Journeyman Electrician Exam (NEC)Control Devices and MotorsEasy
A three-phase Squirrel Cage Induction Motor (SCIM) is operating at 480V, drawing 25A, and has a service factor of 1.15. The motor nameplate indicates a full-load current (FLA) of 23A. According to NEC 430.32(A)(1), what is the maximum continuous current rating for the overload protection device, assuming a continuous duty motor without code letter markings?
- A26.45 A
- B28.75 A
- C31.05 A
- D23.0 A
Show answer & explanationAnswer & explanation
Correct answer: B. 28.75 A
For a continuous duty motor with a service factor of 1.15 or greater, the overload protection device must be sized at not more than 125% of the motor's full-load current (FLA). Therefore, 23A * 1.25 = 28.75A.
Why the other options are wrong
- A. This would be 115% of FLA, which is incorrect for overload protection sizing based on service factor.
- C. This is 135% of FLA, which is too high for a motor with a 1.15 service factor.
- D. This is the motor's FLA, not the overload protection rating.
Motor Overload Protection Factors
The maximum continuous current rating for motor overload protection is determined by the motor's full-load current (FLA) and its service factor or temperature rise rating, as specified in NEC 430.32.
- Motors with a service factor of 1.15 or greater, or a marked temperature rise not over 40°C, are protected at not more than 125% of FLA.
- All other motors are protected at not more than 115% of FLA.
- Overload protection devices prevent motor damage from sustained overcurrents.
Memory trick: Service Factor dictates the Safety Factor for your motor's protection.