Using the 25' tap rule, what is the minimum size copper THWN conductor required for a tap from a 400A, 480V, 3Phase feeder to serve a 150A load?

Using the 25' tap rule, what is the minimum size copper THWN conductor 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 tap sizing uses the 25' tap rule, which requires the tap conductors to be sized for at least 125% of the load they serve. The tap is feeding a 150 A load, so the required ampacity is 150 × 1.25 = 187.5 A. Looking at copper THWN ampacities, using the 90°C rating (typical for THWN), the smallest conductor size that provides at least 187.5 A is 1/0 AWG. A smaller size would not meet the 187.5 A requirement, while larger sizes would still be acceptable but not minimum. The 400 A feeder rating doesn’t determine the tap size here because the tap length is short and the sizing is governed by the tap rule and the load being served. Therefore, the minimum conductor size is 1/0 copper THWN.

The tap sizing uses the 25' tap rule, which requires the tap conductors to be sized for at least 125% of the load they serve. The tap is feeding a 150 A load, so the required ampacity is 150 × 1.25 = 187.5 A. Looking at copper THWN ampacities, using the 90°C rating (typical for THWN), the smallest conductor size that provides at least 187.5 A is 1/0 AWG. A smaller size would not meet the 187.5 A requirement, while larger sizes would still be acceptable but not minimum. The 400 A feeder rating doesn’t determine the tap size here because the tap length is short and the sizing is governed by the tap rule and the load being served. Therefore, the minimum conductor size is 1/0 copper THWN.