Journeyman Electrician Exam (NEC)Control Devices and MotorsEasy
A technician is installing a new three-phase, 480V motor with a full-load current (FLA) of 65A. The motor is equipped with a separate thermal overload relay. According to NEC 430.32(A)(1), what is the maximum allowable setting for the thermal overload relay if the motor has a service factor of 1.15?
- A91.00A
- B80.60A
- C74.75A
- D84.50A
Show answer & explanationAnswer & explanation
Correct answer: C. 74.75A
For a motor with a service factor of 1.15 or greater, or a temperature rise not over 40°C, the maximum setting for a separate thermal overload relay is 125% of the motor's full-load current (FLA). Therefore, 65A * 1.15 = 74.75A.
Why the other options are wrong
- A. This value would be incorrect, potentially using a different percentage or miscalculation.
- B. This value would be incorrect, potentially using a different percentage or miscalculation.
- D. This value would be incorrect, potentially using a different percentage or miscalculation.
Motor Overload Protection Setting (NEC 430.32(A)(1))
NEC 430.32(A)(1) specifies the maximum allowable setting for a separate thermal overload relay based on the motor's full-load current (FLA) and its service factor or temperature rise rating.
- For motors with a service factor of 1.15 or greater, or a temperature rise not over 40°C, the overload device must be sized at not more than 125% of the motor's FLA.
- For all other motors, the overload device must be sized at not more than 115% of the motor's FLA.
- These percentages can be increased if the motor trips, up to a maximum of 140% (for 125% initial setting) or 130% (for 115% initial setting).
Memory trick: Overload Safety: Service Factor Sets the Limit!