Feeder Tap Conductor 10-ft / 25-ft Rule (NEC 240.21(B))

ScopeThe 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 or tablemin_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.

Source and editionMinimum 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.

Free accessNEC 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.

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

API field namesfeeder_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