What is the ampacity for twelve 12 AWG THHN copper conductors in a one-inch EMT at 32°C, consisting of three 4-wire multiwire branch circuits?

To determine the ampacity of twelve 12 AWG THHN copper conductors in a one-inch EMT (Electrical Metallic Tubing) at 32°C, we begin by understanding how ampacity is calculated based on the number of conductors and the specific wiring configuration.

For THHN conductors, the base ampacity at 30°C is typically 30 amps. However, since there are multiple conductors bundled together (twelve in this case), we need to consider adjustments based on the NEC (National Electrical Code) guidelines. When multiple conductors are installed together in a raceway, derating factors must be taken into account.

The NEC specifies derating factors that apply when conductors are installed in a grouping. For twelve conductors, the typical derating factor applicable is 40%. This means the ampacity of each conductor must be reduced due to the heat that builds up when they are bunched together in a conduit.

Calculating based on the base ampacity:

1. Start with the base ampacity for a single 12 AWG copper conductor at 30°C: typically 30 A.

2. Apply the 40% derating factor for twelve conductors:

• Adjusted ampacity = 30 A