If one size conductor is in the box, the cable clamp(s) counts as one of those conductors. This means that one or more internal cable clamps are counted as one conductor. Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest conductor present in the box. Since most boxes contain more than just conductor and wire connectors, it is important to have a good understanding of which items are counted and how much volume is required for those items. Specifications for clamp fill, support fittings fill, device or equipment fill, and equipment-grounding conductor fill are in 314.16(B)(2) through (5). Up to now, no consideration was given to fittings, devices or equipment-grounding conductors in the box-fill calculations. Six 14 AWG conductors can be installed to this box (12 / 2 = 6) (see Figure 3). Finally, divide the available space in the box by the cubic-inch volume for each 14 AWG. As per Table 314.16(B), the minimum volume allowance for each 14 AWG conductor is 2.0 cubic inches. The space remaining in the box is 12 cubic inches (21 - 9). Next, deduct the volume of the existing conductors from the volume of the box. The volume required for each 12 AWG conductor is 2.25 cubic inches, and therefore the volume required for four 12 AWG conductors is 9 cubic inches (4 2.25). Next, calculate the volume for the existing conductors. A 4-inch square box that is 11/2 inches deep has a volume of 21.0 cubic inches. The first step is to find the cubic-inch capacity of the box from Table 314.16(A). How many 14 AWG conductors can be added? The box contains no fittings or devices. For example, a 4-inch square metal box with a depth of 11/2 inches currently contains four 12 AWG conductors. If the new conductors are not the same size as the existing conductors, a volume calculation must be performed. If the new conductors are the same size as the existing conductors, find the maximum number in Table 314.16(A). Occasionally, boxes are already installed and additional conductors must be added. The minimum depth 4-inch square metal box is 2 1/8 inches (see Figure 2). The total volume required for four 10 AWG conductors and six 14 AWG conductors is 22 cubic inches (10 12). Since the volume required for each 14 AWG conductor is 2 cubic inches, the volume needed is 12 cubic inches (6 2). Therefore, the volume required for the 10 AWG conductors is 10 cubic inches (4 2.5). The volume required for each 10 AWG conductor is 2.5 cubic inches. To figure this out, first calculate the volume required for each set of conductors. For example, a box is needed for four 10 AWG conductors and six 14 AWG conductors, so what would be the minimum depth 4-inch square metal box that can be installed? The box will not contain any fittings of devices. When the box contains different size conductors, or the box is not listed in Table 314.16(A), a volume calculation must be performed. 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). The volume allowances range from 1.5 cubic inches (24.6 cubic centimeters) for 18 AWG conductors to 5 cubic inches (81.9 cubic centimeters) for 6 AWG conductors (see Figure 1). The free space required for each conductor is given in cubic centimeters and cubic inches. The volume allowances required per conductor sizes 18 AWG through 6 AWG are listed in Table 314.16(B). This month, the discussion continues with box-fill calculations. Last month's Code In Focus concluded with 314.16(B)(1). Box-volume calculations are in 314.16(A), box-fill calculations are in 314.16(B), and requirements pertaining to conduit bodies enclosing 6 AWG conductors or smaller are in 314.16(C). Procedures for calculating maximum numbers and sizes of conductors (6 AWG and smaller) are in 314.16. 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 C OUTLET, DEVICE, PULL AND JUNCTION BOXES CONDUIT BODIES FITTINGS AND MANHOLES
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