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.

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.

NEC is free to read at nfpa.org/freeaccess. The supplemental 240.21(B) conditions (physical protection, single-OCPD termination for the 25-ft rule, the rating of the supplied device) are stated, not modeled.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: feeder_ocpd_a, tap_length_ft, tap_ampacity_a, min_tap_ampacity_a, margin_a

• Rule fractions 1/10 for the 10-ft tap, 1/3 for the 25-ft tap; the 10 ft and 25 ft boundaries are the exact code lengthsNEC 240.21(B)(1) and (B)(2)
• Tap ampacity basis the 75 C termination column ampacity of the proposed tap conductorNEC 110.14(C) / 310.16