Free E2 Sizing and Protection of Motor Circuits Questions and Answers 1 — 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?
- 34 A
- 42.5 A (Correct answer)
- 51 A
- 37.4 A
Correct answer: 42.5 A
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?
- 125 A
- 150 A
- 200 A
- 175 A (Correct answer)
Correct answer: 175 A
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?
- 40 A
- 46 A
- 50 A (Correct answer)
- 60 A
Correct answer: 50 A
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?
- 95 A
- 118.75 A
- 107.5 A (Correct answer)
- 142.5 A
Correct answer: 107.5 A
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?
- 200 A
- 190 A
- 175 A (Correct answer)
- 225 A
Correct answer: 175 A
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.
Question 6: The disconnecting means for a motor controller and the motor shall have an ampere rating of at least what percentage of the motor's full-load current?
- 100%
- 115% (Correct answer)
- 125%
- 80%
Correct answer: 115%
NEC 430.110(A) requires that the disconnecting means have an ampere rating of at least 115% of the motor's full-load current rating as determined by the FLC tables in Article 430. This ensures the disconnect can safely interrupt the locked-rotor current of the motor. It must also have a sufficient horsepower rating per 430.109.
Free E2 Sizing and Protection of Motor Circuits Questions and Answers 1
What is the minimum ampacity required for the branch-circuit conductors supplying a single 25-hp, 460-volt, 3-phase, continuous-duty motor?