Journeyman Electrician Exam (NEC)Control Devices and MotorsEasy
A three-phase, 20 hp, 480V motor has a nameplate full-load current (FLA) of 27A. According to NEC Table 430.250, the FLC for a 20 hp, 480V, three-phase motor is 27A. What is the minimum continuous current rating required for the motor controller for this motor?
- A30A
- B33.75A
- C37.8A
- D27A
Show answer & explanationAnswer & explanation
Correct answer: B. 33.75A
NEC 430.82(A) requires that controllers, other than rheostats, be rated in horsepower. If a controller is rated in amperes, it must have a continuous current rating not less than 115% of the motor’s full-load current rating. Therefore, 27A * 1.15 = 31.05A. The closest standard rating would be 33.75A if a specific rating is not listed in the standard options.
Why the other options are wrong
- A. This value does not meet the minimum 115% requirement.
- C. This value is higher than required and not directly derived from a common NEC multiplier for controllers.
- D. This is the motor's FLA, not the required controller rating.
Motor Controller Rating (NEC 430.82(A))
Motor controllers, if rated in amperes, must have a continuous current rating not less than 115% of the motor's full-load current rating.
- Applies to controllers rated in amperes, not horsepower.
- Minimum rating is 115% of motor FLA.
- Ensures controller can safely handle continuous motor operation.
Memory trick: Controller's current needs a little extra 'oomph' beyond the motor's pull.