A three-phase, 480V, 50 hp motor is protected by an inverse time circuit breaker for short-circuit and ground-fault protection. The motor nameplate indicates a locked-rotor current (LRC) code of 'J'. According to NEC 430.52(C)(1) Exception No. 2, if the standard maximum rating of 250% of FLA is not sufficient to allow the motor to start, what is the absolute maximum percentage of the motor's full-load current (FLA) that the circuit breaker rating is permitted to be increased to?
- A300%
- B400%
- C600%
- D800%
Show answer & explanationAnswer & explanation
Correct answer: B. 400%
According to NEC 430.52(C)(1) Exception No. 2, if an inverse time circuit breaker sized at 250% of the motor's FLA (as per Table 430.52) is not sufficient to allow the motor to start, its rating can be increased, but it shall not exceed 400% of the motor's full-load current. The locked-rotor code 'J' indicates a relatively high LRC, which might necessitate this increase.
Why the other options are wrong
- A. 300% is the maximum for non-time-delay fuses, not inverse time circuit breakers in this exception.
- C. 600% is incorrect for inverse time circuit breakers under these conditions.
- D. 800% is the maximum for instantaneous trip circuit breakers, not inverse time circuit breakers.
Motor Short-Circuit Protection Adjustment (NEC 430.52 Exceptions)
NEC 430.52(C)(1) includes exceptions that allow increasing the rating of branch-circuit short-circuit and ground-fault protective devices beyond the standard table values if the motor cannot start, with specific upper limits depending on the device type.
- The standard maximum for inverse time circuit breakers is 250% of FLA (Table 430.52).
- If 250% is insufficient for starting, it can be increased, but not to exceed 400% of FLA.
- Similar exceptions exist for other protective device types (e.g., non-time-delay fuses up to 400%).
Memory trick: If 250% isn't enough, an inverse breaker can go up to 400% for that motor's tough start.