Section 240.21 in the National Electrical Code (NEC) contains feeder tap rules. As defined in 240.2, a tap conductor has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4. In other words, a tap conductor has overcurrent protection somewhere ahead of it, but the rating of the overcurrent protection (typically a fuse or breaker) is higher than the rating that is normally required or permitted.
As a general rule (stated in 240.4), conductors must be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). Without the alternative provisions in (A) through (G) of this section, it would always be necessary to select an overcurrent device with an ampere rating that is less than the ampacity of the conductor. The ampacity of the conductor would not just be the ampacity as shown in Table 310.15(B)(16). It would be the ampacity after considering all factors, such as correction and adjustment factors. However, selecting an overcurrent device with an ampere rating less than the ampacity of the conductor is not always necessary because of the alternative provisions in 240.4(A) through (G). See 240.4 for a complete list of provisions that pertain to conductor protection.
One alternative provision in this section pertains to tap conductors. Tap conductors can be protected against overcurrent in accordance with the provisions in 240.4(E)(1) through (E)(6). Rules covering the location and sizing requirements of overcurrent protection for tap conductors are in 240.21. The majority of this section contains requirements for feeder taps and transformer secondary conductors. The first tap rule pertains to feeder tap conductors having a length not exceeding 10 feet. The second tap rule pertains to feeder tap conductors having a length not exceeding 25 feet. The third tap rule also has a maximum length of 25 feet, but this tap rule pertains to transformer primary plus secondary conductors. Previous Code in Focus articles covered these three tap rules.
The fourth tap rule pertains to feeder tap conductors having a length more than 25 feet but not more than 100 feet. Similar to other tap rules, there are specified conditions that must be met before using this tap rule. Unlike other tap rules, however, this tap rule is often overlooked because it only applies to a specific type of occupancy: high-bay manufacturing buildings. As stated in the first sentence of 240.21(B)(4), the height at the walls must be more than 35 feet. In other words, if the height at the walls of a high-bay manufacturing building is 35 feet or less, installing feeder taps in accordance with this provision shall not be permitted.
Besides this occupancy and height condition, there are nine additional conditions, which is more than any other tap rule. To be able to use this tap rule, the conditions of maintenance and supervision must guarantee that only qualified people service the systems. The second condition contains two requirements for length. The length of the tap conductors horizontally shall not be more than 25 feet, and the total length shall not exceed 100 feet. This requirement for horizontal length is not just for a straight run; it includes the total horizontal length such as 90-degree bends. This condition also specifies that the maximum length for the tap conductors is 100 feet (see Figure 1).
The third condition states that the ampacity of the tap conductors shall not be less than one-third the rating of the overcurrent device protecting the feeder conductors. This condition is the same as the first condition for the 25-foot tap rule in 240.21(B)(2). To find the minimum ampacity for the tap conductors when using this tap rule, simply divide the rating of the overcurrent device protecting the feeder conductors by three.
For example, by using the tap rule in 240.21(B)(4), 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? (For this example, all the conditions in 240.21(B)(4) have been met.) 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).
The fourth condition is found in all the feeder tap rules. The tap conductors must terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. It is important to remember that, when using the tap rules, rounding up to the next standard overcurrent device rating is not permitted [240.21(B)].
For example, if 1 AWG copper conductors are installed as tap conductors, the maximum size overcurrent device would be 125A. In accordance with the 75°C column of Table 310.15(B)(16), a 1 AWG conductor has an ampacity of 130A. When the round-up rule in 240.21(B) is permitted, a 1 AWG copper conductor could be protected by an overcurrent device rated 150A. Since the overcurrent device must limit the load to the ampacity of the tap conductors, an overcurrent device rated 150A would not be permitted. The fourth condition also states that the single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
The fifth condition states that the tap conductors must be protected from physical damage by being enclosed in an approved raceway or by other approved means. The last four conditions are not like any of the conditions in the 10-foot or the 25-foot tap rules. In accordance with 240.21(B)(4)(6), the tap conductors must be continuous from end to end and contain no splices. Connections or taps to feeder conductors at the supply side of the tap conductors are permitted, but splices anywhere else in the tap conductors are not permitted. The next condition provides a requirement for the conductor size. The tap conductors must be sized 6 AWG copper or 4 AWG aluminum or larger.
Regardless of the load (when using this tap rule for high-bay manufacturing buildings), the smallest conductors that can be installed are 6 AWG copper or 4 AWG aluminum. For example, a tap will be installed in accordance with 240.21(B)(4), and the tap will terminate in a 30A disconnect (safety switch). Although smaller conductors could carry 30A, 6 AWG copper or 4 AWG aluminum conductors would be required.
The eighth condition states that the tap conductors shall not penetrate walls, floors or ceilings. This condition does not apply to the feeder conductors supplying the tap conductors. They are permitted to penetrate walls, floors and ceilings.
The ninth condition is important because it contains a specification for where the tap is made. The tap shall be made no less than 30 feet from the floor. While most taps are made within 10 feet of the floor, this condition requires the tap to be made at least 30 feet from the floor (see Figure 3).
Next month’s article continues the discussion of sizing conductors.