Determine the ampacity for nine 3/0 AWG THHN aluminum conductors in a three-inch RMC at 35°C with only six current-carrying conductors.

To determine the ampacity for nine 3/0 AWG THHN aluminum conductors in a three-inch rigid metal conduit (RMC) at 35°C with only six current-carrying conductors, it is important to refer to the ampacity values provided in the National Electrical Code (NEC) and to apply any necessary derating factors.

For 3/0 AWG aluminum conductors, the base ampacity at an ambient temperature of 30°C (86°F) is typically around 100 amperes per NEC Table 310.16. However, when there are multiple conductors in a conduit, the ampacity must be derated to prevent overheating due to insufficient cooling.

In the case of having six current-carrying conductors, the NEC requires a derating factor. According to the NEC, for more than three conductors, the ampacity must be reduced based on the number of conductors. When you have six current-carrying conductors, a derating factor of 80% applies.

Calculating the derated ampacity involves multiplying the base ampacity by the derating factor:

• Base ampacity for 3/0 aluminum = 100 A.

• Derating factor for six