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

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

• A. 1/0
• B. 2/0
• C. No. 1
• D. No. 2

Answer: (A)

The 10-foot tap rule lets you size a short tap conductor by the load it serves, not by the feeder’s full rating. In this case, the tap must carry the actual load of 150 A. For copper THWN, the 75°C ampacity of 1/0 AWG is 150 A, which just meets the 150 A requirement. A smaller size would not reliably carry 150 A, while larger sizes (like 2/0 or larger) would be more than necessary for the tap. So the smallest conductor that satisfies the rule is 1/0 AWG copper THWN.

The 10-foot tap rule lets you size a short tap conductor by the load it serves, not by the feeder’s full rating. In this case, the tap must carry the actual load of 150 A. For copper THWN, the 75°C ampacity of 1/0 AWG is 150 A, which just meets the 150 A requirement. A smaller size would not reliably carry 150 A, while larger sizes (like 2/0 or larger) would be more than necessary for the tap. So the smallest conductor that satisfies the rule is 1/0 AWG copper THWN.