Feeder Tap Conductor 10-ft / 25-ft Rule (NEC 240.21(B))
The minimum ampacity an unprotected feeder tap conductor must carry under the 10-ft and 25-ft tap rules of NEC 240.21(B)(1) and (B)(2): at least 1/10 of the feeder overcurrent device for a tap up to 10 ft, or 1/3 for a tap up to 25 ft, with a verdict against the proposed conductor's 75 C ampacity (400 A feeder, 22 ft tap -> 133 A minimum). A tap over 25 ft falls under neither short-tap rule. The other 240.21(B) conditions (physical protection, single-OCPD termination, the supplied-device rating) apply; the AHJ and design engineer govern.
Formula and source
min_tap_ampacity = feeder_OCPD x 1/10 (tap <= 10 ft, 240.21(B)(1)) or x 1/3 (tap <= 25 ft, 240.21(B)(2)); a tap longer than 25 ft falls under neither short-tap rule. The proposed tap conductor's 75 C ampacity must equal or exceed that minimum.
Minimum feeder tap conductor ampacity under the 10-ft and 25-ft tap rules of NEC 2023 240.21(B)(1) and (B)(2), by name. A computational aid for the <= 1000 V case; the AHJ-adopted NEC edition and the design engineer govern.
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