What is the minimum size THWN-2 ungrounded feeder conductor required for a 150-ampere rated panel board in a 208Y/120-volt system?

To determine the minimum size THWN-2 ungrounded feeder conductor required for a 150-ampere rated panel board in a 208Y/120-volt system, consider the National Electrical Code (NEC) guidelines.

For a 150-ampere service, you typically refer to NEC Table 310.16, which outlines the ampacity of conductors based on their size, insulation type, and installation conditions. THWN-2 conductors are rated for wet and dry locations, and their properties must be examined accordingly.

Looking at the NEC Table 310.16, a 1 AWG THWN-2 conductor typically has an ampacity of 130 amperes when installed in a standard raceway and at 30°C. However, taking into account the 75°C conductor insulation rating, 1 AWG is the minimum size conductor that can safely handle the 150-ampere load, assuming proper adjustments for temperature and conduit fill.

This means that while 2 AWG and 3 AWG conductors would be insufficient to handle the full load, the 1 AWG THWN-2 provides sufficient capacity to meet the demand of the 150-ampere panel board while still adhering to safety