A 20 ft tap from a 400 A feeder to a panel. The NEC 240.21(B)(2) one-third rule decides the conductor size.
| Feeder protection | 400 A |
| Tap length | 20 ft, in rigid conduit |
| Tap terminates in | A single breaker at the panel |
| Conductors (75 °C Cu, NEC 310.16) | 1 AWG = 130 A, 1/0 AWG = 150 A |
Length 20 ft ≤ 25 ft and protected — pass. Breaker 125 A ≤ 130 A — pass.
Length and protection pass. 150 A ≥ 133.3 A — pass. Breaker 150 A ≤ 150 A — pass.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Length ≤ 25 ft | ≤ 25 ft | 20 ft | ✓ PASS |
| 1 AWG ampacity ≥ 1/3 of 400 A | ≥ 133.3 A | 130 A | ✗ FAIL |
| 1/0 AWG ampacity ≥ 1/3 of 400 A | ≥ 133.3 A | 150 A | ✓ PASS |
| Terminal breaker ≤ tap ampacity | ≤ 150 A | 150 A | ✓ PASS |
Key insight: The tap must satisfy the one-third rule even if the panel load is small, because until it reaches the terminal breaker the tap is only protected by the 400 A feeder device.
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Open Feeder Tap Rule Calculator (NEC 240.21(B)) calculator →For taps up to 10 ft, the tap ampacity must be at least the terminating device rating and, for field installations leaving the enclosure, at least one-tenth of the feeder OCPD rating.
Yes. Use the ampacity column allowed by the terminal temperature rating, usually 75 °C for equipment over 100 A (NEC 110.14(C)).