What is the ampacity of three 12 AWG THHN and three 14 AWG TW copper current-carrying conductors in a rigid metal conduit at an ambient temperature of 30°C?

To determine the ampacity of the conductors, we first need to refer to the National Electrical Code (NEC) guidelines for the ampacity of copper conductors. Under normal conditions, the ampacity of a 12 AWG copper conductor is typically 20 amperes, and for a 14 AWG conductor, it is usually around 15 amperes.

However, when conducting multiple conductors, there are adjustments that need to be made to account for the heat produced as they carry current together – this is known as the derating factor. In a rigid metal conduit, if you have three conductors, you generally apply a derating factor to account for the increase in temperature due to the closeness of the conductors. The NEC allows for certain types of adjustments based on the number of conductors.

For three 12 AWG conductors, each with an ampacity of 20 A, the overall effect of the derating doesn't significantly reduce their capacity for light residential loads. Therefore, you can still reasonably utilize their full ampacity rating, which remains at 20 A for each conductor.

In the case of three 14 AWG conductors, while a single one has a typical ampacity of 15 A, they