What is the ampacity of each of eighteen 10 AWG THHN copper current-carrying conductors in a metal wireway?

To determine the ampacity of each of eighteen 10 AWG THHN copper current-carrying conductors in a metal wireway, we refer to the National Electrical Code (NEC) guidelines which are used to ascertain the current-carrying capacity of conductors under various conditions.

For 10 AWG THHN copper wire, the nominal ampacity is generally recognized to be around 30 amps for a single conductor in free air at an ambient temperature of 30 degrees Celsius. However, when there are multiple conductors in close proximity—like in a wireway—derating factors must be applied to account for the increase in temperature due to heat build-up among the grouped conductors.

In practice, when multiple conductors are installed together, you would apply a derating factor according to NEC Table 310.15(B)(2)(a). For example, if you have more than three conductors grouped together, the ampacity decreases to prevent overheating.

For eighteen 10 AWG conductors, the derating factor is significant. Upon applying the derating factors appropriate for a grouping of this size, the ampacity of each conductor is approximately reduced. This brings the effective ampacity to around 16 amps per conductor.

Therefore,