Master Electrician Exam (NEC)Load CalculationsEasy
A three-phase, 480V motor has a full-load current (FLA) of 65A. The motor is served by a feeder. What is the minimum continuous current rating for the feeder conductors supplying this motor, according to NEC Article 430.22(A)?
- A81.25 A
- B108.33 A
- C97.5 A
- D65 A
Show answer & explanationAnswer & explanation
Correct answer: A. 81.25 A
According to NEC Article 430.22(A), conductors supplying a single motor used in a continuous duty application must have an ampacity not less than 125% of the motor's full-load current rating. Therefore, 65A * 1.25 = 81.25A.
Why the other options are wrong
- B. Incorrect. This value does not correspond to a standard NEC multiplier for continuous motor loads.
- C. Incorrect. This would be 150% of the FLA, which is typically for overcurrent protection, not continuous conductor sizing.
- D. Incorrect. This is the motor's FLA, not the required conductor ampacity.
Single Motor Feeder Sizing
The calculation of the minimum ampacity required for feeder conductors supplying a single motor, based on its full-load current and duty cycle, as per NEC Article 430.22(A).
- For continuous duty motors, conductors must be sized at 125% of the motor's FLA.
- NEC Article 430 covers motors, motor circuits, and controllers.
- Different multipliers apply for noncontinuous or intermittent duty motors.
Memory trick: Motor Feeder: Continuous Current needs 125% safe Amps.