Table 310.15(B)(16) (formerly Table 310.16) in the National Electrical Code (NEC) provides allowable (or maximum) ampacities for insulated conductors rated up to and including 2,000 volts (V). The allowable ampacities in this table are based on two stipulations: adjacent load-carrying (current-carrying) conductors and ambient temperature.

The first stipulation or condition, in the title of the table, states that 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. Quite often, this adjustment is referred to as derating.

The second condition states that the ampacities are based on an ambient temperature of 30°C. Similar to adjustment factors for adjacent load-carrying conductors, when the ambient temperature is other than 30°C, the table ampacities must be corrected. Although it is not exactly accurate, applying ambient temperature correction factors also is sometimes referred to as derating. It is not an accurate term because not all of the correction factors lower or reduce the allowable ampacity. For ambient temperatures lower than 26°C, the correction factors actually increase the allowable ampacity.

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).

Adjustment factors for more than three current-carrying conductors apply to more than just conductors in raceways. Since ambient temperature can also rise because of adjacent current--carrying conductors in cables, the provision in 310.15(B)(3)(a) also applies to cables. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multi-conductor cables are installed without maintaining spacing for continuous lengths longer than 24 inches and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Some common cables used in construction include armored cable (Type AC), metal-clad cable (Type MC) and nonmetallic-sheathed cable (Types NM, NMC and NMS). Nonmetallic-sheathed cable also is referred to as Romex, which is a federally registered trademark of Southwire.

As an example of how to find the allowable ampacity, three 12–2 with ground nonmetallic-sheathed cables enter the top of a panelboard. Above the panelboard, the three nonmetallic-sheathed cables pass through the same hole in the header (or wood framing). These three cables are grouped together for more than 24 inches between the panelboard and header. Because the cables are grouped together for more than 24 inches, it will be necessary to multiply each conductor’s ampacity by an adjustment factor in Table 310.15(B)(3)(a) (see Figure 1).

If cables are grouped together for a length of more than 24 inches and there are more than three current-carrying conductors, the allowable ampacity of each conductor must be adjusted. For example, three 12–2 with ground nonmetallic-sheathed cables are grouped together above a panelboard for a length of 27 inches. What is the maximum ampacity for each conductor? In accordance with 334.80, the allowable ampacity of Types NM, NMC and NMS cable shall not exceed that of a 60°C rated conductor. The 90°C rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final derated ampacity does not exceed that of a 60°C rated conductor. The allowable ampacity for a 12 AWG conductor, in the 90°C column, is 30 amperes (A). Each 12–2 with ground nonmetallic-sheathed cable has two current-carrying conductors. The total number of current-carrying conductors is six (2 conductors 3 cables = 6). Because the cables are grouped together for more than 24 inches and there are more than three current-carrying conductors, it is necessary to apply an adjustment factor from Table 310.15(B)(3)(a). There are only six current--carrying conductors because grounding and bonding conductors do not count as current-carrying conductors. The adjustment factor for six current-carrying conductors is 80 percent. Now multiply 30A by 0.80 (30 0.80 = 24). The ampacity after applying the adjustment factor is 24A. But in accordance with 334.80, the final derated ampacity shall not exceed that of a 60°C rated conductor. The allowable ampacity for a 12 AWG conductor, in the 60°C column, is 20A. Therefore, the maximum ampacity for each 12–2 with ground nonmetallic-sheathed cable in this example is 20A (see Figure 2).

Even if nonmetallic-sheathed cables are not grouped together for a length greater than 24 inches, it still may be necessary to adjust the ampacity of each conductor because another requirement pertains to three or more cables installed through the same opening. Where more than two NM cables containing two or more current-carrying conductors are installed, without maintaining spacing between the cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(3)(a) [334.80]. Section 334.80 applies even if the cables are not grouped together for a length of more than 24 inches. For example, four 14–2 with ground nonmetallic-sheathed cables are installed, without maintaining spacing between the cables, through the same opening in wood framing. The opening in the wood framing has been sealed with caulk. What is the maximum ampacity for each conductor? The allowable ampacity for a 14 AWG conductor, in the 90°C column, is 25A. Because there are four 14–2 with ground nonmetallic-sheathed cables, the total number of current-carrying conductors is eight (2 conductors 4 cables = 8). Because the four cables containing eight current-carrying conductors are installed in the same hole and the hole has been caulked, it is necessary to apply an adjustment factor from Table 310.15(B)(3)(a). The adjustment factor for eight current-carrying conductors is 70 percent. Now, multiply 25A by 0.70 (25 0.70 = 17.5 = 18). The ampacity after applying the adjustment factor is 18A. But, in accordance with 334.80, the final derated ampacity shall not exceed that of a 60°C rated conductor. The allowable ampacity for a 14 AWG conductor, in the 60°C column, is 15A. Therefore, the maximum ampacity for each 14–2 with ground nonmetallic-sheathed cable in this example is 15A (see Figure 3).

If the number of current-carrying conductors exceeds nine, it may be necessary to install an overcurrent device with a rating lower than the size that is normally installed. For example, five 12–2 with ground nonmetallic-sheathed cables are installed through the same opening in wood framing. After the conductors were installed, the hole was sealed with caulk. What is the maximum ampacity for each conductor, and what is the maximum rating for the overcurrent device? The allowable ampacity for a 12 AWG conductor, in the 90°C column, is 30A. Because there are five 12–2 with ground nonmetallic-sheathed cable, the total number of current-carrying conductors is 10 (2 conductors 5 cables = 10). The Table 310.15(B)(3)(a) adjustment factor for 10 current-carrying conductors is 50 percent. Now multiply 30A by 0.50 (30 0.50 = 15). The maximum ampacity for the 12–2 with ground nonmetallic-sheathed cables in this example is 15A. As stated in 240.4, conductors 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). In typical installations, 20A breakers protect 12 AWG conductors. However, 15A breakers must protect these 12 AWG conductors because the maximum conductor ampacity is only 15A (see Figure 4).

Next month, the discussion of sizing conductors continues.


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.