Are grounding cables included in the calculation of the fill volume of a case?

Yes they are taken into account but not individually, all earth cables for devices within a housing share one maximum volume limit based on the largest cable cross section. For every additional cable over four a quarter of the maximum volume limit is added.

Are wire connectors and branch cables included in the calculation of a housing's fill volume?

No they are not taken into account. Wire connectors do not count and extension cables that stay within the enclosure also don't count. A cable only counts towards the maximum volume if it goes in or out of the enclosure.

Why are switches and sockets counted twice?

NEC 314.16(B)(4) assigns a double volume limit to each point of attachment required for the mounting of equipment. This is because the enclosure takes up space that cannot be used for wiring. When two pieces of equipment are mounted in a double box, this equates to four limits.

How many 12 AWG cables can fit in a box that measures 4 by 4 by 1.5 inches?

The box has a volume of 21 cubic inches and one single 12 AWG cable requires 2.25 cubic inches so the box can hold nine cables without other contents. If you add lugs, devices, and ground wires then it reduces to five cables.

Do cardboard rings or extension rings increase the volume of a box?

Yes, if the volume of the ring is given. Before comparing the total fill amount with the box volume, add the volume indicated in the ring or cover frame to the box volume.

Box Fill Calculator

Conductors in the box

Conductors of this size

Conductor size

The box holds more than one wire size

Adds two more size rows, so each conductor is counted at its own volume.

Long loops or coils

Counting the wires is where most box fill calculations go wrong. A cable with a black, a white and a bare ground gives you two conductors here plus one grounding conductor below.

Clamps, fittings and devices

The box has internal cable clamps

One allowance for the whole set. Clamps outside the box do not count.

Support fitting types

Device or equipment gangs

The device is fed by a smaller conductor

Device fill follows the largest conductor connected to the device, which is not always the largest wire in the box.

Grounding conductors

Equipment grounding conductors

Largest grounding conductor size

An isolated equipment grounding conductor is present

An additional grounding set under 250.146(D) earns one more allowance.

The box you plan to use

How do you know the box volume?

Pick a standard metal box

Enter the volume marked on the box

Standard metal box

Table 314.16(A) covers standard metal boxes. A plaster ring, extension ring or raised cover marked with its own volume adds to the total, so add it to the marked volume instead.

Your box fill result

Total box fill required

How full the box is

Volume allowances counted

Smallest listed box that fits

This box passes. The contents need 22.5 cubic inches and the box gives you 30.3, so you are inside NEC 314.16.

Total box fill required

cu in

Box volume: cu in
How full the box is: %
Room left in the box: cu in

Volume allowances counted
Spare conductors this box still takes

Conductors this box holds on its own
Smallest listed box that fits: cu in

Breakdown and box options

Show the box size comparison

Charts your fill against the standard metal boxes and lists every trade size.

Your box fill worksheet, line by line
Item	Count	Unit volume (cu in)	Volume allowances	Fill (cu in)
Conductors 12 AWG	6	2.25	6	13.5
Internal cable clamps	1	2.25	1	2.25
Devices, 2 per gang 12 AWG	1	2.25	2	4.5
Grounding conductors 12 AWG	2	2.25	1	2.25
Total			10	22.5

What is taking up the box

Every allowance in cubic inches, split by the part of 314.16(B) it comes from.

NEC Table 314.16(A) metal boxes, checked against your fill
Box trade size	Volume (cu in)	Volume (cm3)	Conductors of your largest size	Fits your fill?
4 x 1-1/4 round or octagonal	12.5	205	5	No
4 x 1-1/2 round or octagonal	15.5	254	6	No
4 x 2-1/8 round or octagonal	21.5	353	9	No
4 x 4 x 1-1/4 square	18	295	8	No
4 x 4 x 1-1/2 square	21	344	9	No
4 x 4 x 2-1/8 square	30.3	497	13	Yes
4-11/16 x 1-1/4 square	25.5	418	11	Yes
4-11/16 x 1-1/2 square	29.5	484	13	Yes
4-11/16 x 2-1/8 square	42	689	18	Yes
3 x 2 x 1-1/2 device	7.5	123	3	No
3 x 2 x 2 device	10	164	4	No
3 x 2 x 2-1/4 device	10.5	172	4	No
3 x 2 x 2-1/2 device	12.5	205	5	No
3 x 2 x 2-3/4 device	14	230	6	No
3 x 2 x 3-1/2 device	18	295	8	No
4 x 2-1/8 x 1-1/2 device	10.3	169	4	No
4 x 2-1/8 x 1-7/8 device	13	213	5	No
4 x 2-1/8 x 2-1/8 device	14.5	238	6	No
3-1/4 x 2 x 2-1/2 masonry box or gang	14	230	6	No
3-1/4 x 2 x 3-1/2 masonry box or gang	21	344	9	No
FS single cover or gang (1-3/4 deep)	13.5	221	6	No
FD single cover or gang (2-3/8 deep)	18	295	8	No
FS multiple cover or gang (1-3/4 deep)	18	295	8	No
FD multiple cover or gang (2-3/8 deep)	24	395	10	Yes

