A 10 hp, 230V, three-phase motor is used in a continuous duty application. The motor is protected by an inverse-time circuit breaker for short-circuit and ground-fault protection. According to NEC Table 430.250, the FLC for a 10 hp, 230V, three-phase motor is 28A. What is the maximum allowable setting for the inverse-time circuit breaker, assuming the motor's starting current would not be sufficient to trip it at this setting?
- A98A
- B70A
- C112A
- D84A
Show answer & explanationAnswer & explanation
Correct answer: C. 112A
According to NEC Table 430.52, for an inverse-time circuit breaker protecting a polyphase motor, the maximum setting is 250% of the motor's full-load current. So, 28A * 2.50 = 70A. However, NEC 430.52(C)(1) Exception No. 1 allows for an increase in the rating where the values specified in Table 430.52 are not sufficient for the starting current of the motor. The maximum increase allowed for an inverse-time circuit breaker is up to 400% of the motor's FLC. Therefore, 28A * 4.00 = 112A.
Why the other options are wrong
- A. This value (350% of FLC) is not a standard multiplier for inverse-time circuit breakers.
- B. This is 250% of the FLC, which is the standard maximum setting, but not the absolute maximum when exceptions apply.
- D. This value (300% of FLC) is incorrect for an inverse-time circuit breaker, but correct for a non-time delay fuse.
Motor Branch-Circuit Short-Circuit Protection (Increased Setting)
NEC 430.52(C)(1) Exception No. 1 allows the maximum setting for an inverse-time circuit breaker protecting a motor branch circuit to be increased up to 400% of the motor's FLC if the standard setting (250%) is insufficient for motor starting.
- Standard maximum for inverse-time CB is 250% FLC.
- Can be increased if motor starting current trips the standard setting.
- Absolute maximum increase for inverse-time CB is 400% FLC.
- Higher multipliers exist for other protection types (e.g., fuses).
Memory trick: When the motor 'shocks' the breaker, you can give it a 'fourth' chance.