Master Electrician Exam (NEC)General Electrical KnowledgeEasy
A master electrician is installing a new 480V, 3-phase motor with a full-load current (FLA) of 60A. The motor will be started directly across the line. What is the minimum continuous current rating required for the motor feeder conductors, assuming 75°C terminals?
- A75A
- B60A
- C125A
- D90A
Show answer & explanationAnswer & explanation
Correct answer: A. 75A
Motor feeder conductors must be sized for 125% of the motor's full-load current, as per NEC 430.22(A), to accommodate continuous operation and starting current demands.
Why the other options are wrong
- B. This is the motor's FLA, not the minimum conductor rating for continuous duty.
- C. This value is for the branch circuit overcurrent protection where the motor is not part of a system.
- D. This value would be for the branch circuit overcurrent protective device, not the conductor.
Motor Feeder Conductor Sizing
Motor feeder conductors must be sized to carry at least 125% of the motor's full-load current (FLA) to ensure safe continuous operation and handle starting currents.
- NEC 430.22(A) mandates 125% for single motors.
- This accounts for continuous load and motor starting.
- Temperature ratings of terminals must also be considered.
Memory trick: Motor Feeder: Always Plus a Quarter More Power!