The calculation of the fill amount of an electrical panel box answers the question as to whether all components contained within can be housed in a manner that is compliant with regulations. According to section 314.16 of the NEC (National Electrical Code), individual elements inside the box are converted and added together in cubic inches, then compared against the total volume of the box.

A miscalculation can result in cables being pinched, insulation damaged by sharp edges of the enclosure and no way for heat to escape. This is a significant cause of arcing that can occur in older distribution boards.

The following elements are taken into account when calculating the fill level:

Five types of items have a volume limit each, with the limits corresponding to the amount of free space required for one wire of a given size.

Cables. Each cable strand that enters the box, terminates, connects or passes through is counted once per size.
Integrated cable clamps. A set of cable clamps is considered once. Clamps that are located outside the enclosure are not considered.
Mounts. Fixing screws for lighting and connection elements are considered once per type.
Devices. Switches, sockets or similar devices are considered once per installation location.
Equipment grounding conductors. All grounding conductors share one limit and for each additional conductor over the fourth a quarter of the limit is added.

There are two types of items that often get mistakenly included in the calculation but should not be: cable connectors and extension cables whose total length is within the enclosure, and fittings which are attached outside the enclosure.

Another source of error is the rules for other directions. If an unseparated power conductor reaches a minimum free length at least twice the value required in 300.14, it will be counted as two conductors and not one.

Volume of a single conductor:

Table 314.16(B) shows the required clearances for each wire size. All other numbers used in calculations are multiples of these values.

Conductor size (AWG)	Free space (cu in)	Free space (cm3)
18	1.50	24.6
16	1.75	28.7
14	2.00	32.8
12	2.25	36.9
10	2.50	41.0
8	3.00	49.2
6	5.00	81.9

Be aware of what size applies to each part of the overall fill. For individual wire fills, their own size is used while for clamps and holders the largest wire size within the enclosure is used. For devices the largest size of wires connected to it is used, and for ground fill the largest size of ground cables is used.

Calculation formula:

The total fill volume is the sum of five parts.

V_{\text{total}} = V_{\text{cond}} + V_{\text{clamp}} + V_{\text{fit}} + V_{\text{dev}} + V_{\text{gnd}}

Each section corresponds to a value determined by the number of conductors permitted and the volume of an individual conductor as per Table 314.16(B).

V_{\text{cond}} = \sum_{i} n_i \times V_i \qquadV_{\text{clamp}} = 1 \times V_{\text{largest}} \qquadV_{\text{dev}} = 2 \times g \times V_{\text{device}}

The grounding points have been changed in the 2020 version. One to four ground wires share a limit, after that each additional wire adds a quarter of the limit.

A_{\text{gnd}} = \begin{cases} 1 & n_g \le 4 \\[4pt]1 + \dfrac{n_g - 4}{4} = \dfrac{n_g}{4} & n_g > 4 \end{cases}

If the total fill volume is below the volume of the enclosure then the enclosure meets requirements.

V_{\text{total}} \le V_{\text{box}}

Example calculation:

For example, a metal box with dimensions of 4 by 1-1/2 inches has a volume of 21 cubic inches according to Table 314.16(A). The box contains two cables that are each 12 AWG, all conductors properly terminated, plus one cable that is 14 AWG, two built-in cable clamps, one device yoke, one stud for a light fixture, and one ground wire for each cable.

Item	Allowances	Unit volume	Fill
Four 12 AWG conductors	4	2.25	9.00
Two 14 AWG conductors	2	2.00	4.00
Internal cable clamps	1	2.25	2.25
One device yoke	2	2.25	4.50
One fixture stud	1	2.25	2.25
Grounding conductors	1	2.25	2.25
Total	11		24.25

These elements require 24.25 cubic inches of volume, but the box only has a capacity of 21 cubic inches so this installation is not suitable. The solution would be to use a deeper box. The next larger standard size that can hold 24.25 cubic inches is a rectangular box measuring 4-11/16 by 1-1/4 inches with a volume of 25.5 cubic inches.

Please note the ratings for conduit fittings, studs and devices. All of these values are calculated using 12 AWG cable size. Even if another 14 AWG cable is in a junction box, those limits apply because of maximum allowable conductor fill, not average size.

