All for one and one for all: Basic box fill in the 2023 NEC

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Making sure there is enough room for conductors and devices installed within standard boxes can be easy if you can remember when to count all for one or one for all.

Often, I have just needed to read the entire requirements of section 312.16 each time I attempted the math, just to make sure.

This month I will attempt to bring the basics back and show an easy trick when calculating the volume allowance for Equipment Grounding Conductors. Please keep in mind, box fill is what can be called “fuzzy math.” There are no considerations for larger devices such as GFCI receptacles or lighting dimmers, timers or listed lighting controllers. Any twist-on wire connectors or Wirenuts® are also not counted. Additionally, there are requirements for the minimum depth of a box, with or without devices, that can be reviewed in section 314.24

The Code section starts with affirmation that these requirements only apply when conductors 6 AWG and smaller are installed within standard electrical boxes. The rules allow all assembled parts to be used for volume. This would include mud rings, extension rings and doomed covers. New to the 2017 NEC, are volume allowances for barriers installed which will be ½ in.² if metallic and 1 in.² if plastic, unless marked by the manufacturer.

314.16(A)(1) allows the volume of standard boxes to be in accordance with Table 314.16(A). If other boxes with a volume of less than 100 in.² or plastic boxes are used, markings on the box by the manufacturer shall be used. This would include “Bell Boxes” or plastic “Nail-ons.” The Table describes two types of device boxes. One with the dimension 4 X 2⅛ and the other device box at 3 X 2. The difference is one box, the 3 X 2, has the mounting ears on the outside of the box while the other box has the mounting ears on the inside of the box. This is sometimes called a “handy box.” FS and FD boxes are known as (FS) “Flanged Shallow” and (FD) “Flanged Deep.” Type FS and FD boxes are also known as cast-device boxes and can be used as outlet, device, junction and pull boxes. Both FS and FD boxes are cast-device boxes with a single conduit entry. While the “S” in FS stands for shallow, the “D” in FD stands for deep.

An FSS box contains two conduit entries, while an FC box will have one in the top and one in the bottom of the box. These types of boxes are very durable and are used in mostly industrial locations.

314.16(B) requires the sum of (B)(1) through (B)(6) be added together so the volume allowance does not exceed that of Table 314.16(A). The volume allowances of the components added are in accordance with the conductor Table 314.16(B).

(B)(1) Conductor Fill. Each conductor in the box shall be once. This does not include equipment grounding conductors or pigtails. Each loop counts as one unless the loop is big enough to cut and turn it into two conductors of at least 6 inches. (300.14) The volume allowances for commonly used conductors sizes are as follows:

• 14 AWG – 2.00 cubic inches
• 12 AWG – 2.25 cubic inches
• 10 AWG – 2.50 cubic inches

There is an exception that allows for up to four conductors smaller than 14 AWG entering a box from a domed luminaire or similar canopy to be exempted or not counted. This would include the supply conductors to a ceiling fan.

(B)(2) Clamp Fill. Clamps, whether factory or field installed, count as one volume allowance for all the clamps in the box based on the largest conductor in the box. Clamps with the clamp mechanism on the outside do not count.

(B)(3) Support Fitting Fill. One or more of each type shall be counted as one for each luminaire stud or each luminaire hickey. Today you might encounter only fixture studs while hickeys were used more often in the past. Modern luminaires are typically supplied with only a hollow threaded fixture stud which mounts to the fixture bar Volume allowance is based on the largest conductor in the box.

The 2017 NEC introduced a new device that could be used for mounting luminaires. This is called a Weight-Supporting Attachment Fitting (WSAF) and its companioned device the Weight-Supporting Ceiling Receptacle (WSCR).

(B)(4) Device or Equipment Fill. For each device that can be mounted in one gang or on two 6-32 machine screws, two volume allowances must be made based on the largest conductor connected to that device. For example, a receptacle with 12 AWG conductors connected to it, shall count as 4.5 cubic inches. It does not matter how many wires are connected. Those are counted in (B)(1). If the device needs four screws or takes two gangs to mount, then two double volume allowances are counted based on the largest wire connected to it.

For example, a 120/240-volt, 30-amp dryer receptacle is 4 X 2.5 = 10 cubic inches. Please consider the additional requirements for box depth in 314.24.

(B)(5) Equipment Grounding Conductors. This was changed in the 2020 NEC. The volume allowance is now a calculation. It requires the first four to count as one and each additional EGC count as .25 based on the volume allowance of the largest equipment grounding conductor in the box. This can be simplified by just taking the total number of EGC’s in the box and divide by 4 to derive a multiplier.

For example, seventeen, 12 AWG, equipment grounding conductors in a box.

17 ÷ 4 = 4.25

2.25 X 4.25 = 9.5625 cubic inches total volume allowance for all the equipment grounding conductors in the box. In the past we counted all as one. This has proven to be problematic when ganging multiple device boxes together as with switch locations. Prior to 2020, there was a requirement to count other grounding systems as another EGC. This would be considered the isolated grounding conductors. Now it does not matter. Count them all, divide by four and get your multiplier. Multiply by the largest ECG in the box.

N (B)(6) Terminal Block Fill. New to the 2023 NEC, this section requires terminal blocks to be counted like a wire based on the largest conductor connected to the device.

There is two ways to approach box fill. You can either build the box based on what you are installing, or you can select a box and make the deduction based on volume allowances in accordance with 314.16(B) and Table 314.16(A). With most of our installations being prefabricated and engineered on drawings, this is one area in which we have some say in what size box we select and to select a box that will prevent conductor damage and allow for adequate room for devices installed in the box.

Knowing how to do box fill calculations is a skill most electricians do not understand. Hopefully this month’s column will shed some clarity on when to count all for one or one for all.