As a general rule in the National Electrical Code (NEC), the ampere rating of the overcurrent device (fuse or breaker) must not be less than the ampacity of the conductor. As stated in 240.4, conductors (other than flexible cords, flexible cables and fixture wires) shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). One of the sections in 240.4 that permit the ampere rating of the overcurrent device to be higher than the ampacity of the conductor is commonly referred to as the round-up rule. Where meeting all three conditions in 240.4(B), the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used. Another section in 240.4 that permits the rating of the overcurrent device to be higher than the ampacity of the conductor is 240.4(E). This section references various sections containing rules for tap conductors. 


One of the tap rules referenced is 240.21, which is the section we usually think of regarding tap rules. In accordance with 240.21(B), conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5).


The second feeder tap rule is typically called the 25-foot tap rule. Where the length of the tap conductors is not more than 25 feet and all three provisions in 240.21(B)(2) are met, feeder conductors are not required to be protected at their ampacity. In accordance with the first provision [240.21(B)(2)(1)], the ampacity of the tap conductors shall not be less than one-third of the rating of the overcurrent device protecting the feeder conductors. With the first feeder tap rule in 240.21(B)(1), the ampacity of the tap conductors must be at least one-tenth of the rating of the overcurrent device protecting the feeder conductors, instead of one-third. With the first tap rule, the length is limited to 10 feet.


The second provision states that the tap conductors must terminate in a single circuit breaker or a single set of fuses that limit the load to the tap conductors’ ampacity. Like one of the provisions for the 10-foot tap rule, the tap conductors’ ampacity must not be less than the rating of the overcurrent protective device at the tap conductors’ termination. The second provision also states that the overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.


An example of an overcurrent device supplying additional overcurrent devices would be tap conductors that terminate on the main breaker in a panelboard with breakers on the load side of the main breaker. An example of an overcurrent device not supplying additional overcurrent devices would be tap conductors that terminate in a fused disconnect and the fused disconnect supplying a single load or a single piece of equipment.


The third provision for the 25-foot tap rule states that the tap conductors must be protected from physical damage by being enclosed in an approved raceway or by other approved means. The 25-foot tap rule is permitted where all three provisions in 240.21(B)(2) are met and the length of the tap conductors is not more than 25 feet (see Figure 1).


To find the minimum ampacity for the tap conductors when using the 25-foot tap rule, simply divide the rating of the overcurrent device protecting the feeder conductors by three. For example, by using the 25-foot tap rule, what are the smallest tap conductors that can be connected to 350 kcmil feeder conductors that are protected by a 300-ampere (A) circuit breaker? Because the feeder conductors are protected by a 300A breaker, these tap conductors must have an ampacity of at least 100A (300 ÷ 3 = 100). In accordance with the 75°C column of Table 310.15(B)(16), a 3 AWG conductor has an ampacity of 100A. Therefore, the minimum size tap conductors required for this installation are 3 AWG conductors (see Figure 2).


Like the 10-foot tap rule, feeder conductors under the 25-foot tap rule can have more than one tap. For example, size 500-kcmil THHN copper feeder conductors will terminate in a junction box. A 400A circuit breaker will protect the feeder conductors. A main-breaker, 40-circuit panelboard will be installed within 20 feet of the junction box. The main breaker in this panelboard will have a rating of 250A. The conductors supplying this panelboard will be tap conductors because they will be connected in the junction box to the 500 kcmil feeder conductors. The calculated load on this panelboard is 190A. The length of these tap conductors will be more than 10 feet but not more than 25 feet. The tap conductors between the junction box and panelboard will be enclosed in a raceway. A 100A fused disconnect (with 100A fuses) also will be installed within 20 feet of the junction box. The load on this disconnect will be 78A. The conductors supplying this disconnect also will be tap conductors. The length of these tap conductors also will be more than 10 feet but not more than 25 feet. The tap conductors between the junction box and disconnect also will be enclosed in a raceway. All terminations will be rated 75°C. What size tap conductors are required to supply the panelboard, and what size tap conductors are required to supply the disconnect?


Because the length of the tap conductors is more than 10 feet but not more than 25 feet, size the tap conductors in accordance with the 25-foot tap rule in 240.21(B)(2). Because a 400A breaker protects the feeder conductors, the tap conductors in this installation must have an ampacity of at least 133A (400 ÷ 3 = 133). Because the main breaker in the panelboard will be rated 250A, the tap conductors will have an ampacity that will be more than the minimum 133A. If this was a regular feeder instead of a feeder tap, 4/0 AWG conductors could be installed. In accordance with the 75°C column of Table 310.15(B)(16), 4/0 AWG conductors have an ampacity of 230A. Since the calculated load is only 190A, these conductors would be sufficient to carry the load. And if this were a regular feeder, it would be permissible to use the round-up rule in 240.4(B). But because this is a feeder tap, it is not permissible to use the round-up rule. The ampacity rating of the tap conductors must not be less than the ampacity rating of the main breaker in the panelboard, which is 250A. Therefore, the conductors supplying the panelboard must have an ampacity of at least 250A. In accordance with the 75°C column of Table 310.15(B)(16), 250 kcmil conductors have an ampacity of 255A. The tap conductors required to supply the panelboard are 250 kcmil conductors (see Figure 3).


It also is permissible to use the 25-foot tap rule to size the conductors supplying the disconnect. The ampacity rating of these tap conductors shall not be less than the ampacity rating of the fuses in the disconnect, which is 100A. But, the ampacity of the tap conductors also shall not be less than one-third of the rating of the overcurrent device protecting the feeder conductors. Because the feeder conductors are protected by a 400A breaker, the tap conductors to the disconnect must have an ampacity of at least 133A (400 ÷ 3 = 133). Although the load is only 78A and the rating of the fused disconnect is 100A, the tap conductors must be good for at least 133A. In accordance with the 75°C column of Table 310.15(B)(16), a 1/0 AWG conductor has an ampacity of 150A. Therefore, the minimum size tap conductors required to supply this disconnect are 1/0 AWG conductors (see Figure 4).