The smallest THHN/THWN copper grounded conductor for a service is ___ when the ungrounded conductors are 1/0 THWN/THHN copper and the load on the neutral is expected to be 120 amps.

• A. 6
• B. 3
• C. 2
• D. 1

Answer: (D)

The key idea is sizing the neutral (grounded conductor) to carry the actual neutral current. For THHN/THWN copper, you use the ampacity from NEC Table 310.16 (75°C column). At 75°C, the ampacities are approximately:

• 3 AWG ≈ 100 A

• 2 AWG ≈ 115 A

• 1 AWG ≈ 130 A

• 1/0 AWG ≈ 150 A

Since the neutral is expected to carry 120 A, the smallest conductor that can safely carry that load is 1 AWG. A 2 AWG (115 A) isn’t enough, and a 3 AWG (100 A) isn’t enough either. The 1 AWG size meets and exceeds the required 120 A, while smaller sizes do not.