Tap Conductor Calculator

Under the 25 foot tap rule, a tap off a 400 amp feeder must have an ampacity of at least 133.3 amps, which is 1/0 AWG copper or 2/0 AWG aluminum in the 75 degree C column of NEC Table 310.16. That is one-third of the feeder overcurrent device rating, per NEC 240.21(B)(2)(1). Under the 10 foot rule the answer is usually larger, not smaller, because that rule sizes the tap to the calculated load and to the rating of the device the tap lands in, and only adds a one-tenth-of-the-feeder floor when the tap leaves the enclosure.

The 25 foot rule (NEC 240.21(B)(2)) sets one ratio: the tap conductor must carry at least one-third of the feeder overcurrent device rating. The 10 foot rule (NEC 240.21(B)(1)) sets no such ratio in the general case. Instead it requires the tap to be at least as large as the combined calculated load and at least as large as the rating of the equipment or overcurrent device it terminates in. A one-tenth-of-the-feeder floor is added only for field installations where the tap conductors leave the enclosure or vault in which the tap is made. Sizing a 10 foot tap at one-tenth of the feeder and nothing else is a common and dangerous shortcut: one-tenth of a 400 amp feeder is 40 amps, but a tap landing in a panelboard rated well above that has to be good for the panelboard rating.

Not at the tap point itself. That is the whole purpose of NEC 240.21(B): it permits conductors to be tapped to a feeder without overcurrent protection at the tap, provided the tap meets one of the five rules. Where the protection sits instead depends on the rule. The 25 foot rule, the high bay rule and the outside rule each require the tap to terminate in a single circuit breaker or a single set of fuses that limits the load to the ampacity of the tap conductors, so the protection sits at the end of the tap. The transformer rule places that requirement on the secondary conductors. The 10 foot rule states it differently: it does not name a single device, it requires the tap conductors to be at least as large as the rating of the equipment or overcurrent device they terminate in, and it requires that they not extend beyond the equipment they supply. If the tap does not fit any of the five rules, then you must install overcurrent protection where the conductors receive their supply.

No. NEC 240.21(B) states directly that 240.4(B) is not permitted for tap conductors. The next-standard-size rule that lets you protect a conductor at the next breaker up does not apply here. A tap is sized to an ampacity from Table 310.16, and the conductor you choose must actually meet or exceed the required ampacity rather than land near it and get rounded.

Use NEC 240.21(B)(3). The primary conductors must carry at least one-third of the rating of the overcurrent device protecting the feeder. The secondary conductors must carry at least the primary-to-secondary voltage ratio multiplied by that same one-third. On a 480 to 208 volt transformer tapped off a 400 amp feeder, the primary floor is 133.3 amps and the secondary floor is 480 divided by 208, times 133.3, which is 307.7 amps. Dropping the voltage ratio is the single most dangerous error in this rule, because it undersizes the secondary by more than half. Those are code minimums: if the transformer actually carries more than that, the conductors have to carry the real load, and this calculator applies whichever is larger. The combined primary plus secondary length cannot exceed 25 feet, excluding any portion of the primary that is protected at its ampacity.

Ten feet under 240.21(B)(1), 25 feet under 240.21(B)(2), and 25 feet of combined primary plus secondary under the transformer rule 240.21(B)(3). Longer taps are permitted in only two situations. In a high bay manufacturing building over 35 feet high at the walls, 240.21(B)(4) allows up to 100 feet total, but no more than 25 feet of that horizontally, and the conductors must be at least 6 AWG copper or 4 AWG aluminum. Outside a building, 240.21(B)(5) allows unlimited length provided the disconnect is at a readily accessible location nearest the point of entrance. Anything else needs overcurrent protection at the supply.

No, and confusing them changes the answer. A feeder tap supplying a transformer falls under 240.21(B)(3), and its ratios are taken against the overcurrent device protecting the feeder. Transformer secondary conductors fall under 240.21(C), and those ratios are taken against the overcurrent device protecting the transformer primary. Same-looking arithmetic, different reference device, different result. This calculator implements the feeder tap rules of 240.21(B) only.