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For example, what are the maximum number of 18 through 6 AWG conductors permitted in a 3-inch-by-2-inch-by-2 1/2-inch device box? This box has a volume of 12.5 cubic inches and the maximum conductors permitted are: eight 18 AWG, seven 16 AWG, six 14 AWG, five 12 AWG, five 10 AWG, four 8 AWG and two 6 AWG conductors (See Figure 2). Note that the maximum number is based on the box not containing any fittings or devices, such as fixture studs, cable clamps, hickeys, receptacles or switches. First, find the box and then follow the row across to find the maximum number of conductors permitted. When the same size conductors will be installed in a metal box that is listed in Table 314.16(A), no calculation is needed. As listed in Table 314.16(A), a 4-inch square metal box with a depth of 1 1/2 inches has a volume of 21 cubic inches (See Figure 1). However, this is not the cubic-inch volume of the box. For example, what is the cubic-inch volume of a 4-inch square metal box with a depth of 1 1/2 inches? By using the outside dimensions, the size of the box is 24 cubic inches (4 x 4 x 1.5 = 24). The volume is the actual amount of area or space inside the box.

Volume is not calculated by using the outside dimensions of a box. Table 314.16(A)’s middle section provides the cubic-inch and cubic-millimeter volume for each box trade size. Table 314.16(A) provides minimum volume and maximum number of conductors for certain size metal boxes. The volume of the box, as calculated in 314.16(A), must not be less than the fill calculation as calculated in 314.16(B). While box volume calculations are in 314.16(A), box fill calculations are in 314.16(B).

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Requirements in 314.16(A) and (B) specify how to calculate the maximum number of conductors in outlet, device, pull or junction boxes.

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Just because a cover can be installed on a junction box that is packed full of conductors, does not mean it is Code compliant. After providing an introduction of Article 314, last month’s In Focus covered 314.15 and started discussing provisions in 314.16. Article 314 also includes installation requirements for fittings used to join raceways and to connect raceways and cables to boxes and conduit bodies.

Besides boxes and conduit bodies, this article covers manholes and other electric enclosures intended for personnel entry. Requirements pertaining to the installation and use of all boxes and conduit bodies used as outlet, device, junction or pull boxes are in Article 314 of the National Electrical Code. Article 314: Boxes, Conduit Bodies, Fittings and Manholes

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