What is the minimum size copper THW conductor for branch-circuit conductors supplying a 10-horsepower, 230-volt, single-phase AC motor with terminals rated at 75°C?

What is the minimum size copper THW conductor for branch-circuit conductors supplying a 10-horsepower, 230-volt, single-phase AC motor with terminals rated at 75°C?

• A. 14 AWG
• B. 8 AWG
• C. 10 AWG
• D. 6 AWG

Answer: (D)

Sizing motor branch-circuit conductors starts with the motor’s full-load current and the rule that conductors must carry at least 125% of that current, using the 75°C ampacity when terminals are rated at 75°C. For a 10 HP, 230 V single-phase motor, the full-load current is about 32 A (roughly 10 HP × 746 W ÷ 230 V, with typical efficiency adjusted from the nameplate data). 125% of that is around 40 A. When you look up copper THW ampacities at 75°C, the smallest conductor that meets or exceeds ~40 A is 6 AWG. Therefore, the minimum size is 6 AWG. Notes: 14 AWG and 10 AWG are far below the needed 40 A. 8 AWG is often a borderline choice depending on the exact ampacity table used, but in this context the minimum that satisfies the 125% requirement with 75°C terminals is 6 AWG.

Sizing motor branch-circuit conductors starts with the motor’s full-load current and the rule that conductors must carry at least 125% of that current, using the 75°C ampacity when terminals are rated at 75°C. For a 10 HP, 230 V single-phase motor, the full-load current is about 32 A (roughly 10 HP × 746 W ÷ 230 V, with typical efficiency adjusted from the nameplate data). 125% of that is around 40 A. When you look up copper THW ampacities at 75°C, the smallest conductor that meets or exceeds ~40 A is 6 AWG. Therefore, the minimum size is 6 AWG.

Notes: 14 AWG and 10 AWG are far below the needed 40 A. 8 AWG is often a borderline choice depending on the exact ampacity table used, but in this context the minimum that satisfies the 125% requirement with 75°C terminals is 6 AWG.