Journeyman Electrician Exam (NEC)Equipment and DevicesMedium
A journeyman electrician is installing a new 480-volt, 3-phase, 20-horsepower motor. What is the maximum rating of the inverse time circuit breaker that can be used for branch-circuit short-circuit and ground-fault protection if the motor's full-load current (FLA) is 27 amperes?
- A50 amperes
- B40 amperes
- C70 amperes
- D60 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 70 amperes
For a squirrel-cage motor, the maximum inverse time circuit breaker rating is 250% of the full-load current. Therefore, 27 A * 2.50 = 67.5 A. The next standard size up is permitted if the calculated value does not correspond to a standard overcurrent device rating, which is 70 A.
Why the other options are wrong
- A. This value (185% FLA) is too low for the maximum allowed for an inverse time circuit breaker.
- B. This value (150% FLA) is for instantaneous trip breakers or fuses, not inverse time circuit breakers for this motor type.
- D. This value (222% FLA) is too low, as the maximum allowed is 250% of FLA.
Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (SCGFP) Sizing - Inverse Time Breaker
The maximum rating of an inverse time circuit breaker used for branch-circuit short-circuit and ground-fault protection for a squirrel-cage motor is generally 250% of the motor's full-load current (FLA), rounded up to the next standard size if necessary.
- NEC Table 430.52 specifies maximum OCPD ratings.
- For inverse time circuit breakers on squirrel-cage motors, the maximum is 250% of FLA.
- If the calculated value doesn't match a standard OCPD size, the next higher standard size is permitted.
Memory trick: Motor's protection: full current times protection factor, then find the standard fit.