Master Electrician Exam (NEC)Overcurrent ProtectionMedium
A newly installed industrial motor has a full-load current (FLA) of 60 amperes. Per NEC, what is the maximum rating for the inverse time circuit breaker used for short-circuit and ground-fault protection if the motor is not a special type and is started across the line?
- A150 amperes
- B200 amperes
- C75 amperes
- D175 amperes
Show answer & explanationAnswer & explanation
Correct answer: D. 175 amperes
NEC Table 430.52 allows an inverse time circuit breaker to be sized up to 250% of the motor's full-load current for short-circuit and ground-fault protection. Therefore, 60 A * 2.50 = 150 A. However, if 150 A is not a standard size, the next higher standard size is permitted if it does not exceed 400% of FLA.
Why the other options are wrong
- A. This is 250% of the FLA, but 175A is the next standard size available.
- B. This value exceeds the maximum allowable percentage for an inverse time circuit breaker when the next standard size rule is applied (400% of FLA would be 240A, so 200A is within this, but 175A is the correct next standard size from 150A).
- C. This value is too low and would likely cause nuisance tripping.
Motor Short-Circuit Protection Sizing
The maximum rating of a motor's short-circuit and ground-fault protective device is determined by the motor's full-load current (FLA) and the type of protective device, as specified in NEC Table 430.52.
- Inverse time breakers are generally allowed up to 250% of FLA.
- If the calculated value isn't a standard size, the next higher standard size is permitted.
- The maximum for inverse time breakers, even with the next size rule, is 400% of FLA.
Memory trick: Motor protection is a 'FLA'shy calculation, always check the 'Tab'le.