Which copper conductor size is required for a tap from a 400A, 480V, 3-Phase feeder according to the 25' tap rule?

To determine the required copper conductor size for a tap from a 400A, 480V, 3-Phase feeder, one must apply the 25' tap rule. This rule is found in the National Electrical Code (NEC) and states that a tap conductor can be sized based on the first overcurrent device protecting the conductors. For taps, the required ampacity must cover the tap conductors' maximum current without exceeding the limits specified by the NEC.

In this scenario, since the feeder is rated for 400A, a tap of up to 25 feet would require the conductor to be sized for the tap, not the full feeder rating. The NEC allows for a reduction in conductor size under certain conditions, especially for short distances like a 25-foot tap.

The appropriate ampacity for 1/0 copper conductors at 75°C is 150 amps. In this case, the tap does not need to carry the full 400A load, and thus a size of 1/0 is suitable here as it meets the requirement for connecting to the feeder while adhering to the tap rules. By allowing a smaller conductor size for short distances, the tap can efficiently connect to the main system without violating NEC regulations.