Master Electrician Exam (NEC)Overcurrent ProtectionMedium
A technician is installing a new 480V, 3-phase motor with a full-load current (FLA) of 50 amperes. The motor is protected by a non-time-delay fuse. According to NEC 430.52(C)(1), what is the maximum rating permitted for this fuse?
- A100 amperes
- B60 amperes
- C70 amperes
- D150 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 150 amperes
According to NEC Table 430.52, for a 3-phase motor with an FLA of 50 A, protected by a non-time-delay fuse, the maximum protection is 300% of the FLA. Calculation: 50 A * 300% = 150 A. This value should then be rounded down to the next standard fuse size if it falls between sizes, but 150A is a standard size.
Why the other options are wrong
- A. This is 200% of the FLA, which is also too low.
- B. This would be too low and likely cause nuisance tripping during motor starting.
- C. This is 140% of the FLA, which is too low for a non-time-delay fuse.
Motor Non-Time-Delay Fuse Sizing
For motor branch-circuit short-circuit and ground-fault protection using non-time-delay fuses, the maximum rating is generally 300% of the motor's full-load current (FLA) as per NEC Table 430.52.
- Applies to non-time-delay fuses.
- Maximum rating is 300% of FLA.
- Refer to NEC Table 430.52.
Memory trick: Motors need 'gears' of 'protection' from 'electrical' faults.