Safety is the main reason for the existence of the National Electrical Code (NEC). Its purpose is the practical safeguarding of persons and property from hazards arising from the use of electricity [90.1(A)]. Ensuring conductors and overcurrent protective devices are sized correctly is one aspect of practical safeguarding. It is necessary to reference more than a single section and article to select the correct size conductor and the correct size overcurrent protective device.

Many factors are involved, such as the temperature ratings of terminations, conductors and devices; the connected load or the load calculated in accordance with Article 220; continuous and noncontinuous loads; ambient temperature; and the number of current-carrying conductors. This series on sizing conductors discusses these and other factors that must be considered when sizing conductors.

Up to this point, the discussion has included single insulated conductors, such as THHN but has not included nonmetallic-sheathed cable. Ampacity requirements are a little different for nonmetallic-sheathed cable. Article 334 covers the use, installation and construction specifications of nonmetallic-sheathed cable [334.1]. As required by 334.112, the conductor insulation shall be rated at 90°C. However, the allowable ampacity cannot come from the 90°C column or even the 75°C column of Table 310.15(B)(16) (formerly Table 310.16). The allowable ampacity shall not exceed that of a 60°C rated conductor [334.80]. For example, what is the maximum ampacity for a 10 AWG copper nonmetallic-sheathed cable? 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 ambient temperature correction factors is not necessary. Although the insulation is rated at 90°C, the allowable ampacity for a nonmetallic-sheathed cable shall not exceed that of a 60°C rated conductor, which is 30 amperes (A). Therefore, the maximum ampacity of a 10 AWG nonmetallic-sheathed cable is 30A (see Figure 1).

Since Table 310.15(B)(16) is based on an ambient temperature of 30°C, it is necessary to apply ampacity corrections factors if the ambient temperature is anything other than 26–30°C. As previously mentioned, the allowable ampacity for a nonmetallic-sheathed cable shall not exceed that of a 60°C rated conductor, but using the ampacity in the 60°C column is not required when there are correction and/or adjustment calculations. Like termination temperature provisions in 110.14(C), it is permissible to use the ampacity in the 90°C column for correction and/or adjustment calculations. 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 [334.80]. For example, what is the maximum ampacity for an 8 AWG copper nonmetallic-sheathed cable? The terminations on both ends are rated at least 75°C. The maximum ambient temperature will be 56°C. Although the final derated ampacity must not exceed the ampacity in the 60°C column, it is permissible to use the ampacity in the 90°C column for correction and/or adjustment calculations. The allowable ampacity for an 8 AWG nonmetallic-sheathed cable in the 90°C column is 55A. The Table 310.15(B)(2)(a) correction factor, in the 90°C column for an ambient temperature of 56°C, is 0.71. The maximum ampacity after applying the adjustment factor is 39A (55 0.71 = 39.05 = 39). In accordance with 334.80, the ampacity for this conductor shall not exceed the ampacity listed in the 60°C column, which is 40A. However, because of the ambient temperature, the maximum ampacity for the 8 AWG nonmetallic-sheathed cable in this example is 39A (see Figure 2).

A requirement and table pertaining to circular raceways on rooftops was added to the 2008 Code. Where conductors or cables are installed in circular raceways exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(3)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b) [310.15(B)(3)(c)]. Because the ambient temperature inside raceways installed on or above rooftops will be higher than raceways not installed on or above rooftops, the outdoor temperature or ambient temperature must be adjusted. Table 310.15(B)(3)(c) stipulates how much temperature in degrees Celsius or Fahrenheit to add to the maximum outdoor temperature. For example, what is the maximum ampacity for a 1 AWG copper THHN conductor in a raceway with two other current--carrying conductors? 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 107°F. The conductors will be installed in a circular raceways exposed to direct sunlight on the rooftop. The distance above the roof to the bottom of the conduit will be

3 inches. Although the raceway contains four conductors, there are only three current-carrying conductors because grounding and bonding conductors do not count as current-carrying conductors. Therefore, there is no adjustment factor for current-carrying conductors. The Table 310.15(B)(3)(c) adjustment for a circular raceway installed 3 inches above the roof is 40°F. Because of the temperature adder, the ambient temperature is now 147°F (107 + 40 = 147). The Table 310.15(B)(2)(a) correction factor, in the 90°C column for an ambient temperature of 147°F, is 0.65 (see Figure 3).

Now that the ambient temperature correction factor (including the temperature adder for the circular raceway on the rooftop) is known, apply the correction factor to find the conductor’s allowable ampacity. The ampacity for a 1 AWG copper THHN conductor, as listed in the 90°C column of Table 310.15(B)(16), is 145A. After applying the correction factor, the ampacity is 94A (145 0.65 = 94.25 = 94). The maximum ampacity for the 1 AWG THHN conductors in this example is 94A (see Figure 4).

Caution is advised because the ampacity for this conductor has been reduced by 51A in the 90°C column and by 36A in the 75°C column. If the load supplied by these conductors is greater than 94A, these conductors shall not be installed.

This discussion will continue next month.


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.