NEC 240.21(B) · Commercial

Feeder taps: the 10-foot and 25-foot rules

Feeder tap conductors may be smaller than the feeder if they follow length-based rules. A tap of 10 ft or less is sized for what it serves; a tap of 25 ft or less needs at least 1/3 of the feeder overcurrent device's rating.

Diagram of feeder taps on a 300 A feeder: 10 ft tap at least 30 A if it leaves the enclosure, 25 ft tap at least 100 A ending in one breaker. — The next-size-up allowance in 240.4(B) does not apply to tap conductors, so size the tap from the rule itself, not from the nearest standard breaker. Diagram: Farmer Works Electric, free to share under CC BY 4.0 · Download PNG · How to credit

What the code requires

Feeder taps may be made anywhere on the load side of the feeder's overcurrent device, if they meet one of the 240.21(B) rules. The next-size-up allowance in 240.4(B) does not apply to tap conductors.

10-ft rule, (B)(1): the tap's ampacity must be at least the combined calculated load and at least the rating of the equipment or overcurrent device it ends in. Where listed equipment such as an SPD comes with instructions on minimum conductor size, a tap supplying it may be sized by those instructions. The tap cannot extend beyond the equipment it supplies and must be in a raceway except at the tap point. For field installations where the tap leaves the enclosure, ampacity must be at least 1/10 of the feeder overcurrent device rating.

25-ft rule, (B)(2): the tap's ampacity must be at least 1/3 of the feeder overcurrent device rating. It must end in a single breaker or set of fuses that limits the load to the tap's ampacity, and be in a raceway or other approved protection.

High-bay manufacturing buildings with walls over 35 ft, (B)(4): up to 25 ft horizontal and 100 ft total, qualified-person maintenance and supervision, ampacity of at least 1/3 of the feeder overcurrent device, termination in a single breaker or set of fuses that limits the load to the tap's ampacity, protection in a raceway or other approved means, minimum 6 AWG copper or 4 AWG aluminum, continuous with no splices, no wall, floor or ceiling penetrations, and the tap made at least 30 ft above the floor.

The code also covers taps that supply a transformer, with primary plus secondary no longer than 25 ft, in (B)(3), and outside taps of unlimited length in (B)(5). Each has its own conditions.

On the job

10-ft tap conductors that leave the enclosure with an ampacity below 1/10 of the feeder overcurrent device rating are a common correction.
New in 2023, 215.15 requires barriers in panelboards, switchboards, switchgear and motor control centers fed by these taps. Missing barriers on the line side of a tap-fed panel's main is a common correction.