Allowable ampacities for insulated conductors rated up to and including 2,000 volts (V) are in Table 310.15(B)(16). This table underwent a number change in the 2011 edition of the National Electrical Code (NEC). Before, this table was numbered 310.16. While this was not a change in electrical installations, it was significant because this ampacity table has been 310.16 (or 310-16) since the 1975 edition of the NEC.

More is involved than just selecting a conductor based on the ampacity shown in the table. The allowable ampacities are based on no more than three current--carrying conductors in a raceway, cable or earth (directly buried). The ampacities also are based on an ambient temperature of 30°C. When there are more than three current-carrying conductors and/or the ambient temperature is other than 30°C, the table ampacities must be adjusted and/or corrected.

Last month’s column concluded by covering ambient temperature correction factors. This month, the discussion continues with adjustment factors for more than three current-carrying conductors in a raceway, cable or earth (directly buried).

In accordance with 310.15(A)(3), no conductor shall be used in such a manner that its operating temperature exceeds the temperature for which the type of insulated conductor involved is designated. The first informational note (formerly fine print note) under 310.15(A)(3) provides some helpful information about temperature ratings of conductors and the principal determinates of operating temperature.

Ambient temperature is one factor that can raise the operating temperature. Heat generated internally in the conductors because of load current flow, including fundamental and harmonic currents, can also raise operating temperature. If the heat generated in the conductor is not able to dissipate into the ambient medium, the heat could grow to a level above the operating temperature. Thermal insulation that covers or surrounds conductors is one factor that can affect the rate of heat dissipation. The fourth principal determinate of operating temperature is adjacent load-carrying (current-carrying) conductors. Raising the ambient temperature is not the only problem with adjacent current-carrying conductors in a raceway or cable. Since heat is increasing because of adjacent current-carrying conductors, heat dissipation is hindered.

A Code regulation is in place to counteract an increase in operating temperature due to adjacent current-carrying conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)-(a). When there are more than three current-carrying conductors, the maximum conductor ampacity must be reduced or lowered. 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 terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 30°C. 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. 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 there will be more than three current-carrying conductors in this raceway, it is 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 conductors. In accordance with 310.15(B)(6), a grounding or bonding conductor shall not be counted when applying the provisions of 310.15(B)(3)(a). The adjustment factor for six current-carrying conductors is 80 percent. Now multiply 115A by 0.80 (115 0.80 = 92). The maximum ampacity for the 2 AWG THW copper conductor in this example is 92A (see Figure 1).

With an increase in the number of current-carrying conductors comes an increase in operating temperature, which hinders heat dissipation even more. As a result, the adjustment factor percentage decreases as the number of current-carrying conductors increases. As the adjustment factor percentage decreases, so does the maximum ampacity allowed for a conductor. The adjustment factor for four, five or six current-carrying conductors is 80 percent. The adjustment factor for seven, eight or nine current-carrying conductors is 70 percent. For example, using all the information and conditions in Figure 1 except changing the number of current-carrying conductors, what is the maximum ampacity for the 2 AWG THW copper conductor if the total number of current-carrying conductors will be eight? In accordance with Table 310.15(B)(16), the allowable ampacity for a 2 AWG THW copper conductor is 115A. The adjustment factor for eight current-carrying conductors is 70 percent. Multiply 115A by 0.70 (115 0.70 = 80.5 = 81). Before, with six current-carrying conductors, the maximum ampacity for this 2 AWG conductor was 92A. Now, with eight current--carrying conductors, the maximum ampacity for this 2 AWG THW copper conductor is 81A (see Figure 2).

As I previously discussed in this series, temperature ratings of conductors must be considered. In accordance with 110.14(C), the temperature rating associated with the ampacity of a conductor shall not exceed the lowest temperature rating of any connected termination, conductor or device. The next sentence is key when applying temperature correction and/or adjustment factors. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both [110.14(C)]. This passage means that 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 six current--carrying conductors in this raceway. There also will be an equipment-grounding conductor in this raceway. The terminations on both ends are rated at least 75°C. The maximum ambient temperature for these conductors will be 30°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 is 75A. The Table 310.15(B)(3)(a) adjustment factor for six current-carrying conductors is 80 percent. The maximum ampacity after applying the adjustment factor is 60A (75 0.80 = 60). 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 terminations will allow for an ampacity of 65A, the adjustment factor for six current-carrying conductors will not. Because of the six current-carrying conductors, the maximum ampacity for the 6 AWG conductors in this example is 60A (see Figure 3).

Next month’s column will continue the discussion of sizing conductors.


MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician’s Exam Prep Manual.” For more information, visit his Web site at www.charlesRmiller.com. He can be reached by telephone at 615.333.3336 or at charles@charlesRmiller.com.