Published In August 2000
The conductor fill requirements for a conduit or an outlet box are calculated in the field on a regular basis. However, field people seldom determine the conductor fill requirements for a surface metal or nonmetallic raceway (Article 352), a metal or nonmetallic wireway (Article 362), or the wire space for a cabinet or cutout box (Article 373. The calculation is neither difficult nor mysterious if a few simple rules within the National Electrical Code (NEC) are followed. Manufacturers design surface metal and nonmetallic raceways to contain a specific number of conductors within the raceway. The number of conductors will be stamped in the raceway or on the shipping carton, or included with the installation instructions for the raceway. Since the manufacturer’s instructions provide the information on raceway fill, calculating the fill is not necessary. Determining whether to derate the conductors based upon the number of conductors in the raceway does require a calculation. Section 352-4, second paragraph states that derating for the number of conductors in the raceway based upon Section 310-15(b)(2)(a) [the old Note 8 for more than three current-carrying conductors in a raceway] is not necessary for surface metal raceways. However, three conditions must be met. (1) The cross-sectional area of the metal raceway must be greater than 4 square inches; (2) the number of current-carrying conductors cannot exceed 30, and (3) the sum of the cross-sectional area of all contained conductors cannot exceed 20 percent of the cross-sectional area of the raceway. For example, a surface metal raceway containing 10 No. 2/0 THHN conductors, eight No. 8 THHN conductors, and 12 No. 12 THHN conductors would normally require a 45 percent adjustment factor of the current-carrying capacity of the conductors in the raceway, based upon Section 310-15(b)(2)(a). The first step is to calculate the total square inch area the conductors will occupy. Dividing the total square-inch area of conductor fill by 20 percent will provide the minimum-size metal surface raceway required, not to exceed the 20 percent cross-section requirement. Example: 2.6754 square inches area of total conductors /20 percent =13.377-square-inch wireway (This exceeds the four-square-inch minimum required in the first requirement.) A metal raceway that measures approximately 33/4 inches x 33/4 inches would allow the 20 percent maximum fill so that a 45 percent adjustment factor would not be required for the current-carrying conductors. The same calculation method can be used for metal wireways based on Section 362-5 provided there are no more than 30 conductors within the wireway and the 20 percent maximum fill has not been exceeded. For both the surface metal raceway and the metal wireway, if the number of conductors exceeds 30, or the cross-sectional area has been filled to more than 20 percent, the conductors must be derated. Section 362-19 requires current-carrying conductors installed in nonmetallic wireways to be derated in accordance with Section 310-15(b)(2)(a), even where the fill doesn’t exceed 20 percent. The metal wireway dissipates heat better than the nonmetallic wireway, which is likely to be affected by excessive heat from the conductors. Cabinets and cutout boxes, as covered by Article 373, are usually designed with swinging doors to enclose small transformers, switches, overcurrent protection devices, meters, or control devices. These enclosures are intended to enclose these devices. In addition, conductors must be connected to the devices. Section 373-7 requires these enclosures to have sufficient space within the enclosure to install the necessary conductors to connect to these devices. Section 373-8 does not permit these enclosures used for switches or overcurrent devices to be used as junction boxes, auxiliary gutters, or raceways for conductors feeding through or being tapped to other switches or overcurrent devices. However, a provision in Section 373-8 permits these extra conductors to be spliced or tapped in the cabinet if sufficient space is provided. The tapped or spliced conductors cannot fill the wiring space to more than 40 percent of the cross-sectional area of the wiring space. Furthermore, the conductors, taps, and splices cannot fill the wiring cross-section space to more than 75 percent of the cross-sectional area of the wiring space. For example, if the wiring space in a cabinet is 4 inches by 4 inches, the total wiring cross-sectional space is 16 square inches. The total fill is limited to 40 percent of the 16 square inches of available space or 16 square inches x .40 = 6.4 square inches. The total area of conductors that can be installed in the wire space of the cabinet can be calculated by the area of the conductors based upon the type of insulation and size of conductors in accordance with Table 5 in Chapter 9. These calculations are relatively simple and can be accomplished in the field using the NEC and a calculator. ODE is a staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or by e-mail at firstname.lastname@example.org.