What is the smallest size THHN/THWN copper conductor permitted for a feeder protected by a 150 A breaker?

The smallest size THHN/THWN copper conductor permitted for a feeder protected by a 150 A breaker is indeed 1/0 AWG. This is based on the National Electrical Code (NEC) guidelines, which dictate the minimum conductor sizes based on the ampacity ratings and the type of conductor insulation.

For a 150 A circuit, the ampacity requirements dictate that you need to use a conductor that can safely handle that amount of current while accounting for factors like temperature rating and installation conditions. According to NEC Table 310.16, 1/0 AWG copper wire has a sufficient ampacity to handle 150 A under typical conditions.

While other conductor sizes like 2 AWG and 3 AWG can be utilized for lower amperage circuits, they do not meet the necessary ampacity for a 150 A load. The larger sizes, such as 2/0, exceed the minimum requirement but are not the smallest conductor that meets code. Hence, 1/0 AWG is the proper choice as it provides both safety and compliance with the regulatory standards for this size of breaker.