What is a tap conductor?

A tap is a conductor connected to a feeder that is smaller than the feeder and is protected at its origin only by the feeder's overcurrent device. NEC 240.21(B) carves out specific exceptions that let these unprotected taps exist within strict length and ampacity limits.

What does the 10-ft tap rule require?

Under 240.21(B)(1) the tap may not exceed 10 ft, its ampacity must be at least 10 percent of the feeder overcurrent device rating, it must equal or exceed the rating of the device it supplies, and it must stay within the enclosure it serves.

What does the 25-ft tap rule require?

Under 240.21(B)(2) the tap may run up to 25 ft, its ampacity must be at least one-third of the feeder overcurrent device rating, and it must terminate in a single overcurrent device rated no higher than the tap ampacity.

Is there a tap rule with no length limit?

Yes. The outside-tap rule in 240.21(B)(5) allows unlimited length when the tap runs outside the building except at its termination, is protected from physical damage, and terminates in a single overcurrent device located where the conductors enter the building.

Is the tap ampacity the bare wire table value?

No. Tap ampacity is the conductor's allowed current after all temperature and conduit-fill derating. Use the derated value when comparing against the 10 percent or one-third thresholds, not the raw ampacity table figure.

NEC Tap Rule Calculator (210% and 100 A Rules)

Feeder taps let a large feeder branch off into a smaller conductor without its own overcurrent device — but only inside the narrow exceptions NEC 240.21(B) allows. This calculator checks a proposed tap against the 10-ft, 25-ft, and outside-tap rules at once so you know whether the conductor is legal before you pull it.

How it works

Each rule sets a minimum tap ampacity tied to the feeder’s overcurrent device rating, with a length ceiling:

10-ft rule:   length ≤ 10 ft   tap ampacity ≥ 10% of feeder OCPD
25-ft rule:   length ≤ 25 ft   tap ampacity ≥ 1/3 of feeder OCPD
outside rule: any length       single OCPD at building entry, damage-protected

The tool computes both thresholds from the feeder rating and reports which rules the tap satisfies. Passing any one rule, with its termination and enclosure conditions met, makes the tap compliant.

Example and notes

A 175 A tap off a 400 A feeder, run 20 ft, fails the 10-ft rule on length and fails the 25-ft rule because one-third of 400 A is 133 A — wait, 175 A exceeds 133 A, so it actually passes the 25-ft rule as long as it terminates in a single device rated 175 A or less. Shorten that same tap to 8 ft and it must instead carry at least 40 A (10 percent of 400 A), which it easily does. Always confirm the termination and physical-protection conditions, and remember tap ampacity is the derated value, not the bare table figure.