Electrical System Design_branch Ckt

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CENTER FOR PROFESSIONAL DEVELOPMENT IN ENGINEERING & TECHNOLGY

TRAINING – WORKSHOP IN

ELECTRICAL SYSTEM DESIGN Motor Circuit Calculation

About the Speaker: Gener G. Restubog  

Registered Electrical Engineer 2009 President – IIEE Northern Laguna Chapter

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Education • M. of Engg – Elect Engg Adamson University, Manila • M.Sci in Elect Engg (Units Earned), University of the Phils – Diliman QC • B.Sci in Elect Engg, Central Colleges of the Phils, QC

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Current Job/Affiliations • Sr Partner & Head, Training & Consulting – Resonanz TG Inc • Consultant - Tricore Solutions, Inc • Engineering Faculty – De Lasalle University– Dasmarinas • Engineering Faculty – Technological Inst of the Phils - Quezon City • Engineering Faculty – University of Perpetual Help System – Laguna

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Former Electrical Design Manager – GHD Pty Limited Former Facility Design Manager – Intel Technology Phils Former Designer Proj Manager – Trans-Asia Phils. Framegroup Mgt System Former Electrical Design Engineer – Trans-Asia Phils, Kinhill-Tan S’pore, TCGI Engrs Former Engineering Faculty Member Lyceum Univ-Laguna, PLM, MFI, CCP

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Experience • Over 24 years experience in engineering design, consultancy, construction, and teaching of electrical engineering

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Design Calculation Motor Branch Ckts & Feeders

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AC Motors

Branch Circuits & Feeders

Source: NEC Handbook: W.M. Earley

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Motor Branch Ckts & Feeders

-Design Calculation Motor Circuit Conductor Single Motor, PEC 4.30.2.2 Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current rating, as determined by 4.30.1.6(A)(1), or not less than specified in 4.30.2.2 (A) through (F). Several Motors or a Motor(s) and Other Load(s). PEC 4.30.2.4 Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than the sum of each of the following: •125 percent of the full-load current rating of the highest rated motor, plus the sum of the full-load current ratings of all the other motors in the group, as determined by 4.30.1. 6(A) plus the sum of other loads

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Motor Branch Ckts & Feeders

-Design Calculation Motor and Branch-Circuit Overload Protection Continuous-Duty Motors - More Than 1 Horsepower

A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating: Motors with a marked service factor 1.15 or greater Motors with a marked temperature rise 40°C or less All other motors

125% 125% 115%

Larger Than 1500 Horsepower. For motors larger than 1500 hp, a protective device having embedded temperature detectors that cause current to the motor to be interrupted when the motor attains a temperature rise greater than marked on the nameplate in an ambient temperature of 40°C.

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Motor Branch Ckts & Feeders

-Design Calculation

Motor and Branch-Circuit Short Circuit and Ground Fault Protection

PEC Section 4.30.4.2 In Accordance with Table 4.30.4.2. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 4.30.4.2 shall be used. (With exceptions)

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Motor Branch Ckts & Feeders

-Design Calculation Table 4.30.4.2 Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices

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Motor Branch Ckts & Feeders

-Design Calculation Motor Feeder Short Circuit and Ground Fault Protection Rating or Setting — Motor Load and Other Load(s). Where a feeder supplies a motor load and other load(s), the feeder protective device shall have a rating not less than that required for the sum of the other load(s) plus the following (PEC Section 4.30.5.3) (1) For a single motor, the rating permitted by 4.30.4.2

(2) For a single hermetic refrigerant motor-compressor, the rating permitted by 4.40.3.2 ICB =< 175% - 225% of motor rated-load current or branch-circuit selection current, whichever is greater

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Motor Branch Ckts & Feeders

-Design Calculation Motor Feeder Short Circuit and Ground Fault Protection (3) For two or more motors, the rating permitted by 4.30.5.2

a)

ICB =< largest setting of branch-circuit short circuit and ground fault protection plus the sum of the full load currents of the other motors in the group (with exception)

b) ICB = ampacity of feeder ( if feeder ampacity > PEC Section 4.30.2.4) Exception: Where the feeder overcurrent device provides the overcurrent protection for a motor control center, the provisions of 4.30.8.3 shall apply.

