A maintenance technician is replacing a fuse in a motor circuit. The motor nameplate indicates a service factor (SF) of 1.15. If the continuous full-load current (FLA) of the motor is 40 amperes, what is the maximum continuous current that the motor can safely draw without tripping its separate overload protection device, assuming the overload device is sized per NEC 430.32(A)(1)?
- A46 amperes
- B40 amperes
- C57.5 amperes
- D50 amperes
Show answer & explanationAnswer & explanation
Correct answer: A. 46 amperes
According to NEC 430.32(A)(1), for motors with a service factor of 1.15 or greater, the separate overload device shall be selected to trip at no more than 125% of the motor's nameplate full-load current. Therefore, 40 A * 1.15 (service factor) = 46 A. Then, the overload protection is sized based on the FLA, not the service factor adjusted FLA, but the service factor dictates the percentage multiplier for the overload device. So, 40 A * 1.25 (for SF 1.15+) = 50 A. However, the question asks for the maximum continuous current the motor can safely draw, not the overload trip setting. The maximum continuous current is determined by FLA * SF = 40A * 1.15 = 46A. The overload device would trip at 50A, protecting it up to 46A.
Why the other options are wrong
- B. This is the FLA, but the motor can safely draw more due to its service factor.
- C. This value incorrectly applies the 1.15 service factor to the 125% overload setting, or misinterprets the question.
- D. This would be the maximum trip setting for the overload device (125% of FLA), not the maximum continuous current the motor itself can safely draw.
Motor Service Factor (SF)
A motor's service factor (SF) indicates how much overload it can safely tolerate above its nameplate full-load current (FLA) for short periods without damage. An SF of 1.15 means it can run at 115% of FLA.
- SF of 1.15 or greater allows overload device sizing at 125% of FLA.
- SF less than 1.15 (or not marked) requires overload device sizing at 115% of FLA.
- The motor can continuously draw FLA x SF without overheating.
Memory trick: Motor 'SF' is like a 'Super-FLA' boost, but a '125%' trip is the guard rail.