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Feeder Tap Conductor Size Calculator

Size a feeder tap conductor under the NEC 240.21(B) tap rules. Enter the feeder overcurrent device rating and the tap length; the tool applies the 10-foot rule (240.21(B)(1)) or the 25-foot rule (240.21(B)(2)), reports the required minimum ampacity, and returns the smallest copper or aluminum conductor from Table 310.16 that meets it.

Feeder tap

25-ft rule (240.21(B)(2))

Conductor

Defaults (75 °C termination, 86 °F ambient, 3 conductors) match the published base tap examples. Enter real conditions to derate per Table 310.15.

How to use this calculator

  1. Enter the rating of the overcurrent device protecting the feeder you are tapping.
  2. Enter the tap length. The tool selects the 10-foot or 25-foot rule automatically and shows which one governs.
  3. On the 10-foot path, enter the rating of the device the tap terminates in and whether the tap leaves the enclosure (which adds the one-tenth-of-feeder floor).
  4. Pick the conductor material and termination temperature column, and adjust ambient temperature or conductor count if your install is not at the base conditions.
  5. Read the required ampacity, the smallest qualifying conductor, and the remaining code conditions you still have to meet.

NEC reference

This calculator follows NEC 2020 Section 240.21(B). The 10-foot rule is 240.21(B)(1): the tap ampacity must equal the calculated load and the rating of the device at its termination, and where the tap leaves the enclosure the feeder device may not exceed 10 times the tap ampacity. The 25-foot rule is 240.21(B)(2): the tap ampacity must be at least one-third of the feeder overcurrent device rating and the tap must terminate in a single overcurrent device. Conductor ampacity comes from Table 310.16, with Table 310.15(B)(1) ambient correction and Table 310.15(C)(1) adjustment when you enter non-base conditions. The tap-rule text and the 1/10 and 1/3 fractions are identical across the 2017, 2020, and 2023 editions.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

The two feeder tap rules

A tap conductor connects to a feeder at a point with no overcurrent device. Normally a conductor is protected at its supply end at its ampacity; a tap deliberately is not. NEC 240.21(B) makes that acceptable by capping the tap length and forcing a minimum ampacity that is a fixed fraction of the feeder overcurrent device, so a fault on the tap is still cleared quickly by the feeder device or the device at the tap end.

The 10-foot rule (240.21(B)(1)) is the tighter one: the tap ampacity has to at least match the device it feeds, and where it leaves the enclosure it also has to be at least one-tenth of the feeder OCPD. The 25-foot rule (240.21(B)(2)) trades a larger minimum ampacity, one-third of the feeder OCPD, for the longer allowable run, and requires the tap to land on a single breaker or fuse set that limits its load.

Worked example: 400 A feeder, 25-foot tap

  1. The feeder is protected by a 400 A device and the tap runs 25 feet, so the 25-foot rule applies.
  2. Required tap ampacity is one-third of the feeder OCPD: 400 / 3 = 133.3 A.
  3. In the 75 degrees C column of Table 310.16, 1 AWG copper is 130 A, which falls short of 133.3 A.
  4. 1/0 copper is 150 A, which meets it, so the minimum tap conductor is 1/0 copper.
  5. The tap still has to terminate in a single device rated no more than 150 A and be protected from physical damage.

Why 240.4(D) does not limit a tap conductor

For an ordinary branch conductor, 240.4(D) caps overcurrent protection at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG copper regardless of the table ampacity. Those caps do not apply to feeder taps: 240.4 makes an exception for conductors sized under 240.21, so a tap is sized on its full Table 310.16 ampacity. That is why a 10 AWG copper tap can be used at its 35 A table value here even though a 10 AWG branch conductor would be held to 30 A of protection. This calculator applies that exception and does not impose the 240.4(D) cap.

Conditions beyond ampacity

Meeting the ampacity fraction is necessary but not sufficient. The results panel lists the remaining conditions for the applied rule: for a 25-foot tap, termination in a single overcurrent device and protection from physical damage; for a 10-foot tap, not extending beyond the equipment supplied and, where it leaves the enclosure, the 10-times feeder-device limit. A tap that meets the ampacity but fails a condition is not a compliant tap.

Frequently asked questions

What is the 10-foot tap rule?

NEC 240.21(B)(1) allows a feeder tap up to 10 feet with no device at the tap point, as long as the tap ampacity is at least the rating of the device it feeds, the tap does not extend beyond that equipment, and it is in a raceway. Where the tap leaves the enclosure, the feeder device may not exceed 10 times the tap ampacity, i.e. the tap is at least one-tenth of the feeder OCPD.

What is the 25-foot tap rule?

NEC 240.21(B)(2) allows a feeder tap up to 25 feet where the tap ampacity is at least one-third of the feeder overcurrent device, the tap terminates in a single breaker or fuse set that limits its load, and the conductors are protected from physical damage.

What size wire do I need for a feeder tap?

Compute the required ampacity for the applicable rule, then choose the smallest conductor whose Table 310.16 ampacity meets it. A 400 A feeder with a 25-foot tap needs 133 A, which is 1/0 copper at 75 degrees C.

Does NEC 240.4(D) apply to tap conductors?

No. 240.4 excepts conductors sized under 240.21 from the small-conductor limits, so a tap is sized on its full table ampacity rather than the 15, 20, or 30 A caps for 14, 12, and 10 AWG copper.

Do tap conductors need overcurrent protection at the tap point?

No. The tap is intentionally unprotected where it meets the feeder; the length limit and the minimum ampacity are what keep it safe. This is the defining feature of a tap under 240.21.

Related calculators

Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.

Running an electrical contracting business? Jobber handles scheduling, invoicing, and job management. Start a 14-day free trial.

For deeper NEC training on feeder taps and NEC 240.21 overcurrent protection, Mike Holt's NEC courses are the industry standard.

Working from the printed code? NEC code books and tab sets are available from Mike Holt.

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