Motors and Motor Circuits Flashcards

What is the minimum ampacity required for the branch-circuit conductors supplying a 10 HP, 230-volt, 3-phase squirrel-cage motor?

Answer: 35A

First, find the motor's Full-Load Current (FLC) from NEC Table 430.250, which is 28A. NEC 430.22 requires branch-circuit conductors to have an ampacity of at least 125% of the motor FLC. The calculation is 28A x 1.25 = 35A.

What is the maximum standard size inverse-time circuit breaker that can be used for branch-circuit, short-circuit, and ground-fault protection for a 10 HP, 230V, 3-phase motor (FLC 28A)?

Answer: 70A

NEC Table 430.52 specifies the maximum rating of a motor branch-circuit protective device. For a polyphase squirrel-cage motor and an inverse-time breaker, the maximum is 250% of the FLC. The calculation is 28A x 2.50 = 70A. Since 70A is a standard size per 240.6(A), this is the maximum size permitted.

A feeder supplies two 3-phase, 230V motors: a 10 HP (FLC 28A) and a 5 HP (FLC 15.2A). What is the minimum required ampacity for the feeder conductors?

Answer: 50.2A

Per NEC 430.24, the feeder ampacity must be at least 125% of the largest motor's FLC plus the sum of the FLCs of the other motors. The calculation is (28A x 1.25) + 15.2A = 35A + 15.2A = 50.2A. The conductors must have an ampacity of at least 50.2A.

Using the same motors from the previous question (10 HP and 5 HP), what is the maximum standard size inverse-time circuit breaker permitted for the feeder protection?

Answer: 80A

NEC 430.62(A) states the feeder protective device rating shall not be greater than the largest branch-circuit protective device plus the FLC of the other motors. The largest branch device (for the 10 HP motor) is 70A (28A x 250%). The calculation is 70A + 15.2A = 85.2A. Since this is not a standard size, you must round down to the next standard size, which is 80A.

What is the maximum setting for a separate overload protection device for a continuous-duty motor with a nameplate FLC of 40A, a service factor of 1.15, and a temperature rise of 40°C?

Answer: 50A

According to NEC 430.32(A)(1), for a motor with a marked service factor of 1.15 or greater or a marked temperature rise of 40°C or less, the overload device shall be selected at not more than 125% of the motor nameplate FLC. The maximum setting is 40A x 1.25 = 50A.

A motor control circuit is tapped from the load side of a 60A branch-circuit breaker and the conductors do not leave the enclosure. What is the minimum size copper conductor required for this control circuit?

Answer: 14 AWG

NEC 430.72(B) and Table 430.72(B) provide the requirements for motor control circuit conductor protection. Since the conductors do not extend beyond the enclosure, Column B applies. For a protective device rated at 60A, the minimum control circuit conductor size is 14 AWG copper.