When sizing conductors, the rating of the overcurrent device must also be considered. In accordance with 240.4 in the National Electrical Code (NEC), 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). Although this main rule states that conductors must be protected in accordance with their ampacities, there are some alternative provisions. An example would be for overcurrent devices rated 800 amperes (A) or less. This rule, in 240.4(B), is often referred to as the “round-up” rule.

Without alternative provisions, the rating of the overcurrent protective device could not be above the ampere rating of the conductor. For example, a 4 AWG copper conductor, listed in the 75°C column of Table 310.15(B)(16), shows an allowable ampacity of 85A, which is not a standard ampere rating. Standard ampere ratings for fuses and inverse-time circuit breakers are listed in 240.6(A). Without alternative provisions, such as the round-up rule, the maximum rating for the overcurrent device protecting this conductor would be 80A.

When sizing conductors, certain factors, such as the number of current--carrying conductors, ambient temperature, terminations and continuous loads, must be considered. The overcurrent device must also be considered when sizing conductors. For example, what size THHN copper conductors are required to supply a branch circuit under the following conditions? The load will be a 39A, nonmotor, continuous load. These branch-circuit conductors will be in a raceway. There will be a total of six current-carrying conductors and an equipment grounding conductor in this raceway. The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 40°C. In accordance with 210.19(A)(1), the branch-circuit conductor shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. Since the entire load is continuous, multiply the entire load by 125 percent. The minimum ampacity after multiplying by 125 percent is 49A (39  125% = 48.75 = 49). Although the conductors are rated 90°C, the allowable ampacity shall not exceed the 75°C column because of the terminations [see 110.14(C)(1)(a)]. An 8 AWG copper conductor, in the 75°C column, has an allowable ampacity of 50A. Based only on the temperature ratings of the terminations and on the load being continuous, the minimum size conductors are 8 AWG copper conductors
(see Figure 1).

Because the ambient temperature will be higher than 30°C and there will be more than three current-carrying conductors in the raceway, correction and adjustment factors must be applied to the conductors. Since 8 AWG conductors were selected to satisfy the requirements for continuous loads and for the terminations, check if the ampacity of these conductors will equal or exceed the load after applying correction and adjustment factors. The temperature rating for a THHN conductor is 90°C. In accordance with 110.14(C), conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both. The Table 310.15(B)(16) ampacity for an 8 AWG THHN conductor is 55A. The maximum ambient temperature in this example is 40°C. Although the terminations limit the ampacity to the 75°C column, it is permissible to use the ampacity in the 90°C column for correction and adjustment. Therefore, the Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 40°C is 0.91. The Table 310.15(B)(3)(a) adjustment factor for six current-carrying conductors in the raceway is 80 percent (or 0.80). After derating because of ambient temperature and adjacent current-carrying conductors, 8 AWG THHN conductors have a maximum ampacity of 40A (55  0.91  0.80 = 40.04 = 40). After derating, these conductors have an ampacity greater than the load. Based only on the ambient temperature and the number of current-carrying conductors, the minimum size conductors are 8 AWG THHN copper conductors (see Figure 2).

The conductor selected shall be protected in accordance with 240.4 [210.20(B)]. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load [210.20(C)]. (Note, there is an exception for overcurrent devices that are listed for operation at 100 percent of their rating.) Because the entire load in this example is continuous, multiply the entire load by 125 percent. The overcurrent device must have a rating of at least 49A (39  125% = 48.75 = 49). In accordance with 240.6(A), the next standard ampere rating above 49 is 50A. Therefore, the minimum size fuse or circuit breaker protecting the conductors in this example is 50A. After derating because of ambient temperature and the number of current-carrying conductors, the 8 AWG copper conductors in Figure 2 have an ampacity of only 40A. Since the overcurrent device must be rated at least 50A, the 8 AWG THHN conductors (that were previously determined to be adequate for this installation) will not be permitted (see Figure 3).

Because the overcurrent device shall not be less than 50A and the 8 AWG conductors from Figure 2 shall be protected at 40A or less, it is necessary to select a larger conductor. After selecting the next larger size conductor, apply the adjustment and correction factors to verify that the conductor can be protected by a 50A overcurrent device. The next larger size conductor is 6 AWG THHN, and it has an ampacity of 75A. The Table 310.15(B)(2)(a) correction factor, in the 90°C column, for an ambient temperature of 40°C is 0.91. The Table 310.15(B)(3)(a) adjustment factor for six current-carrying conductors in the raceway is 80 percent (or 0.80). After applying the adjustment and correction factors, 6 AWG THHN conductors have an ampacity of 55A (75  0.91  0.80 = 54.6 = 55). Size 6 AWG copper conductors are required to supply the branch circuit in Figure 1. In this example, the rating of the overcurrent device was a determining factor in sizing these branch-circuit conductors (see Figure 4).

The minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load [210.19(A)(1)]. After correcting/adjusting the ampacity because of ambient temperature and/or the number of current-carrying conductors, the conductors must have an allowable ampacity that equals or exceeds the actual load, not 125 percent of the load. For example, what size THHN copper conductors are required to supply a branch circuit under the following conditions? The load will be a 39A, nonmotor, continuous load. These branch-circuit conductors will be in a raceway. There will be a total of eight current-carrying conductors and an equipment grounding conductor in this raceway. The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 40°C. This example is identical to the example in Figure 1, except there are eight current-carrying conductors instead of six. Like the first example, the overcurrent device in this example must also have a rating of at least 50A because the load is continuous (39  125% = 48.75 = 49). The ambient temperature correction factor of 0.91 is the same. The Table 310.15(B)(3)(a) adjustment factor for eight current-carrying conductors in the raceway is 70 percent (or 0.70). This adjustment factor is more than the adjustment factor in the first example. Therefore, apply the correction and adjustment factors to the 6 AWG THHN conductors to see if they would be permitted. After applying the correction and adjustment factors, 6 AWG THHN conductors have an ampacity of 48A (75  0.91  0.70 = 47.775 = 48). Although the continuous load is 49A, the conductors are only required to have a rating of the actual load of 39A. Even with eight current-carrying conductors in the raceway, 6 AWG THHN copper conductors are permitted to supply this branch circuit (see Figure 5).

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.