Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A technician is installing a new 50 hp, 460V, three-phase motor. The motor has a full-load current (FLA) of 65A. The motor branch-circuit short-circuit and ground-fault protective device is a non-time-delay fuse. According to NEC Table 430.52, what is the maximum rating permitted for this protective device?
- A150 A
- B175 A
- C110 A
- D125 A
Show answer & explanationAnswer & explanation
Correct answer: B. 175 A
According to NEC Table 430.52, for a three-phase motor, the maximum rating for a non-time-delay fuse used as a branch-circuit short-circuit and ground-fault protective device is 300% of the motor's full-load current (FLA). Therefore, 65A * 3.00 = 195A. Since 195A is not a standard fuse size, the next lower standard size is 175A, as per NEC 430.52(C)(1) Exception No. 1.
Why the other options are wrong
- A. This is too low and would likely cause nuisance tripping.
- C. This is too low and would likely cause nuisance tripping.
- D. This is too low and would likely cause nuisance tripping.
Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (NEC 430.52)
NEC 430.52 specifies the maximum ratings for short-circuit and ground-fault protective devices for motor branch circuits, which vary based on the type of motor and the protective device used (e.g., fuses, circuit breakers).
- These devices protect the conductors and motor from short circuits and ground faults, not overloads.
- Multipliers are applied to the motor's full-load current (FLA) to determine the maximum permitted rating.
- Different multipliers exist for non-time-delay fuses, time-delay fuses, instantaneous trip circuit breakers, and inverse time circuit breakers.
Memory trick: Fuse your motor for faults, not just for fun; multiply FLA by the right one!