Which size copper THWN conductor is required for a tap from a 400A, 480V, 3-Phase feeder to serve a 150A load using the 10' tap rule?

To determine the appropriate size of the copper THWN conductor for a tap from a 400A, 480V, 3-Phase feeder to serve a 150A load, applying the 10' tap rule is crucial. The 10' tap rule allows for a smaller conductor size when the tap is less than 10 feet in length, and the load is less than 10% of the feeder's rating.

In this case, since the load is 150A, we should first check if it is less than 10% of the feeder’s capacity. The feeder is rated at 400A, and 10% of that is 40A. Therefore, since 150A is significantly larger than this threshold, the tap needs to be sized appropriately based on the load rather than solely on the 10% rule.

According to the National Electrical Code (NEC) guidelines, for a 150A load, the appropriate conductor size must be based on the ampacity of the conductor. For copper THWN insulation, 150A typically requires No. 1 AWG copper wire when adhering to the standard calculations for conductors rated for continuous loads (taking into account the temperature rating and installation conditions).

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