Master Electrician Exam (NEC)General Electrical KnowledgeEasy
A master electrician is planning the installation of a new 480V, 3-phase motor with a full-load current (FLA) of 60A. According to NEC Article 430, what is the minimum continuous current rating required for the branch-circuit conductors supplying this motor if it is not a hermetic refrigerant motor?
- A75A
- B90A
- C125A
- D60A
Show answer & explanationAnswer & explanation
Correct answer: A. 75A
For other than hermetic refrigerant motors, branch-circuit conductors supplying a single motor must have an ampacity not less than 125% of the motor's full-load current rating, as per NEC 430.22(A). Therefore, 60A * 1.25 = 75A.
Why the other options are wrong
- B. This value would be 150% of the FLA, which is typically for overcurrent protection, not conductor sizing.
- C. This value would be 208% of the FLA, which is significantly higher than the NEC requirement for conductors.
- D. This is the motor's full-load current (FLA), but NEC requires an overcurrent factor for conductors.
Motor Branch-Circuit Conductor Sizing
NEC requires motor branch-circuit conductors to be sized at a minimum percentage of the motor's full-load current to safely carry continuous operating currents and accommodate starting currents.
- For non-hermetic motors, conductors must be sized at 125% of the motor FLA.
- This factor accounts for continuous operation and motor starting currents.
- Refer to NEC Article 430 for specific requirements.
Memory trick: Motor Conductors Need Extra Amps for Smooth Starts.