If the ungrounded service conductors are 1/0 AWG copper, what is the minimum size of the grounded conductor?

• A. 6 AWG
• B. 4 AWG
• C. 8 AWG
• D. 2 AWG

Answer: (A)

Grounded conductor sizing is driven by the service rating, not just by the size of the ungrounded conductors. The neutral (grounded) conductor must be able to carry the unbalanced current that can flow in a 120/240 V service without overheating, and the NEC provides minimum sizes based on the overcurrent protection (service rating). When the ungrounded conductors are 1/0 copper, that typically corresponds to about a 150 A service. For a 150 A service, the minimum size permitted for the grounded (neutral) conductor is 6 AWG copper. Using a smaller conductor would not meet the required ampacity for the neutral under that OCPD, while a larger size would still be acceptable but isn’t required. If the service rating were higher, the required neutral size would increase accordingly.