Ambient temperature and the number of current-carrying conductors installed in a raceway or cable are two factors to consider when sizing conductors. In accordance with 310.15(A)(3) in the National Electrical Code (NEC), no conductor shall be used in such a manner that its operating temperature exceeds the limit designated for the type of insulated conductor involved.

In no case shall conductors be associated together in such a way, with respect to type of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of any conductor is exceeded. The allowable or maximum ampacities listed in Table 310.15(B)(16) are based on an ambient temperature of 30°C. When the ambient temperature is something other than 30°C, the Table 310.15(B)(16) ampacities shall be corrected.

This table is also based on a maximum of three current-carrying conductors installed in a raceway or cable. Adjacent load-carrying conductors affect operating temperature in two ways: the ambient temperature can be raised, and heat dissipation can be impeded. When there are four or more current-carrying conductors in a raceway, cable or earth (directly buried), the table ampacities must be adjusted. This is often referred to as derating.

Previously, I have covered ambient temperature and the number of current-carrying conductors separately, but when the number of current-carrying conductors (in a raceway, cable or directly buried in the earth) exceeds three and the ambient temperature is something other than 30°C, it is necessary to apply both the ambient temperature correction factor and the adjustment factor for current--carrying conductors. This is often referred to as double derating. For example, what is the maximum ampacity for a 2 AWG THW copper conductor in a raceway under the following conditions? There will be a total of six current-carrying conductors in this raceway. There also will be an equipment grounding conductor in this raceway. The load is noncontinuous. The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 41°C.

Start by finding the conductor and the listed ampacity in Table 310.15(B)(16). The allowable ampacity for a 2 AWG THW copper conductor is 115 amperes (A). Because the ambient temperature is other than 30°C, the table ampacity must be multiplied by the appropriate ambient temperature correction factor. Type THW conductor has a temperature rating of 75°C; therefore, look for the correction factor in the 75°C column of Table 310.15(B)(2)(a). Follow down the 75°C column, and find the row that intersects with the maximum ambient temperature. The correction factor, in the 75°C column, for an ambient temperature of 41°C is 0.82.

Next multiply 115A by 0.82 (115 x 0.82 = 94.3). After derating because of ambient temperature, this conductor has a maximum ampacity of 94.3A. Because there will be more than three current-carrying conductors in this raceway, it is also necessary to apply the adjustment factor in Table 310.15(B)(3)(a). Although this raceway contains seven conductors, there are only six current-carrying conductors because grounding and bonding conductors do not count as current-carrying. The adjustment factor for six current-carrying conductors is 80 percent. Now multiply 94.3A by 0.80 (94.3 x 0.80 = 75.44 = 75). Because of the ambient temperature and the number of current-carrying conductors in the raceway, the maximum ampacity for the 2 AWG THW copper conductor in this example is 75A (see Figure 1).

If the conductor’s ampacity is not equal to or greater than the load after applying correction and/or adjustment factors, a different installation will be required. For example, the six conductors in Figure 1 will supply two three-phase, 3-wire loads that will draw 92A each. After applying correction and adjustment factors, the maximum ampacity for the 2 AWG THW copper conductor is only 75A. Since 75A is less than a 92A load, a different installation will be required. One option is to divide the conductors into two separate raceways. With only three current-carrying conductors and an equipment grounding conductor in each raceway, no Table 310.15(B)(3)(a) adjustment factor is required. Because the ambient temperature will still be 41°C, apply the ambient temperature correction factor (115 x 0.82 = 94.3 = 94). After derating because of ambient temperature only, the 2 AWG THW copper conductor has a maximum ampacity of 94A, which is enough to carry the 92A load (see Figure 2). Another option would be to increase the size of the conductors.

As previously discussed in this series, temperature ratings of conductors must be considered. The temperature rating associated with the ampacity of a conductor shall not exceed the lowest temperature rating of any connected termination, conductor or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both [110.14(C)]. If it is necessary to apply correction and/or adjustment factors, it is permissible to use the conductor’s ampacity straight out of the table, even if the ampacity is higher than what is allowed for the terminations.

For example, what is the maximum ampacity for a 6 AWG THHN copper conductor in a raceway under the following conditions? There will be a total of eight current-carrying conductors in this raceway. There also will be an equipment grounding conductor in this raceway. The load is noncontinuous. The terminations on both ends are rated at least 75°C. The maximum ambient temperature for these conductors will be 40°C. The temperature rating for a THHN conductor is 90°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/or adjustment calculations. The allowable ampacity for a 6 AWG THHN conductor, in the 90°C column, is 75A. The Table 310.15(B)(2)(a) correction factor, also in the 90°C column, for an ambient temperature of 40°C is 0.91. The maximum ampacity after applying the correction factor is 68.25A (75 0.91 = 68.25).

Because there will be more than three current-carrying conductors in this raceway, it also is necessary to apply the adjustment factor in Table 310.15(B)(3)(a). The Table 310.15(B)(3)(a) adjustment factor for eight current-carrying conductors is 70 percent. The maximum ampacity after applying the adjustment factor is 48A (68.25 x  0.70 = 47.775 = 48). In accordance with 110.14(C)(1)(a)(3), the ampacity for this conductor shall not exceed the ampacity listed in the 75°C column, which is 65A. Although the temperature limitations of the terminals will allow for an ampacity of 65A, the correction factor and adjustment factors will not. Because of the eight current-carrying conductors and the ambient temperature of 38°C, the maximum ampacity for the 6 AWG THHN conductor in this example is 48A (see Figure 3).

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]. The conductors in Figure 3 started with an allowable (or maximum) ampacity of 75A. But, after applying correction and adjustment factors, the maximum dropped to 48A. The standard ampere ratings in 240.6(A) do not list 48 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 48 is 50A. The maximum size fuse or breaker permitted to protect the conductors in Figure 3 is 50A.
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.