard test conditions.

load factor

The ratio of the average load over a designated period of time to the peak load occurring in that period is called load factor. For Saudi Aramco purposes, load factor should be considered to be 100 percent.

listed

Equipment or materials that are included in a list published by an organization (e.g., UL) acceptable to the authority having jurisdiction, and concerned with product evaluation and that maintains periodic inspection of production of listed equipment or materials, and whose listing states either that the equipment or material meets appropriate designated standards, or that has been tested and found suitable for use in a specified manner.

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main bonding jumper

A conductor that serves to connect the grounded circuit conductor and the equipment grounding conductor at the service entrance. The main bonding jumper is one of the most critical elements in the grounding system because it provides the bonding conductor connection link between the grounded service conductor, the equipment grounding conductor, and the grounding electrode conductor. The main bonding jumper connection carries the fault current for the service enclosure as well as from the equipment system.

NEC

National Electric Code

NEMA

National Electrical Manufacturer’s Association

normal operation

The highest conductor temperature rating that is allowed by any part of a cable under operating current load.

overcurrent

Any current that is in excess of the rated current of equipment or the ampacity of a conductor. Overcurrent may result from overload, short circuit, or ground fault.

overload

Operation of equipment that is in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.

panelboard

A single panel or group of panel units that are designed for assembly in the form of a single panel. A panelboard contains buses and automatic overcurrent devices, and may or may not be equipped with switches for the control of light, heat, or power circuits. A panelboard is designed to be placed in a cabinet or cutout box placed in or against a wall or partition, and accessible only from the front.

power cable

A conductor or group of conductors that supplies current for the operation of a machine, apparatus, or

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system. PVC

Polyvinylchloride

raceway

Any channel for holding wires, cables, or busbars. Raceways include metallic and non-metallic conduit, busways, wireways, electrical metallic tubing, and cable trays.

receptacle

A receptacle is a contact device that is installed at the outlet for the connection of a single attachment plug.

service

The conductors and equipment that deliver energy from the electricity supply system to the wiring system of the premises served.

service conductors

The supply conductors that extend from the street main or from transformers to the service equipment of the premises supplied.

service-entrance conductors (overhead system)

The service conductors that are between the terminals of the service equipment and a point usually outside the building, clear of building walls, where they are joined by tap or splice to the service drop.

service-entrance conductors (underground system)

The service conductors that are between the terminals of the service equipment and the point of connection to the service lateral.

service equipment

The necessary equipment, usually consisting of a circuit breaker or switch and fuses and their accessories, and that are located near the point of entrance of supply conductors to a building or other structure, or in an otherwise defined area, and intended to constitute the main control and means of cutoff for the supply.

short circuit

The highest conductor temperature rating that is allowed by any part of a cable when under short circuit conditions.

stranded conductor

A conductor that is composed of a group of wires or any combination of groups of wires. The wires in a stranded conductor are usually twisted or braided

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together in the form of either a concentric, rope, or bunch lay. thermoplastic

A family of insulation materials that will soften when heated. Common examples are polyvinyl-chloride (PVC) and cross-linked polyethylene (XLPE).

UL

Underwriter’s Laboratories, Incorporated

wet locations

Installations underground or in concrete slabs or masonry that are in direct contact with the earth, subject to saturation with water or other liquids, and exposed to weather and unprotected, for example, outdoor conduit and cable tray.

wireways

A sheet metal trough with hinged or removable covers for housing and protecting electric wires and cable.

XLPE

Cross-linked polyethylene

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