Journeyman Electrician Exam (NEC)Control Devices and MotorsMedium
A technician is installing a new 25 hp, 460V, three-phase motor. The motor has a nameplate full-load current (FLA) of 34A and a Service Factor (SF) of 1.15. According to NEC 430.32(A)(1), what is the maximum setting for the continuous-duty thermal overload protection for this motor?
- A34.0 A
- B39.1 A
- C42.5 A
- D47.6 A
Show answer & explanationAnswer & explanation
Correct answer: B. 39.1 A
According to NEC 430.32(A)(1), for continuous-duty motors with a Service Factor (SF) of 1.15 or greater, the thermal overload device must be sized at not more than 125% of the motor's full-load current.
Why the other options are wrong
- A. This is 100% of the FLA, which is too low for a motor with an SF of 1.15 or greater.
- C. This option represents 125% of the FLA (34A * 1.25 = 42.5A), which is incorrect for a motor with a Service Factor of 1.15. The 125% factor applies to motors with a Service Factor of 1.15 or greater, but the calculation itself should be FLA * 1.25.
- D. This value is too high and does not correspond to any standard NEC multiplier for overload protection.
Motor Thermal Overload Sizing (SF ≥ 1.15)
NEC 430.32(A)(1) specifies that for continuous-duty motors with a Service Factor (SF) of 1.15 or greater, the thermal overload protection device must be sized at not more than 125% of the motor's full-load current (FLA).
- Applies to continuous-duty motors.
- Applies when the motor's Service Factor (SF) is 1.15 or greater.
- Maximum setting is 125% of the motor's nameplate Full-Load Amps (FLA).
Memory trick: Service Factor 1.15 or more, add a quarter more!