Which of the following THHN/THWN copper conductors is the smallest size permitted for a feeder where a 150 A breaker protects the feeder?

The smallest size of THHN/THWN copper conductor permitted for a feeder protected by a 150 A breaker is indeed 1/0. This is determined based on the National Electrical Code (NEC) Table 310.16, which specifies the ampacity of conductors based on their size and insulation type.

For feeders, it is necessary to select a conductor size that can safely handle the load without overheating or causing damage to the electrical system. According to the NEC, a 1/0 copper conductor has an ampacity of 150 amps, matching the rating of the breaker. This means it can continuously carry the full load current without exceeding its temperature limit.

Conductor sizes larger than 1/0, such as 2/0, 3/0, and 4/0, would also be suitable for a 150 A application, but they would not be the smallest permissible size. Understanding the correct sizing based on NEC ampacity tables is crucial for ensuring safety and compliance in electrical installations.