When sizing conductors, more is involved than just looking at Table 310.15(B)(16) in the National Electrical Code (NEC) and selecting a copper or aluminum conductor with the right ampacity. Of all the tables in Article 310, Table 310.15(B)(16) is referenced the most. The allowable (or maximum) ampacities listed in this table—for insulated conductors rated up to and including 2,000 volts (V)—are conditional. One condition is the number of load-carrying (current-carrying) conductors. As stated in the title, the ampacities are based on not more than three current-carrying conductors in a raceway, cable or earth (directly buried).

When there are more than three current-carrying conductors, the table ampacities must be adjusted by factors listed in Table 310.15(B)(3)(a). The other condition is ambient temperature. The ampacities in Table 310.15(B)(16) are based on an ambient temperature of 30°C. Similar to adjustment factors for more than three current-carrying conductors, the table ampacities must be corrected when the ambient temperature is other than 30°C. Last month’s column concluded by covering adjustment factors for adjacent load-carrying conductors. This month, the discussion continues with adjustment factors for more than three current--carrying conductors in a raceway, cable or earth (directly buried).

Section 310.15(B)(3)(a)(4) pertains to armored cable (Type AC) and metal-clad cable (Type MC). Type AC cable is a fabricated assembly of insulated conductors in a flexible interlocked metallic armor [320.2]. Article 320 covers the use, installation and construction specifications for AC cable. Type MC cable is a factory assembly of one or more insulated circuit conductors with or without optical fiber members enclosed in an armor of interlocking metal tape or a smooth or corrugated metallic sheath [330.2]. Article 330 covers the use, installation and construction specifications of MC cable. Adjustment factors shall not apply to Type AC cable or to Type MC cable under the four conditions listed in 310.15(B)(3)(a)(4)a through (4)d.

The first condition is that the cables do not have an overall outer jacket. This does not mean that cables with outer jackets cannot be installed. It just means that it will be necessary to apply Table 310.15(B)(3)(a) adjustment factors to jacketed cables with more than three current-carrying conductors or to multiconductor cables that are installed without maintaining spacing for a continuous length longer than 24 inches. The next condition is that each cable must not have more than three current-carrying conductors. The third condition pertains to conductor size. To use this adjustment factor exemption, the conductors must be 12 AWG copper. The last condition is that not more than 20 current--carrying conductors are installed without maintaining spacing, are stacked or are supported on “bridle rings.”

For example, 10 Type MC cables, two-conductor (with an additional equipment grounding conductor), 12 AWG THHN copper will be installed to power general-purpose receptacles in an office building. None of the cables will have an overall outer jacket. The cables will be bundled together and installed as a group, without maintaining spacing, for a distance of 50 feet. What is the Table 310.15(B)(3)(a) adjustment factor for each conductor in this example?

Because they are metal-clad cables, check to see if this installation will meet all four conditions listed in 310.15(B)(3)(a)(4). Since none of the cables will have an overall outer jacket, the first condition will be met. The second and third conditions will be met because each cable will have only two current-carrying conductors, and each cable will be 12 AWG copper. The fourth condition will be met because there will not be more than 20 current-carrying conductors bundled together without maintaining spacing. Since all four of these conditions will be met, it is not necessary to apply an adjustment factor from Table 310.15(B)(3)(a) (see Figure 1).

If all four of the conditions in 310.15(B)(3)(a)(4) are not met, it may be necessary to apply an adjustment factor to Type AC or MC cables. For example, three 12 AWG THHN copper Type MC cables will be installed. Two of the cables will be two-conductor (with an additional equipment grounding conductor), and one cable will be four-conductor (with an additional equipment grounding conductor). The cables will be bundled together and installed as a group, without maintaining spacing, for a distance of 20 feet. The neutral conductor in the four-conductor cable will be counted as a current--carrying conductor. There will be a total of eight current-carrying conductors. None of the cables will have an overall outer jacket. What is the Table 310.15(B)(3)(a) adjustment factor for each conductor in this example? In this example, all four conditions in 310.15(B)(3)(a)(4) will not be met. The second condition is that each cable has no more than three current-carrying conductors. Since one of the cables has four current-carrying conductors, it is not permitted to use Section 310.15(B)(3)(a)(4). Because the multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 inches, it will be necessary to adjust the ampacity of each conductor. The Table 310.15(B)(3)(a) adjustment factor for eight current-carrying conductors is 70 percent (see Figure 2).

Section 310.15(B)(3)(a)(5) also pertains to Type AC cable and Type MC cable. An adjustment factor of 60 percent shall be applied to Type AC or Type MC cable meeting the three stipulations in 310.15(B)(3)(a)(5)a through (5)c. The first stipulation is that the cables do not have an overall outer jacket. The second stipulation is that there are more than 20 current-carrying conductors. The last stipulation is the cables are stacked or bundled longer than 24 inches without maintaining spacing.

With this provision, the conductors can be any size or any combination of sizes. Also, this provision does not set a limit for the number of current-carrying conductors in a cable. For example, 11 Type MC cables will be installed. Six of the cables will be two-conductor (with an additional equipment grounding conductor) 12 AWG THHN copper. Five of the cables will be two-conductor (with an additional equipment grounding conductor) 10 AWG THHN copper. None of the cables will have an overall outer jacket. The cables will be bundled together and installed as a group, without maintaining spacing, for 50 feet. There will be 22 current-carrying conductors in this group. The terminations on both ends will be rated at least 75°C. The ambient temperature will not be higher than 30°C. The loads are not continuous. What is the maximum ampacity for each 12 AWG conductor and each 10 AWG conductor in this example?

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 Table 310.15(B)(16), the allowable ampacity for a 12 AWG THHN copper conductor is 30 amperes (A), and the maximum ampacity for a 10 AWG THHN conductor is 40A. Because this example does not meet all four conditions, it is not permitted to use the adjustment factor exemption in 310.15(B)(3)(a)(4). The Table 310.15(B)(3)(a) adjustment factor for 22 conductors is 45 percent, but this installation meets the stipulations in 310.15(B)(3)(a)(5). Therefore, it is only required to use an adjustment factor of 60 percent. Multiply 30A by 0.60 (30 0.60 = 18). The maximum ampacity for the 12 AWG conductors is 18A. Multiply 40A by 0.60 (40 0.60 = 24). The maximum ampacity for the 10 AWG conductors is 24A (see Figure 3).

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.