motors 2-7 motor branch circuit protection
An inverse-time circuit breaker is used for branch-circuit short-circuit and ground-fault protection for a 30-horsepower, 208-volt, 3-phase squirrel-cage motor. Calculate the maximum rating permitted for the circuit breaker.
225 A Table 430.250, 3-phase motor30 hp, 208 V = 88 ATable 430.52, inverse-time circuit breaker = 250%Rating = 88 × 2.50 = 220 A430.52(C)(1), Exception No. 1Next higher OCPD permitted240.6(A) standard sizes = 225 A
A 40-horsepower, 230-volt, 3-phase synchronous motor uses non-time-delay fuses for motor branch-circuit short-circuit and ground-fault protection. Calculate the maximum rating for a non-time-delay fuse.
250 A Table 430.250, 3-phase, synchronous motors (right side of Table)40 hp, 230 V (sync.) = 83 ATable 430.52, sync. motorNon-time-delay fuse = 300%Rating = 83 × 3.00 = 249 A430.52(C)(1), Exception No. 1Next higher OCPD permitted240.6(A) standard size = 250 A
What is the maximum standard rating for a time-delay fuse used as the motor branch-circuit short-circuit and ground-fault protection for a 230-volt, 3-phase hermetically-sealed compressor motor with a branch-circuit selection current (BCSC) of 38 amperes?
440.22(A), not to exceedOCPD rating = BCSC × 175% = 38 × 1.75 = 66.5 ANext lower OCPD240.6(A) standard sizes = 60 A
Match the following motor branch-circuit conductor ampacity to the correct copper wire size using 75°C insulation type THWN.
62 A → 6 AWG 83 A → 4 AWG 156 A → 2/0 AWG 211 A → 4/0 AWG
A single-phase, 10-horsepower, 240-volt, capacitor-start motor is installed. Calculate the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device using a nonadjustable inverse-time circuit breaker for this application.
Table 430.248 (10 hp at 240 V = 50 A)Table 430.52, inverse-time circuit breaker = 250%Rating = 50 × 2.50 = 125 A
What is the maximum standard rating for a time-delay (dual-element) fuse permitted to be used for motor branch-circuit short-circuit and ground-fault protection for a 10-horsepower, 208-volt, single-phase capacitor motor when starting current is a problem?
Table 430.248, single-phase motor10-hp, 208 V = 55 ATable 430.52(C)(1), Exception No. 2(2)Time-delay dual-element fuse = 225%Rating = 55 × 2.25 = 123.75 ANext lower standard size OCPD240.6(A) standard size = 110 A
What is the maximum standard rating or setting of the motor branch-circuit short-circuit and ground-fault protective device for a five-horsepower, 240-volt, single-phase capacitor motor using non-time-delay fuses when the starting current is a problem?
Table 430.248, single-phase motor5 hp, 240 V = 28 A430.52(C)(1), Exception No. 2(1)Non-time-delay fuse = 400% max.OCPD = 28 × 4.00 = 112 ANext lower standard size OCPD = 110 A non-time delay fuseNote: When Exception No. 2 is used and the value does not correspond to a standard OCPD size in 240.6(A), round down because the exception states that the OCPD shall "in no case exceed" the given percentage. The correct answer is: 110 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for branch-circuit short-circuit and ground-fault protection for a 15-horsepower, 440-volt, 3-phase squirrel-cage motor. Calculate the maximum instantaneous trip circuit breaker rating permitted when starting current is a problem and where the need for further adjustment has been demonstrated by engineering evaluation.
Table 430.250 3-phase15 hp 440 V = 21 A430.52(C)(3), Exception No.1Max. rating instantaneous trip circuit breaker = 1,300%Max. rating = 21 × 13.00 = 273 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for branch-circuit short-circuit and ground-fault protection for a 15-horsepower, 440-volt, 3-phase squirrel-cage motor. Calculate the maximum setting for the instantaneous trip rating of this circuit breaker.
Table 430.250 3-phase15 hp, 440 V = 21 ATable 430.52, instantaneous trip circuit breaker = 800%Rating = 21 × 8.00 = 168 A
An inverse-time circuit breaker is used for branch-circuit short-circuit and ground-fault protection for a 30-horsepower, 208-volt, 3-phase squirrel-cage motor. Calculate the maximum standard rating permitted for the inverse-time circuit breaker when starting current is a problem.
