Journeyman Electrician Practice Exam

What is the minimum size THW copper branch-circuit conductor required for a 3-phase, continuous-duty AC motor with a full-load current of 70 amperes?

1. 2 AWG

2. 3 AWG

3. 4 AWG

4. 5 AWG

The correct answer is: 3 AWG

To determine the minimum size THW (Thermoplastic High Water Resistant) copper branch-circuit conductor needed for a 3-phase, continuous-duty AC motor with a full-load current of 70 amperes, several factors must be considered. For motor circuits, the National Electrical Code (NEC) provides guidelines for the sizing of conductors based on full-load current ratings. According to NEC Article 430, the minimum conductors must often be sized according to the motor's full-load current and then adjusted for the type of insulation and ambient temperature. The NEC specifies that for motors, the conductor may be sized based on a percentage of the full-load current, and typically a derating factor may be applied based on continuous operation. For continuous loads, conductors should be sized at 125% of the motor's full-load current. Calculating for this motor: - Full-load current = 70 amperes - For a continuous load, the required current = 70 A x 1.25 = 87.5 amperes. Now, looking at the wire gauge options available, 3 AWG copper conductors can safely carry 100 amperes according to NEC ampacity charts, which means it can adequately handle the calculated requirement for