A technician is installing a new 480-volt, 3-phase, 20-horsepower motor. The motor nameplate indicates a full-load current (FLA) of 27 amperes. What is the maximum standard inverse-time circuit breaker size permitted for branch-circuit short-circuit and ground-fault protection (SCGFP) for this motor, assuming no special conditions?
- A50 amperes
- B40 amperes
- C30 amperes
- D35 amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 50 amperes
According to NEC Table 430.52, for an inverse-time circuit breaker protecting a 3-phase motor, the maximum percentage of FLA is 250%. For an FLA of 27 A, the maximum OCPD is 27 A * 2.50 = 67.5 A. NEC 430.52(C)(1) Exception No. 1 allows going up to the next standard size if the calculated value does not correspond to a standard size. The standard overcurrent device sizes are listed in NEC 240.6(A). The next standard size below or at 67.5 A is 50 A.
Why the other options are wrong
- B. 27 A * 2.00 (200%) = 54 A. The next standard size up to 54 A is 50 A. However, the maximum is 250%.
- C. 27 A * 1.50 (150%) = 40.5 A. The next standard size up to 40.5 A is 40 A. However, the maximum is 250%.
- D. 27 A * 1.75 (175%) = 47.25 A. The next standard size up to 47.25 A is 45 A. However, the maximum is 250%.
Motor SCGFP Sizing (Inverse-Time Breaker)
Determining the maximum inverse-time circuit breaker rating for motor branch-circuit short-circuit and ground-fault protection based on the motor's full-load current (FLA) and NEC rules.
- NEC Table 430.52 provides multiplier percentages.
- Inverse-time circuit breakers have a maximum of 250% of FLA.
- NEC 240.6(A) lists standard overcurrent device ratings.
- Exception allows using the next higher standard size if calculated value isn't standard.
Memory trick: Motor protection is a two-step dance: calculate, then find the standard size.