CSLB C-10 Electrical ContractorEquipment and InstallationEasy
A client requests a new dedicated 20-ampere, 120-volt branch circuit for a single-phase, continuous-duty motor. The motor has a nameplate full-load current (FLC) of 15 amperes. What is the minimum size copper THWN-2 conductor required for this branch circuit?
- A8 AWG
- B12 AWG
- C14 AWG
- D10 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 12 AWG
Motor branch-circuit conductors must be sized at not less than 125% of the motor’s FLC. For a 15-ampere motor, 125% is 18.75 amperes. A 12 AWG THWN-2 conductor is rated for 20 amperes at 75°C, which is sufficient.
Why the other options are wrong
- A. 8 AWG is significantly oversized and not the minimum required for this application.
- C. 14 AWG is rated for 15 amperes, which is less than 125% of the motor's FLC.
- D. 10 AWG is oversized and not the minimum required for this application.
Motor Branch-Circuit Conductor Sizing
Conductors supplying a single motor used in continuous duty must have an ampacity of at least 125% of the motor's full-load current.
- Applies to continuous-duty motors.
- Minimum ampacity = FLC x 1.25.
- Conductor ampacity must be chosen from appropriate temperature column (e.g., 75°C for THWN-2).
Memory trick: Motor Conductors Need Extra Juice: Multiply FLC by 1.25.