When sizing conductors, the number of current-carrying conductors installed in a raceway or cable must be considered. No conductor shall be used in such a manner that its operating temperature exceeds that which is designated for the type of insulated conductor involved [310.15(A)(3)].
Adjacent load-carrying conductors affect operating temperature in two ways: the ambient temperature can be raised, and heat dissipation can be impeded. Table 310.15(B)(16) in the National Electrical Code (NEC) provides allowable (or maximum) ampacities for insulated conductors, rated up to and including 2,000 volts (V), when there are three or fewer current-carrying conductors. When there are four or more current-carrying conductors in a raceway, cable or earth (directly buried), the table ampacities must be adjusted. This process is often referred to a derating.
Last month’s column concluded by covering requirements for load-carrying conductors as they pertain to cables (armored cable, metal-clad cable and nonmetallic-sheathed cable). This month, the discussion continues with adjustment factors for more than three current-carrying conductors in a raceway, cable or earth (directly buried).
When there are more than three current-carrying conductors in a raceway or cable, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). This table underwent some changes in the 2011 edition of the NEC, one of which was renumbering. Because the ambient temperature tables are now 310.15(B)(2)(a) and (b), the table containing adjustment factors for more than three current-carrying conductors had to be renumbered to Table 310.15(B)(3)(a). Another change was to the left column heading. In the 2008 edition, as well as all editions since the 1993 edition, the heading was “Number of Current-Carrying Conductors.” Now, it is just “Number of Conductors.” A footnote also was added to this table. The footnote states that the number of conductors is the total number of conductors in the raceway or cable adjusted in accordance with 310.15(B)(5) and (6). Because of this change, it is now necessary to count spare conductors. Conductors are sometimes installed for future expansion. For example, a raceway containing six current-carrying conductors and three spare conductors has been installed. What is the adjustment factor for the conductors in this installation? Although the spare conductors are not “current-carrying” at the time of installation, they could be at a later date. Therefore, the three spare conductors must be counted as current-carrying conductors. Because the total number of current-carrying conductors in this example is nine (6 + 3 = 9), the adjustment factor is 70 percent (see Figure 1).
It may not be necessary to count all conductors installed in a raceway as current-carrying conductors. Under normal conditions, grounding and bonding conductors do not carry current. In accordance with 310.15(B)(6), it is not required to count grounding and bonding conductors when applying the provisions of 310.15(B)(3)(a). For example, a raceway containing seven conductors has been installed. Six of the conductors are current-carrying, and one is an equipment-grounding conductor. What is the adjustment factor for the conductors in this installation? Although there are seven conductors in this raceway, it is not required to count the grounding conductor as a current-carrying conductor. The footnote under Table 310.15(B)(3)(a) states that the number of conductors is the total number of conductors in the raceway or cable adjusted in accordance with 310.15(B)(5) and (6). Since 310.15(B)(6) states that it is not required to count the grounding conductor, there are six current-carrying conductors in the raceway in this example. The adjustment factor for six conductors is 80 percent (see Figure 2).
Neutral conductors may or may not be required to count as current-carrying conductors. Rules for neutral conductors are in 310.15(B)(5). Neutral conductor provisions are divided into three groups. A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a) [310.15(B)(5)(a)]. For example, a multiwire branch circuit consisting of five conductors has been installed in a raceway. Three of the conductors are ungrounded (hot), one conductor is a neutral, and one conductor is an equipment ground. The multiwire branch circuit supplies power to incandescent lighting. The power system is a three-phase, 4-wire wye-connected system, and the voltage is 208/120V. Each phase or leg of the multiwire branch draws 12 amperes (A) at 120V. What is the adjustment factor for the conductors in this example? In this installation, the circuit is balanced, and the neutral conductor carries only the unbalanced current. When all loads are energized, the current on the neutral conductor will be zero amperes. Therefore, it is not required to count the neutral as a current-carrying conductor. Since the equipment ground does not count, there are only three current-carrying conductors. There is no adjustment factor for three conductors because Table 310.15(B)(16) is based on three or fewer current-carrying conductors (see Figure 3).
In the last example, as long as all three circuits are energized, there will not be any current on the neutral conductor, but what if one of the circuits is de-energized or turned off? If one circuit is de-energized in Figure 3’s example, the neutral conductor will become a current-carrying conductor. With one circuit de-energized, there are still only three current-carrying conductors, and there is no adjustment factor for three current-carrying conductors.
Section 310.15(B)(5)(a) also applies to single-phase systems. For example, two 12-2 and two 12-3 with ground nonmetallic-sheathed cables are installed, without maintaining spacing between the cables, through the same opening in wood framing. The opening in the wood framing has been sealed with caulk. The power system is a 120/240V, single--phase, 3-wire system. The two 12-3 with ground cables are supplying multiwire branch circuits. The neutral conductor in each multiwire branch circuit carries only the unbalanced current from the other conductors of the same circuit. What is the maximum ampacity for each conductor? Not counting grounding conductors, there are 10 conductors (six ungrounded, two grounded, and two neutral conductors). Because the neutral conductor (in each multiwire branch circuit) carries only the unbalanced current, it is not required to count the neutrals as current-carrying conductors. Since the grounding conductors do not count, there are eight current-carrying conductors (six ungrounded and two grounded conductors). The allowable ampacity for a 12 AWG conductor, in the 90°C column, is 30A. Because the four cables containing eight current-carrying conductors are installed in the same hole and the hole has been caulked, it is necessary to apply an adjustment factor from Table 310.15(B)(3)(a) [334.80]. The adjustment factor for eight current-carrying conductors is 70 percent. Now multiply 30A by 0.70 (30 0.70 = 21). The ampacity after applying the adjustment factor is 21A. But in accordance with 334.80, the final derated ampacity shall not exceed that of a 60°C rated conductor. The allowable ampacity for a 12 AWG conductor, in the 60°C column, is 20A. Therefore, the maximum ampacity for the 12 AWG cables in this example is 20A (see Figure 4).
Next month’s column continues the discussion of sizing conductors.
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, charles@charlesRmiller.com and www.charlesRmiller.com.