The overcurrent device rating is a key factor when determining the correct size conductor. Article 240 in the National Electrical Code (NEC) provides general requirements for overcurrent protection and overcurrent protective devices. As a general rule, after the application of adjustment and correction factors, a conductor’s ampacity rating shall not be less than the ampacity rating of the overcurrent device.


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). Some of the rules in 240.4(A) through (G) are alternative rules. These alternative rules permit the rating of the overcurrent protective device to be higher than the ampacity of the conductor.


The rule that is probably best known and used most often pertains to overcurrent devices rated 800 amperes (A) or less. This rule in 240.4(B) is often referred to as the round-up rule. This section also contains alternative rules for tap conductors. Tap conductors also are included in the types of conductors that may not be required to be protected against overcurrent in accordance with their ampacities.


Section 240.4(E) references various sections throughout the Code covering rules for tap conductors. One of those sections referenced is 240.21, which is the section we usually think of when we think of tap rules. In accordance with 240.21, overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 240.21(A) through (H). While other types of taps are covered, most of this section covers tap conductors in feeders and transformers. 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 first feeder tap rule is typically called the 10-foot tap rule. Feeder conductors are not required to be protected at their ampacity if the tap-conductor length does not exceed 10 feet and the tap conductors comply with all of the requirements in 240.21(B)(1)(1) through (4). It is essential to have a good understanding of the fourth provision in this section because it pertains to sizing the conductor.


In accordance with 240.21(B)(1)(4), the ampacity of the tap conductor shall not be less than one-tenth of the rating of the overcurrent device protecting the feeder conductors. While the ampacity of the tap conductors shall not be less than one-tenth of the rating of the overcurrent device protecting the feeder conductors, the tap conductors can be larger than one-tenth of the rating of the overcurrent device. For example, size 500 kcmil THHN copper feeder conductors will terminate in a metal wireway or trough. The feeder conductors will be protected by a 400A circuit breaker. A main-breaker panelboard will be installed above the trough. The main breaker in this panelboard will have a rating of 200A and will be supplied by 3/0 AWG THHN copper conductors. These 3/0 conductors will be tap conductors because they will be connected in the trough to the 500 kcmil feeder conductors. The calculated load on this panelboard is 155A. The length of the tap conductors will not exceed 10 feet. The tap conductors between the trough and the panelboard will be enclosed in a raceway. All terminations will be rated 75°C. Will this installation be permitted?


Because the tap conductors will not exceed 10 feet, it shall be permissible to use the 10-foot tap rule in 240.21(B)(1). 
Now check to see if this installation meets all the provisions for the 10-foot tap rule. The first provision in 240.21(B)(1)(1) states that the ampacity of the tap conductors must not be less than the load. The first provision also states that the ampacity of the tap conductors must not be less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors. In this installation, the rating of the overcurrent device (circuit breaker) is not more than the ampacity of the conductors because, in accordance with the 75°C column of Table 310.15(B)(16), the maximum ampacity of 3/0 AWG conductors is 200A. In this installation, the second provision is not applicable. This installation complies with the third provision because the tap conductors are enclosed in a raceway, which extends from the tap to the panelboard. The fourth provision pertains to field installations, and this installation will be a field installation. If the tap conductors leave the enclosure or vault in which the tap is made, the ampacity of the tap conductors shall not be less than one-tenth of the rating of the overcurrent device protecting the feeder conductors. Because a 400A breaker protects the feeder conductors, the tap conductors in this installation must have an ampacity of at least 40A (400 ÷ 10 = 40). In this installation, the ampacity of the tap conductors are one-half the rating of the overcurrent device protecting the feeder conductors. Therefore, this installation is permitted (see Figure 1).


As long as the feeder conductor is installed in accordance with 215.2, more than one tap can be made to the feeder. In accordance with 215.2, feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV and V of Article 220. This section also states that the minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.


For example, four fused disconnects will be installed above a metal trough. The load on each disconnect will be 40A, and each disconnect will be fused at 50A. Each disconnect will be supplied by 8 AWG THHN conductors. The 8 AWG conductors will be connected in the trough to 3/0 AWG THHN feeder conductors. The feeder conductors will be protected by a 200A circuit breaker. None of these tap conductors will exceed 10 feet in length. The tap conductors between the trough and the disconnect will be enclosed in a raceway. All terminations will be rated 75°C. Protecting 8 AWG conductors with a 200A circuit breaker seems odd because 8 AWG conductors are usually protected at 40 or 50A. But this installation will be permitted because the tap conductors do not exceed 10 feet and this installation complies with all the provisions in 240.21(B)(1)(1) through (4).


This installation complies with the first part of the first provision because the ampacity of the tap conductors is not less than the load. The load on each disconnect is 40A, and in accordance with the 75°C column of Table 310.15(B)(16), the maximum ampacity of an 8 AWG conductor is 50A. This installation complies with the second part of the first provision because the ampacity of the tap conductors is not less than the rating of the overcurrent protective device at the termination of the tap conductors. In this installation, the second provision is not applicable. This installation complies with the third provision because the tap conductors are enclosed in a raceway, which extends from the tap to the disconnect. This installation complies with the fourth provision because the ampacity of the tap conductors is not less than one-tenth of the rating of the overcurrent device protecting the feeder conductors. Because a 200A breaker protects the feeder conductors, the tap conductors in this installation must have an ampacity of at least 20A (200 ÷ 10 = 20). Although these 8 AWG conductors are connected to 3/0 AWG conductors that are protected at 200A, this installation is permitted (see Figure 2).


While the ampacity of the tap conductors shall not be less than one-tenth the rating of the overcurrent device protecting the feeder conductors, the rating of the overcurrent device can be less than one-tenth the rating of the overcurrent device protecting the feeder conductors. For example, a parallel set of 500 kcmil THHN feeder conductors will terminate in a metal trough. An 800A overcurrent device will protect these feeder conductors. A 30A fused disconnect (with 30A fuses) will be installed to supply a 21A load. The length of the tap conductors will not exceed 10 feet. The tap conductors between the trough and the disconnect will be enclosed in a raceway. All terminations will be rated 75°C. What size tap conductors are required for this installation? Using the tap rule in 240.21(B)(1) is permitted because the tap conductors do not exceed 10 feet. The first provision states that the ampacity of the tap conductors must not be less than the load. In this installation, the load is 21A. The first provision also states that the ampacity of the tap conductors must not be less than the rating of the overcurrent protective device at the termination of the tap conductors. In this installation, the rating of the overcurrent protective device is 30A. By looking at this first provision only, 10 AWG conductors would be permitted, but this is not the only provision that shall be met. This installation complies with the third provision because the tap conductors are enclosed in a raceway, which extends from the tap to the disconnect. Because the tap conductors are field-installed, compliance with the fourth provision is required. The ampacity of the tap conductors shall not be less than one-tenth of the rating of the overcurrent device protecting the feeder conductors. Because an 800A overcurrent device protects the feeder conductors, the tap conductors in this installation must have an ampacity of at least 80A (800 ÷ 10 = 80). Although the load is only 21A and the rating of the fused disconnect is 30A, the tap conductors must have an ampere rating of at least 80A. In accordance with the 75°C column of Table 310.15(B)(16), a 4 AWG conductor has an ampacity of 85A. Therefore, the minimum size tap conductors required for this installation are 4 AWG conductors (see Figure 3).


Next month’s column continues the discussion of sizing conductors.