Article 240 in the National Electrical Code (NEC) provides general requirements for overcurrent protection and overcurrent protective devices. In accordance with 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). The main rule states that the rating of the overcurrent protective device shall not be more than the ampacity of the conductor. But, because of alternative provisions in 240.4(A) through (G), there are times when the conductor ampacity can be less than the rating of the overcurrent protective device. One example of an alternative provision is the round-up rule in 240.4(B). The small-conductor rule in 240.4(D) is another example of an alternative provision for the protection of 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. Instead of 240.4(E) providing specific provisions on tap conductors, this section provides references located in other parts of the Code book.
The first types listed are tap conductors for household ranges and cooking equipment; the reference is 210.19(A)(3). The provision in 210.19(A)(3) states that branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. This is similar to the main rule in 240.4, which states that the rating of the overcurrent protective device shall not be more than ampacity of the conductor. But like the alternative provisions, there are two exceptions for household ranges and cooking equipment. The first exception is the section that pertains to tap conductors. Conductors tapped from a 50-ampere (A) branch circuit supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than 20A and shall be sufficient for the load to be served [210.19(A)(3) Exception No. 1]. As also stated in this exception, the tap conductors include any conductors that are a part of the leads supplied with the appliance that are smaller than the branch-circuit conductors. For example, one 50A branch circuit will be installed to supply power to one counter-mounted cooking unit (cooktop) and one wall-mounted oven. Size 6 AWG copper conductors will terminate in a junction box in the cabinet under the cooktop. The leads supplied with the cooktop and with the wall-mounted oven are 10 AWG copper conductors. Although a 30A overcurrent device is usually used to protect 10 AWG conductors, these conductors can be protected at 50A because of the first exception to 210.19(A)(3) (see Figure 1). Use caution with this type of installation because some jurisdictions require a separate circuit for each cooking appliance.
Another reference in 240.4(E) is for the tap rules that are located in 240.21. This is the section we usually think of when we think of tap rules. It briefly mentions branch-circuit tap conductors. Like the section referenced in 240.4(E), 240.21 also states that branch-circuit tap conductor requirements are in 210.19. While branch-circuit conductors and service conductors are mentioned, most of the requirements in 240.21 pertain to 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). Rules in this section are very detailed and specific. The first feeder tap rule is known as 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). The first provision is divided into two provisions that pertain to the ampacity of the tap conductor. First, the tap conductors’ ampacity must not be less than the combined calculated loads on the circuits supplied by the tap conductors. This is similar to the provision in 215.2(A)(1), which states that 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. The second part of the first provision 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. Sometimes, there is confusion about the section that states tap conductors must not be less than the rating of the device supplied by the tap conductors. It would seem that tap conductors could terminate directly onto a device or equipment without first terminating or landing onto an overcurrent protective device (fuse or circuit breaker), but this is not the case. Since 240.21(B) covers feeder taps, it is helpful to look at the definition of a feeder and also a branch circuit. Feeders are all circuit conductors between the service equipment, the source of a separately derived system or other power supply source, and the final branch-circuit overcurrent device (Article 100—Definitions). A branch circuit is defined as the circuit conductors between the final overcurrent device protecting the circuit and the outlet(s) (Article 100—Definitions). Since the overcurrent device ahead of the feeder tap is not the final overcurrent device protecting the circuit, feeder tap conductors must terminate on an overcurrent device. The overcurrent device at the termination of the tap can either supply power to a single circuit or to multiple loads (see Figure 2).
In accordance with 240.21(B)(1)(2), tap conductors shall not extend beyond the switchboard, panelboard, disconnecting means or control devices they supply. For field-installed tap conductors, this provision is not applicable. The third provision states that, except at the point of connection to the feeder, the tap conductors must be enclosed in a raceway, which shall extend from the tap to the enclosure of an enclosed switchboard, panelboard or control devices or to the back of an open switchboard.
The fourth provision pertains to field installations. As previously mentioned, this provision will override the second provision. 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.
To find the minimum ampacity for the tap conductors when using the 10-foot tap rule, simply divide by 10 the rating of the overcurrent device protecting the feeder conductors. For example, using the 10-foot tap rule, what are the smallest tap conductors that can be connected to 350 kcmil feeder conductors that are protected by a 300A circuit breaker? Because the feeder conductors are protected by a 300A breaker, these tap conductors must have an ampacity of at least 30A (300 ÷ 10 = 30). In accordance with Table 310.15(B)(16), a 10 AWG conductor has an ampacity of 30A. Therefore, the minimum size tap conductors required for this installation are 10 AWG conductors (see Figure 3).
For electricians studying for an electrical exam, knowing where to find these tap rules and understanding them is critical. Questions pertaining to the tap rules are on most master electrician exams and on quite a few journeyman electrician exams.
Next month’s column continues the discussion of sizing conductors.