314.16(B) Box-Fill Calculations

The National Electrical Code contains provisions for the maximum numbers and sizes of conductors that can be installed in boxes and conduit bodies. The 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. Calculation specifications for sizing outlet, device and junction boxes (and conduit bodies) are in 314.16. While box-volume calculations are in 314.16(A), box-fill calculations are in 314.16(B). Box-fill calculations in 314.16 are applicable for conductor sizes 18 AWG through 6 AWG. Box-fill requirements pertaining to conduit bodies enclosing 6 AWG conductors or smaller are in 314.16(C). Last month's Code In Focus concluded with 314.16(B)(2). This month, the discussion begins with support-fittings fill.

314.16(B)(3) Support-Fittings Fill

When one or more luminaire (fixture) studs or hickeys are present in the box, a single volume allowance in accordance with Table 314.16(B) shall be made for each type of fitting based on the largest conductor present in the box. The most common reference to a hickey is that of a conduit bender, used to bend small sizes of rigid-metal conduit. The obvious question is this: How can a conduit bender (hickey) be located inside an outlet box? The answer, of course, is that it cannot. A less-often-used type of hickey, which can be installed in boxes, is a type of coupling. Hickeys are often used to support large chandeliers. This mechanical device acts as a coupling by joining the luminaire (fixture) stem to the luminaire (fixture) stud. First the hickey screws onto a luminaire (fixture) stud, and then, a luminaire (fixture) stem attaches to the hickey. The luminaire (fixture) conductors, routed through the luminaire (fixture) stem, exit the hickey's opening and enter the outlet box. Once in the box, the luminaire (fixture) conductors are spliced to the circuit conductors, which are then concealed by the luminaire (fixture) canopy.

One or more luminaire (fixture) studs, within the box, count as one conductor. If one size conductor is in the box, the luminaire (fixture) stud counts as that size conductor. Likewise, one or more hickeys, within the box, count(s) as one conductor. If one size conductor is in the box, the hickey counts as one of these conductors. Luminaire (fixture) studs and hickeys are quite often installed in the same box, and where they are, they are counted as two conductors. For example, a metal octagon box contains one luminaire (fixture) stud and one hickey. One 12-2 with ground nonmetallic sheathed cable is installed in the box. Instead of the box containing three 12 AWG conductors, five 12 AWG conductors must be counted.

If the box contains more than one size conductor, the luminaire (fixture) stud and hickey must each be counted as one of the largest-size conductors in the box. For example, a 4 * 2 1/8-inch metal octagon box contains one luminaire (fixture) stud and one hickey. One 12-2 with grounded nonmetallic sheathed cable and two 14-2 with grounded nonmetallic sheathed cables are installed in the box. Because there are different size conductors in the box, the luminaire (fixture) stud counts as one 12 AWG conductor, and the hickey counts as one 12 AWG conductor.

314.16(B)(4) Device or Equipment Fill

For each yoke or strap containing one or more devices or equipment, a double-volume allowance in accordance with Table 314.16(B) shall be made for each yoke or strap based on the largest conductor connected to a device(s) or equipment supported by that yoke or strap. A mounting yoke or strap can contain one or more devices (or equipment), such as a single receptacle, a duplex receptacle, a single switch, a double switch, a triple switch or any combination.

Unlike clamp and support fittings, which count as one, a mounting yoke or strap must be counted as two conductors. The double-volume allowance for the device is determined by the conductors connected to the device. For example, a duplex receptacle with 12 AWG conductors connected to it counts as two 12 AWG conductors. A duplex receptacle with 14 AWG conductors connected to it counts as two 14 AWG conductors. A single-pole switch with 12 AWG conductors connected to it counts as two 12 AWG conductors. A single-pole switch with 14 AWG conductors connected to it counts as two 14 AWG conductors.

When more than one size conductor is connected to the mounting yoke or strap, it must be counted as two of the largest conductors that are connected to that device. For example, a double switch is fed from two different circuits. One circuit is a 15-ampere circuit and the other is a 20. The top switch has 12 AWG conductors connected to it and the bottom switch has 14 AWG conductors connected to it. Since the largest conductors connected to the switch are 12 AWG conductors, this device counts as two 12 AWG conductors.

Unlike clamp and support fittings, which count as one of the largest conductors in the box, a mounting yoke or strap is counted as two of the largest conductors connected to the device or equipment supported by that yoke or strap. The yoke or strap is not necessarily counted as the largest size conductors in the box. For example, a four-inch square metal box with a single-gang plaster (mud) ring will support a single-pole switch. Two 14 AWG conductors will be connected to the switch. The two 14 AWG conductors will enter the box through a 1/2-inch electrical metallic tubing (EMT). Two other raceways are also connected to the box. Two 12 AWG conductors are in each of the other two raceways. The 12 AWG conductors from one EMT are spliced to the other 12 AWG conductors from the other EMT. Because the switch counts as two 14s, the conductors counted in this box are four 14 AWG and four 12 AWG conductors.

Next month's Code in Focus continues the discussion of box-fill calculations.

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