Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A circuit for a 3-phase, 480V, 20 HP motor requires a feeder. The motor has a full-load current (FLC) of 27A. The feeder conductors are copper, installed in conduit, and rated for 75°C. What is the minimum required ampacity for the feeder conductors supplying this motor?
- A34A
- B27A
- C41A
- D48A
Show answer & explanationAnswer & explanation
Correct answer: A. 34A
According to NEC 430.22(A), conductors supplying a single motor used in a continuous duty application must have an ampacity of not less than 125% of the motor's FLC. Calculation: 27A (FLC) * 1.25 = 33.75A. The next standard conductor ampacity would be 34A or higher. This ensures the conductors can handle the motor's continuous operating current safely.
Why the other options are wrong
- B. 27A is the FLC, not the minimum required ampacity for continuous duty, which requires 125% of FLC.
- C. 41A is higher than required, leading to unnecessary cost.
- D. 48A is significantly higher than required, leading to unnecessary cost.
Motor Feeder Conductor Sizing
Conductors supplying a single motor used in a continuous duty application must have an ampacity of not less than 125% of the motor's full-load current (FLC), as per NEC 430.22(A).
- Applies to continuous duty motors.
- Requires 125% of the motor's FLC.
- FLC is found in NEC Tables 430.247-430.250.
- Ensures conductors can handle sustained motor operation.
Memory trick: Motor Wires: Full-load current, plus a safety margin, ensures it's right.