There is more to sizing a conductor than just selecting a conductor from Table 310.15(B)(16) (formerly Table 310.16) in the National Electrical Code (NEC). Table 310.15(B)(16) is referenced in Section 310.15(B), which states that ampacities for conductors rated 0 to 2,000 volts (V) shall be specified in the Allowable Ampacity Table 310.15(B)(16) through Table 310.15(B)(19) and Ampacity Table 310.15(B)(20) and Table 310.15(B)(21). Section 310.15(B) continues by stating the ampacities might need to be modified because of provisions in 310.15(B)(1) through (B)(7). To select the appropriate conductor, a number of factors must be considered, and some of them are in 310.15(B). They include the temperature ratings of terminations, conductors and devices; the connected load or the load calculated in accordance with Article 220; continuous and noncontinuous loads; ambient temperature; the number of current-carrying conductors; and the overcurrent device rating. The latter must be coordinated with the conductor it protects. 


As a general rule, a conductor must be protected at its ampacity, but there are some alternative rules in 240.4(A) through (G). One of the alternative rules pertains to tap conductors, of which 240.4(E)(1) through (E)(6) mentions several types. 


Rules covering the location and sizing requirements of overcurrent protection for tap conductors are in 240.21. Most of the rules in this section pertain to feeder taps and transformer secondary conductors. The rule in 240.21(B)(5) is the last feeder tap rule, and it covers outside feeder taps of unlimited length. The fifth feeder tap rule shall be permitted where the conductors are located outdoors of a building or structure, except at the point of load termination. Compliance with four conditions is required when using this tap rule. The first condition states that the tap shall be protected from physical damage in an approved manner [240.21(B)(5)(1)]. This is similar to a condition in each of the other feeder tap rules, but the other tap rules state that the tap conductors shall be protected from physical damage by being enclosed in an approved raceway or by other approved means. The first condition for outside feeder taps simply says to protect the conductors in an approved manner. Besides enclosing the conductors in a raceway, conductors could be protected from physical damage where installed as overhead conductors. See 225.18 and 225.19 for clearance requirements for overhead feeder conductors.


Other feeder tap rules share the second condition. 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. Remember, in accordance with 240.21(B), rounding up to the next standard overcurrent device rating is not permitted when using the tap rules. For example, outdoor feeder tap conductors will terminate at a single set of fuses in a fused disconnect. The fuses in the disconnect will have a rating of 125 amperes (A). The load on these outside feeder tap conductors will be 103A, noncontinuous. The feeder tap conductors will be THWN-2 copper conductors and all terminations will be rated 75°C. If all the conditions in 240.21(B)(5) will be met, what is the minimum size tap conductor required to supply the disconnect? If these conductors were feeder conductors and not feeder tap conductors and the rating of the overcurrent device protecting the feeder was 125A, size 2 AWG conductors could be installed. In accordance with the 75°C column of Table 310.15(B)(16), a 2 AWG conductor has an ampacity of 115A, which is more than the load in this example. When the round-up rule in 240.21(B) is permitted, an overcurrent device rated 125A can be installed to protect a 2 AWG copper conductor. But, the conductors in this installation are tap conductors, and using the round-up rule is not permitted. Since the overcurrent device must limit the load to the ampacity of the tap conductors, the conductors must be rated at least 125A. In accordance with the 75°C column of Table 310.15(B)(16), a 1 AWG conductor has an ampacity of 130A (see Figure 1).


The second condition also states that the single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side. Tap conductors that terminate on the main breaker in a panelboard with breakers on the load side are an example of an overcurrent device supplying additional overcurrent devices. The third condition states that the overcurrent device for the tap conductors must be an integral part of the disconnecting means or shall be located immediately adjacent thereto. This condition is similar to a provision in 230.91 that pertains to a service disconnecting means.


The fourth condition states that the disconnecting means for the tap conductors must be installed at a readily accessible location and must comply with either 240.21(B)(5)(4)(a), (4)(b) or (4)(c). In accordance with (4)(a), the disconnect can be installed on the outside of a building or structure. In accordance with (4)(b), the disconnect can be installed on the inside of a building or structure as long as the disconnect is installed nearest the point of entrance of the tap conductors. The last condition in this section states that the disconnecting means can be installed in accordance with 230.6, nearest the point of entrance of the tap conductors. Section 230.6 provides descriptions of what shall be considered outside of a building or structure. The disconnect location requirements for outside feeder tap conductors and for outside secondary conductors are the same as those for services in 230.70(A)(1) and those for outside branch circuits and feeders in 225.32.


In all four tap rules listed before the outside tap rule, length is a factor. With outside taps, length is not a factor. This tap rule states in the title that the length is unlimited. With these outside tap rules, there is no minimum conductor size other than having an ampere rating equal to or greater than the rating of the overcurrent device. For example, by using the outside tap rule, what are the smallest tap conductors that can be installed to supply an outdoor fused disconnect with fuses rated 100A? The tap conductors will be connected to 500 kcmil feeder conductors that are protected by a 400A circuit breaker. All of the conductors will be protected from physical damage in an approved manner. The disconnecting means will be installed in a readily accessible location on the outside of a building. The load on these tap conductors will be 75A, continuous. The feeder tap conductors will be THWN-2 copper conductors, and all terminations will be rated 75°C. The feeder tap conductors will be 50 feet long. Since all the conditions in 240.21(B)(5) will be met, simply select a conductor with an ampere rating equal to or greater than the rating of the fuses in the disconnect. Since the fuses will be rated 100A, the tap conductors must be rated at least 100A. In accordance with the 75°C column of Table 310.15(B)(16), a 3 AWG conductor has an ampacity of 100A 
(see Figure 2).


Except for the title, the tap rule in 240.21(C)(4) is exactly the same as the fifth feeder tap rule. The rule in 240.21(B)(5) covers outside feeder taps of unlimited length, and the rule in 240.21(C)(4) covers outside transformer secondary conductors. Like the length of the outside feeder tap conductors, the length of the outside secondary conductors is not a factor because there is no mention of length. All of the conditions in both of these tap rules are matching. The conductors must be protected from physical damage in an approved manner. The conductors must terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. The overcurrent device for the conductors must be an integral part of the disconnecting means or shall be located immediately adjacent thereto (see Figure 3). The disconnecting means for the conductors must be installed at a readily accessible location and must be compliant with one of the following requirements:


• Outside of a building or structure


• Inside as long as the disconnect is nearest the point of entrance of the transformer secondary conductors

• In accordance with 230.6, nearest the point of entrance of the transformer secondary conductors

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