Table 430.250, 3-phase motor30 hp, 208 V = 88 A430.52(C)(1), Exception No. 2(3)Max. inverse-time circuit breaker = 400%Max. rating = 88 × 4.00 = 352 ANext lower standard size OCPD240.6(A) standard size = 350 A
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for the branch-circuit short circuit and ground-fault protection for a 50-horsepower, 230-volt, 3-phase Design B energy-efficient motor. What is the maximum instantaneous trip circuit breaker rating permitted for this application?
Table 430.250, 3-phase motor50 hp, 230 V = 130 ATable 430.52, Design B energy-efficient motor, instantaneous trip circuit breaker = 1,100%Rating = 130 × 11.00 = 1,430 A
Time-delay fuses are used for motor branch-circuit short-circuit and ground-fault protection for a 125-horsepower, 460-volt, 3-phase wound-rotor motor. What is the maximum rating permitted where time-delay fuses are used for this application?
Table 430.250, 3-phase wound rotor motor125 hp, 460 V = 156 ATable 430.52, wound-rotor motorTime-delay fuse = 150%Rating = 156 × 1.50 = 234 A430.52(C)(1), Exception No. 1Next higher OCPD permitted240.6(A) standard sizes = 250 A
A nonadjustable inverse-time circuit breaker is used for the motor branch-circuit short-circuit and ground-fault protection for a 1 1/2-horsepower, 460-volt, 3-phase squirrel-cage motor. What is the maximum rating permitted where an inverse-time circuit breaker is used? (Based on the NEC 2020 and earlier)
Table 430.250, 3-phase11/2 hp, 460 V = 3 ATable 430.52, inverse time circuit breaker = 250%Rating = 3 × 2.5 = 7.5 A430.52(C)(1), Exception No. 1Next higher OCPD permitted240.6(A) standard sizes = 15 A
What is the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device for a 7 1/2-horsepower, 208-volt, 3-phase squirrel-cage induction motor using time-delay fuses?
The correct answer is: 45 A Table 430.250, 3-phase motor71/2 hp, 208 V = 24.2 A430.52(C)(1), Table 430.52Squirrel-cage motor with time-delay fuse = 175%Rating = 24.2 × 1.75 = 42.35 A430.52(C)(1), Exception No. 1Next higher OCPD permitted240.6(A) standard sizes = 45 A
Match the following motor circuit conductors to the correct size of the raceway specified.
Three 2/0 AWG THWN in a PVC Schedule 80 Conduit → 1 1/2" Three 2/0 AWG THWN in a Rigid Metal Conduit → 1 1/2" Three 4/0 AWG THWN in a PVC Schedule 80 Conduit → 2" Three 4/0 AWG THWN in a Rigid Metal Conduit → 2"
Match the following motor circuit conductors to the correct size of the raceway specified.
Three 4 AWG THWN in a PVC Schedule 80 Conduit → 1" Three 4 AWG THWN in a Rigid Metal Conduit → 1" Three 6 AWG THWN in a PVC Schedule 80 Conduit → 3/4" Three 6 AWG THWN in a Rigid Metal Conduit → 3/4"
A 60-horsepower, 3-phase, 208-volt squirrel-cage motor is installed with a motor nameplate full-load current rating of 165 amperes. The minimum ampacity of the branch-circuit conductors is calculated to be 211 amperes. The branch-circuit conductors are required to be not less than 4/0 AWG THWN copper, and they are installed in a two-inch rigid metal conduit. The maximum rating of the motor branch-circuit short-circuit and ground-fault protective device using time-delay fuses has been calculated to be not greater than a 350-ampere time-delay fuse when starting current is a problem. Therefore, this motor with 4/0 AWG THWN copper conductors and a full-load current of 169 amperes can be protected by a 350-ampere time-delay fuse.
True Note: This is true due to motor load characteristics. This overcurrent protective device is permitted to be over 200% larger than motor full-load current.430.52(C)(1), Exception No. 2(2)Max. time-delay fuse = 225%Max. rating = 169 × 2.25 = 380.25 ANext lower standard size OCPD240.6(A) standard size = 350 A
For torque motors, the rated current shall be ? current, and this nameplate current shall be used to determine the ampacity of the branch-circuit conductors covered in 430.22 and 430.24, the ampere rating of the motor overload protection, and the ampere rating of motor branch-circuit short-circuit and ground-fault protection in accordance with 430.52(B).
locked-rotor 430.6(B)
