Electrician Red Seal · Question
A three-phase, 208V, 10 HP motor has a nameplate FLA of 30A. What is the minimum size of THHN conductor required for the motor circuit, assuming an ambient temperature of 30°C and no more than three current-carrying conductors in the conduit?
According to CEC Rule 28-106(1), continuous duty motor branch circuit conductors must have an ampacity not less than 125% of the motor's FLA. 125% of 30A is 37.
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Question: A three-phase, 208V, 10 HP motor has a nameplate FLA of 30A. What is the minimum size of THHN conductor required for the motor circuit, assuming an ambient temperature of 30°C and no more than three current-carrying conductors in the conduit?
Answer options:
- No. 12 AWG ✅ No. 10 AWG
- No. 8 AWG
- No. 6 AWG
Correct answer: No. 10 AWG
Explanation: According to CEC Rule 28-106(1), continuous duty motor branch circuit conductors must have an ampacity not less than 125% of the motor's FLA. 125% of 30A is 37.5A. From CEC Table 2 (using 75°C column for THHN as per 4-006(6)), No. 10 AWG copper conductor has an ampacity of 35A (for 60°C insulation, 40A for 75°C, 50A for 90°C). Due to the 125% factor, we need at least 37.5A. Checking Table 2 for 75°C (THHN), 10 AWG is 40A, which satisfies 37.5A. Therefore, No. 10 AWG is the minimum size.
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