Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A technician is installing a 25 hp, 460V, three-phase motor. The motor nameplate indicates a full-load current (FLA) of 34A and a service factor (SF) of 1.15. What is the maximum permissible setting for the continuous-duty thermal overload protection device, assuming it is a separate device?
- A46.2 Amperes
- B40.8 Amperes
- C39.1 Amperes
- D42.5 Amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 39.1 Amperes
According to NEC 430.32(A)(1), for motors with a service factor of 1.15 or greater, the overload protection device must be set at not more than 125% of the motor's full-load current (FLA). Therefore, 34A * 1.25 = 42.5A.
Why the other options are wrong
- A. This option is incorrect. This calculation uses the wrong multiplier for the service factor.
- B. This option is incorrect. This calculation uses the wrong multiplier for the service factor.
- D. This is the correct answer. The maximum setting is 125% of the FLA for a motor with a service factor of 1.15 or greater.
Motor Overload Protection Setting (SF ≥ 1.15)
For continuous-duty motors with a service factor of 1.15 or greater, separate overload protection devices must be set at no more than 125% of the motor's full-load current (FLA) as marked on the nameplate.
- Applies to continuous-duty motors.
- Service factor must be 1.15 or greater.
- Maximum setting is 125% of FLA.
- Ensures motor protection without nuisance tripping.
Memory trick: Overload's Factor: If SF is high, multiply by 1.25 to stay safe.