As stated in 90.1(A) of the National Electrical Code (NEC), the purpose of the Code is the practical safeguarding of people and property from hazards arising from the use of electricity. Because improperly sized conductors can create electrical hazards, it is crucial to understand how to select the correct size conductor.

First, conductors (branch-circuit, feeder and service) must be sized to carry the load. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served [210.19(A)(1)].

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 [215.2(A)(1)]. Loads for service-entrance conductors shall be determined in accordance with Part III, IV or V of Article 220, as applicable [230.42(A)]. Once the load is determined, other things must be considered, such as if the load, or any part of the load, is continuous. Will the installation comply with the temperature limitation provisions in 110.14(C)? Is it necessary to reduce the conductor’s ampacity because of adjacent current-carrying conductors or ambient temperature? Is the small-conductor rule in 240.4(D) applicable? Is it permissible to round up to the next standard size overcurrent device? Will these conductors be feeder taps or transformer secondary conductors?

Since I have already discussed some of these considerations in this series on sizing conductors, I will discuss other considerations in this and upcoming issues.

The last sentence in the first paragraph of 310.15(B)(3)(a) mentions paralleled conductors, which are electrically joined at both ends. Normally, paralleled conductors are installed in separate raceways. If the paralleled conductors are installed in separate raceways and there are three or fewer current-carrying conductors in the raceway, it will not be necessary to reduce the conductor’s ampacity because of Table 310.15(B)(3)(a). For example, a paralleled set of 1/0 AWG THW conductors will be installed to supply a 208/120-volt (V), three-phase, 4-wire panelboard. The paralleled conductors will be installed in separate raceways. Each raceway will contain three ungrounded (hot) conductors, one grounded (neutral) conductor and one equipment grounding conductor. The major portion of this load does not consist of nonlinear loads. The terminations on both ends are rated at least 75°C. The ambient temperature will not be higher than 30°C. This load is not continuous.

What is the maximum ampacity per phase for these paralleled conductors? Since the ambient temperature in this example is 30°C and Table 310.15(B)(16) is based on 30°C, applying an ambient temperature correction factor is not necessary. In accordance with 310.15(B)(5)(a), it is not necessary to count the neutral as a current-carrying conductor. Because the neutral conductor is not counted, there are only three current-carrying conductors in each raceway. There is no need to apply a Table 310.15(B)(3)(a) adjustment factor because Table 310.15(B)(16) is based on not more than three current-carrying conductors. The allowable ampacity, from Table 310.15(B)(16), for a 1/0 AWG THW conductor is 150 amperes (A). To find the total ampacity of paralleled conductors, multiply the allowable ampacity of one conductor by the number of conductors in the paralleled set. Because this installation has two conductors in parallel, multiply the conductor’s ampacity by two (150 2 = 300). The maximum ampacity per phase for these paralleled conductors is 300A (see Figure 1).

Unless otherwise permitted or required in 240.4(A) through (G), conductors shall be protected against overcurrent in accordance with their ampacities specified in 310.15 [240.4]. (For requirements that pertain to flexible cords, flexible cables and fixture wires, see 240.4.) The conductors in Figure 1 have a maximum ampacity of 300A. The standard ampere ratings for fuses and inverse-time circuit breakers are listed in 240.6(A). Because 300 is a standard ampere rating, the paralleled conductors in Figure 1 must be protected by a 300A overcurrent device (see Figure 2).

If paralleled conductors are installed in the same raceway and there are four or more current-carrying conductors in the raceway, it will be necessary to reduce the conductor’s ampacity by an adjustment factor in Table 310.15(B)(3)(a). Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor [310.15(B)(3)(a)]. For example, a paralleled set of 1/0 AWG THW conductors will be installed to supply a 208/120V, three-phase, 4-wire panelboard. All of the paralleled conductors will be installed in the same raceway. The raceway will contain six ungrounded (hot) conductors, two grounded (neutral) conductors and one equipment grounding conductor. The major portion of this load does not consist of nonlinear loads. The terminations on both ends are rated at least 75°C. The ambient temperature will not be higher than 30°C. This load is not continuous.

What is the maximum ampacity per phase for these paralleled conductors that are installed in the same raceway? Since the ambient temperature in this example is 30°C and Table 310.15(B)(16) is based on 30°C, applying an ambient temperature correction factor is not necessary. In accordance with 310.15(B)(5)(a), it is not necessary to count the neutral conductors as current-carrying conductors. Because the paralleled conductors will be installed in the same raceway, there will be six current--carrying conductors. The Table 310.15(B)(3)(a) adjustment factor for six current-carrying conductors is 80 percent. The allowable ampacity, from Table 310.15(B)(16), for a 1/0 AWG THW conductor is 150A. The maximum ampacity (per conductor) after applying the adjustment factor is 120A (150 0.80 = 120). Because the conductors will be paralleled, multiply the ampacity by two (120 2 = 240). Because these paralleled conductors will be installed in the same raceway, the maximum ampacity per phase is 240A (see Figure 3).

The maximum ampacity of the conductors in Figure 3 was reduced because of the number of current-carrying conductors in the raceway. Therefore, the maximum ampacity per phase is now only 240A. The standard ampere ratings in 240.6(A) do not list 240 as a standard size. In accordance with 240.4(B), it is permissible to round up to the next standard overcurrent device. The next standard size above 240 is 250A. The maximum size fuse or breaker permitted to protect the conductors in Figure 3 is 250A (see Figure 4).

In Figure 4, if a 300A feeder is needed instead of a 250A feeder, a different installation will be required. Some of the different installations that would permit a 300A feeder include installing the paralleled conductors in separate raceways (like the installation in Figure 1), installing 90°C conductors instead of 75°C conductors, or increasing the size from 1/0 AWG to 2/0 AWG.

Conductors for each phase, polarity, neutral or grounded circuit can be connected in parallel but only in sizes 1/0 AWG and larger. (Note: see 310.10(H) for two exceptions for smaller conductors permitted to be run in parallel.) Paralleled conductors also shall be installed in accordance with the requirements in 310.10(H)(2) through (6). The paralleled conductors in each phase, polarity, neutral, grounded-circuit conductor, equipment grounding conductor or equipment bonding jumper shall be the same length, consist of the same conductor material, be the same size in circular mil area, have the same insulation type and be terminated in the same manner [310.10(H)(2)]. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and the same electrical characteristics [310.10(H)(3)]. This section also states that conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor. Conductors installed in parallel shall comply with the provisions of 310.15(B)(3)(a) [310.10(H)(4)]. The last two sections in 310.10(H) pertain to equipment grounding conductors and equipment bonding jumpers [310.10(H)(5) and (H)(6)].

Next month’s column will continue 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.