Master Electrician Exam (NEC)Overcurrent ProtectionHard
A master electrician is designing the overcurrent protection for a 480V, 3-phase, 100 HP squirrel-cage motor with a full-load current (FLA) of 124 amperes. The motor is supplied by a feeder with 75°C rated terminals. If the engineer specifies a time-delay fuse for short-circuit and ground-fault protection, what is the maximum standard size fuse permitted by the NEC?
- A225 amperes
- B300 amperes
- C200 amperes
- D250 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 250 amperes
According to NEC Table 430.52, for a squirrel-cage motor with time-delay fuses, the maximum rating is 175% of the FLA. Calculation: 124 A * 1.75 = 217 A. Since 217 A is not a standard fuse size, NEC 430.52(C)(1) Exception No. 1 permits the next higher standard size, which is 250 amperes (from NEC 240.6).
Why the other options are wrong
- A. 225A is not a standard fuse size according to NEC 240.6.
- B. This size (300A) is higher than the next standard size and would exceed the permissible maximum if 217A was used directly, potentially allowing motor damage. It also exceeds 225% of FLA if using the next size rule (279A).
- C. 200A is a standard size, but it is below the calculated 217A, so it might cause nuisance tripping.
Motor Fuse Sizing (Time-Delay)
Time-delay fuses for motor short-circuit and ground-fault protection are typically sized at 175% of the motor's full-load current (FLA), with the next higher standard size permitted if the calculated value is not standard.
- Base percentage for time-delay fuses is 175% of FLA (NEC Table 430.52).
- Next higher standard size is allowed if calculated value is not standard (NEC 240.6, 430.52 Exception).
- Maximum for time-delay fuses, even with next size rule, is 225% of FLA.
Memory trick: Motor fuses are 'Timed' to perfection, '175' is the magic number, then go 'Next' if needed.