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Motor Branch Ckts & Feeders

-Design Calculation – Single Motor Example 1 A three-phase, 460 volts, 60Hz, 25-Hp squirrel cage induction motor is to be connected for full voltage starting. It has a full load current of 31.6 amperes, a service factor of 1.15 and a Code Letter “F” in its nameplate. The ambient temperature of the place of installation is 40° C. Total Load : (See Table 4.30.14.4) Full load current of 3-phase, 25-Hp motor = 34 Amperes Note : The full load current value used to determine the ampacity of conductors for motors shall be based on Table 4.30.14.4

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Table 4.30.14.4 Full-Load Current , Three Phase AC Motors

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AC Motors

Namplate Rating

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Motor Branch Ckts & Feeders

-Design Calculation – Single Motor Branch Circuit Conductors : [See Section 4.30.2.2(a)] 125% of Full Load Current: 34 Amperes x 1.25 = 42 Amperes Use 3-14mm² THW wires

Note : Ampacity of 14mm² THW at 40° C ambient is 55 Amperes x 0.88 = 45 Amperes. See Table 3.10.1.16 for ampacity correction factors. Motor and Branch Circuit Overload Protection : [See Section 4.30.3.2(a)] 125% of Rated Full Load Current : 31.6 Amperes x 1.25

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Motor Branch Ckts & Feeders

-Design Calculation – Single Motor Branch Circuit Short-Circuit and ground Fault Protection: (See Section 4.30.4.2)

The branch circuit maybe protected by any of the four common types of protective devices, namely: Nontime delay fuse, Time delay fuse, Instantaneous trip breaker and Inverse time breaker. For the protection of a squirrel cage breaker with Code Letter “F” (See Table 4.30.4.2) the protective device can be any of the following: Nontime Delay (NTD) Fuses: ( See Table 4.30.4.2) 300% of Full Load Current: 34 Amperes x 3 = 102 Amperes

Use 100 Amperes NTD fuse.

Note : If the 100-A fuse is not sufficient for the starting current of the motor, see Section 4.30.4.2(c)(1) Exception No. 2a. RESONANZ

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Motor Branch Ckts & Feeders

-Design Calculation Time Delay (TD) Fuses : (See Table 4.30.4.2) 175% of Full Load Current: 34 Amperes x 1.75 =59 Amperes Use 60 Amperes TD fuse ( See section 4.30.4.2(c) Exception No. 2b). Instantaneous Trip (IT) Breaker : (See Table 4.30.4.2) 175% of Full Load Current: 34 Amperes x 7 =238 Amperes Use 250 Amperes IT circuit breaker (See Section 4.30.4.2(c) Exception No.1) Inverse Time Delay (ITD) Breaker : (See Table 4.30.4.2) 250% of Full load Current : 34 Amperes x 2.5 = 85 Amperes Use 90 Amperes ITD circuit breaker.

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Motor Branch Ckts & Feeders

-Design Calculation – Multiple Motors Example 2 Three squirrel cage induction motor: 25-Hp, 20-Hp and 10-Hp, all rated at 460 volts, 3-phase, 60-Hz are to be served by a feeder circuit. Ambient temperature does not exceed 30° C. Total Load : ( see Table 4.30.14.4) One 3-phase, 460 volts, 25-Hp motor One 3-phase, 460 volts, 20-Hp motor One 3-phase, 460 volts, 10-Hp motor 25% of Largest Motor ( see Section 4.30.2.4) 34 Amperes x 0.25) Net Computed Current

= 34 Amperes = 27 Amperes = 14 Amperes = 8.5 Amperes = 83.5 Amperes

Feeder Conductors : ( See Table 3.10.1.16) Use 3-22mm² THW wires. RESONANZ

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Motor Branch Ckts & Feeders

-Design Calculation – Multiple Motors Feeder Circuit Protection : [See Section 4.30.4.3(c)] Largest Protective Device: The 25-Hp motor being the largest motor, has the largest protective device. Nontime Delay (NTD) Fuses:

Largest NTD fuse Full load current of 20-Hp motor Full load current of 10-Hp motor Total Computed Current

= 100 Amperes = 27 Amperes = 14 Amperes = 141 Amperes

Use 150 Amperes NTD fuse.

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Motor Branch Ckts & Feeders

-Design Calculation Time Delay (TD) Fuses: Largest TD fuse Full load current of 20-Hp motor Full load current of 10-Hp motor Total Computed Current

= 60 Amperes = 27 Amperes = 14 Amperes = 101 Amperes

Use 150 Amperes TD fuse. Inverse Time Delay (ITD) Breaker: Largest NTD fuse Full load current of 20-Hp motor Full load current of 10-Hp motor Total Computed Current

= 90 Amperes = 27 Amperes = 14 Amperes = 131 Amperes

Use 125 Amperes NTD breaker. Note : Only fuses and/or inverse time circuit breakers are allowed for this type of installation. [(See Section 4.30.4.3(c)(1)]. RESONANZ

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Motor Branch Ckts & Feeders

-Design Calculation

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Motor Branch Ckts & Feeders

-Design Calculation

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Motor Branch Ckts & Feeders

-Design Calculation

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References: 1. 2. 3. 4. 5. 6. 7. 8.

IEEE Std 241-1990 Recommended Practice for Electric Power System in Commercial Buildings IEEE Std 141-1993 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants Philippine Electrical Code 2009 National Electrical Code 2011 Eaton Cuttler-Hammer Catalogue Designing Electrical System, James Stallcup NEC Handbook, McPartland NEC Handbook, W.M Earley