FREE E2 Sizing and Protection of Motor Circuits Questions and Answers

Question 1

What is the minimum ampacity required for the branch-circuit conductors supplying a single 25-hp, 460-volt, 3-phase, continuous-duty motor?

Accepted Answer

42.5 A

Answer

First, find the motor's Full-Load Current (FLC) from NEC Table 430.250, which is 34A for a 25-hp, 460V, 3-phase motor. Per NEC 430.22, conductors for a single continuous-duty motor must have an ampacity of at least 125% of the motor FLC. The calculation is 34A * 1.25 = 42.5A.

Question 2

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 50-hp, 460-volt, 3-phase motor?

Accepted Answer

175 A

Answer

The motor's FLC from Table 430.250 is 65A. Per Table 430.52, the maximum percentage for an inverse-time breaker is 250% of the FLC. The calculation is 65A * 2.50 = 162.5A. Since 162.5A is not a standard size, NEC 430.52(C)(1) Exception No. 1 permits rounding up to the next standard size, which is 175A.

Question 3

A continuous-duty motor has a nameplate current rating of 40A and a service factor of 1.15. What is the maximum rating for a separate overload protection device for this motor?

Accepted Answer

50 A

Answer

According to NEC 430.32(A)(1), for a motor with a service factor of 1.15 or greater, the separate overload device shall be selected to trip at no more than 125% of the motor nameplate current rating. The calculation is 40A * 1.25 = 50A. This protects the motor from sustained overcurrents without nuisance tripping.

Question 4

A feeder supplies three motors with FLCs of 50A, 30A, and 15A. What is the minimum required ampacity for the feeder conductors?

Accepted Answer

107.5 A

Answer

NEC 430.24 requires the feeder conductor ampacity to be the sum of all motor FLCs plus 25% of the largest motor's FLC. The calculation is (50A * 1.25) + 30A + 15A = 62.5A + 30A + 15A = 107.5A. The conductors must have an ampacity of at least 107.5A.

Question 5

A feeder supplies two motors. The largest motor is protected by a 150A branch-circuit breaker, and the second motor has an FLC of 40A. What is the maximum size circuit breaker allowed for the feeder protection?

Accepted Answer

175 A

Answer

Per NEC 430.62(A), the feeder overcurrent device rating shall not be greater than the largest branch-circuit protective device rating plus the sum of the FLCs of the other motors on the feeder. The calculation is 150A (largest branch OCPD) + 40A (FLC of other motor) = 190A. Since 190A is not a standard size, the next lower standard size of 175A must be used.

(E2) E2 Commercial Electrical Inspector Practice Test

(E2) E2 Commercial Electrical Inspector Practice Test

FREE E2 Sizing and Protection of Motor Circuits Questions and Answers