HOLT Electricians Practice Exam

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What is the required size of a THHN conductor for a 2 horsepower, 230-volt, single phase motor?

No. 12
No. 14
No. 10
No. 8

The correct answer is: No. 14

To determine the correct size of a THHN conductor for a 2 horsepower, 230-volt, single phase motor, it is essential to calculate the full-load current of the motor. A standard method for calculating this is by using the formula for single-phase motors: Full-load current (Amps) = (Horsepower × 746) / (Voltage × Efficiency × Power Factor) Assuming typical values for efficiency (90%) and power factor (0.8) for a motor, the calculation would yield approximately: Full-load current = (2 × 746) / (230 × 0.9 × 0.8) = 9.82 Amps (rounded to 10 Amps). According to the National Electrical Code (NEC), a conductor needs to be sized for a certain percentage above the full-load current to account for safe operation, typically using a 125% rule for motors. This results in: 10 Amps × 1.25 = 12.5 Amps. Based on NEC's ampacity tables for conductors, a No. 12 THHN conductor is rated for 20 Amps, which is suitable for this application. Therefore, while options that might suggest smaller conductor sizes are