What is the adjusted ampacity for six No. 4/0 THHN copper conductors in a single conduit?

To determine the adjusted ampacity for six No. 4/0 THHN copper conductors in a single conduit, it’s essential to consider both the NEC (National Electrical Code) guidelines and the applicable adjustment factors based on the filling of the conduit.

The ampacity of a single No. 4/0 THHN copper conductor is typically found in NEC tables, and it indicates a standard ampacity rating under normal conditions. However, because multiple conductors are placed in a single conduit, adjustment factors must be applied to account for the increased heat generated when conductors are bundled together.

According to NEC Table 310.16, the initial ampacity of a 4/0 THHN copper conductor is around 230 amps at 75 degrees Celsius. When you have more than three conductors (which you do with six), NEC Table 310.15(B)(3)(a) states that you need to apply a correction factor to account for the extra heat. For six conductors, the adjustment factor is typically 80%.

The calculation to find the adjusted ampacity would look like this:

1. Base ampacity for one No. 4/0 THHN copper conductor = 230A

2. Apply the adjustment