For a motor circuit using a 120/240-volt, 3Ø, 4-wire panelboard with 75°C terminations, the smallest acceptable AWG for the conductor is which of these?

For a motor circuit using a 120/240-volt, 3Ø, 4-wire panelboard with 75°C terminations, the smallest acceptable AWG for the conductor is which of these?

• A. 8
• B. 4
• C. 6
• D. 12

Answer: (B)

Sizing motor conductors relies on carrying at least 125% of the motor’s full-load current, using the ampacity value that matches the termination temperature rating of the equipment. With a 75°C-terminated panelboard, you read ampacities from the 75°C column. For each conductor size, you compare its 75°C ampacity to the required 1.25 × motor FLC. The smaller sizes in the options have lower ampacities, and would fall short of the 125% requirement, while the next size up does meet it. Specifically, AWG sizes compatible with 75°C terminations have roughly: #12 around 30 A, #8 around 55 A, #6 around 65 A, and #4 around 85 A. If the motor’s 1.25 × FLC is at or below about 85 A, AWG 4 is sufficient, making it the smallest conductor that meets the requirement. Hence, AWG 4 is the correct choice.

Sizing motor conductors relies on carrying at least 125% of the motor’s full-load current, using the ampacity value that matches the termination temperature rating of the equipment. With a 75°C-terminated panelboard, you read ampacities from the 75°C column. For each conductor size, you compare its 75°C ampacity to the required 1.25 × motor FLC. The smaller sizes in the options have lower ampacities, and would fall short of the 125% requirement, while the next size up does meet it. Specifically, AWG sizes compatible with 75°C terminations have roughly: #12 around 30 A, #8 around 55 A, #6 around 65 A, and #4 around 85 A. If the motor’s 1.25 × FLC is at or below about 85 A, AWG 4 is sufficient, making it the smallest conductor that meets the requirement. Hence, AWG 4 is the correct choice.