What is the smallest size of THHN/THWN copper conductor allowed for a 150 A feeder to a subpanel?

For a 150 A feeder to a subpanel, the National Electrical Code (NEC) provides guidelines for conductor sizing to ensure that the electrical system operates safely without overheating. The minimum size of THHN/THWN copper conductor that may be used for this application is 1/0 AWG.

When determining the appropriate conductor size, it's essential to consider not only the ampacity of the conductor but also factors like temperature ratings and installation conditions. According to NEC Table 310.16, a 1/0 copper conductor has an ampacity that is adequate for carrying 150 A in typical conditions. Additionally, 1/0 AWG copper wire accounts for some safety margin, ensuring that the wire can handle the load without overheating during continuous operation.

Selecting a larger conductor, such as 2/0 or 3/0, would provide more capacity, but since the question asks for the smallest size allowed, 1/0 is the correct choice. This ensures compliance with code requirements while meeting the electrical load needs of the subpanel.