Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsHard
A manufacturing facility is installing a new 20 HP, 480V, 3-phase squirrel-cage induction motor. The motor has a nameplate full-load current rating of 27A. What is the maximum inverse-time circuit breaker rating permitted for branch-circuit short-circuit and ground-fault protection for this motor, assuming it does not exceed the controller's short-circuit rating?
- A70 Amperes
- B60 Amperes
- C80 Amperes
- D50 Amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 70 Amperes
According to NEC Table 430.250, a 20 HP, 480V, 3-phase motor has a FLC of 27A. NEC 430.52(C)(1) allows an inverse-time circuit breaker to be sized at 250% of the motor's FLC. So, 27A * 2.50 = 67.5A. Since 67.5A is not a standard OCPD size, the next higher standard size is permitted, which is 70A (NEC 240.6(A) and 430.52(C)(1) Exception No. 1).
Why the other options are wrong
- B. This is too low based on the calculation of 250% of FLC (67.5A).
- C. This exceeds the maximum permissible rating for this motor.
- D. This is too low based on the calculation.
Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (Inverse-Time Breaker)
This protection device protects the motor branch-circuit conductors, motor control apparatus, and the motor itself from short-circuits and ground-faults.
- Sized based on the motor's full-load current (FLC) from NEC Tables 430.247-430.250.
- Maximum ratings are specified in NEC Table 430.52.
- Inverse-time circuit breakers are typically allowed up to 250% of FLC.
Memory trick: Motor protection: FLC, then multiplication, then standard size.