How to use this calculator:

First enter the cable size. All other values will be calculated based on this size. If there are multiple cable diameters, then enable the fields for second and third diameter and fill out the respective quantities into those fields.

Then fill out the form. For switchboards with integrated pipe clamps, activate the switch for the pipe clamp. For multiple switchboards, count the mounting positions instead of the devices themselves and enter the maximum size for the earthing conductors.

Finally select the panel. You can either pick a standard size from Table 314.16(A) or enter the volume given for a non-metallic panel. The results window will show the total fill, percentage of capacity used by the panel, number of additional cables that can be added and the smallest permitted panel that can hold the calculated values.

Standard metal switchboards

Table 314.16(A) lists the volume of standard metal enclosures and the maximum number of cables that each enclosure can accommodate when no other items are contained within it. The values in the "Cables" column are obtained by dividing the volume of the enclosure by the volume of a single cable of this size, with the resulting value rounded down.

Box trade size	cu in	cm3	Max 14 AWG	Max 12 AWG	Max 10 AWG
4 x 1-1/4 round or octagonal	12.5	205	6	5	5
4 x 1-1/2 round or octagonal	15.5	254	7	6	6
4 x 2-1/8 round or octagonal	21.5	353	10	9	8
4 x 4 x 1-1/4 square	18.0	295	9	8	7
4 x 4 x 1-1/2 square	21.0	344	10	9	8
4 x 4 x 2-1/8 square	30.3	497	15	13	12
4-11/16 x 1-1/4 square	25.5	418	12	11	10
4-11/16 x 1-1/2 square	29.5	484	14	13	11
4-11/16 x 2-1/8 square	42.0	689	21	18	16
3 x 2 x 1-1/2 device	7.5	123	3	3	3
3 x 2 x 2 device	10.0	164	5	4	4
3 x 2 x 2-1/4 device	10.5	172	5	4	4
3 x 2 x 2-1/2 device	12.5	205	6	5	5
3 x 2 x 2-3/4 device	14.0	230	7	6	5
3 x 2 x 3-1/2 device	18.0	295	9	8	7
4 x 2-1/8 x 1-1/2 device	10.3	169	5	4	4
4 x 2-1/8 x 1-7/8 device	13.0	213	6	5	5
4 x 2-1/8 x 2-1/8 device	14.5	238	7	6	5
3-1/4 x 2 x 2-1/2 masonry box or gang	14.0	230	7	6	5
3-1/4 x 2 x 3-1/2 masonry box or gang	21.0	344	10	9	8
FS single cover or gang (1-3/4 deep)	13.5	221	6	6	5
FD single cover or gang (2-3/8 deep)	18.0	295	9	8	7
FS multiple cover or gang (1-3/4 deep)	18.0	295	9	8	7
FD multiple cover or gang (2-3/8 deep)	24.0	395	12	10	9

All enclosures not listed here that are less than 100 cubic inches or non-standard enclosures must list their volume. Also recessed boxes, extension boxes and covers must also be listed with a volume which is added to the total enclosure volume.

Common mistakes when calculating the fill volume of housings:

The most common mistake is to calculate each earthing separately which will result in the enclosure appearing overcrowded, even though it meets the actual requirements of the standard. All earthing conductors for equipment share up to five conductors a limit.

Another mistake that inspectors often find is the simple rather than double counting of devices. As each outlet point equates to a double restriction, surface mounted boxes already use up four restrictions before any cable has been run in, even if only two devices are being connected.

There are two other minor errors. Cables that run through the case but aren't connected to it must also be taken into account, while pigtails added to facilitate connection should not be counted at all.

This calculation tool is based on section 314.16 of the NEC and applies to enclosures with a volume up to 100 cubic inches. Local codes and standards used in your jurisdiction take precedence. Enclosure boxes and junction boxes that exceed 100 cubic inches are determined by section 314.28. Have this work inspected by the appropriate authorities before closing the enclosure.

Frequently asked questions

Are grounding cables included in the calculation of the fill volume of a case?: Yes they are taken into account but not individually, all earth cables for devices within a housing share one maximum volume limit based on the largest cable cross section. For every additional cable over four a quarter of the maximum volume limit is added.
Are wire connectors and branch cables included in the calculation of a housing's fill volume?: No they are not taken into account. Wire connectors do not count and extension cables that stay within the enclosure also don't count. A cable only counts towards the maximum volume if it goes in or out of the enclosure.
Why are switches and sockets counted twice?: NEC 314.16(B)(4) assigns a double volume limit to each point of attachment required for the mounting of equipment. This is because the enclosure takes up space that cannot be used for wiring. When two pieces of equipment are mounted in a double box, this equates to four limits.
How many 12 AWG cables can fit in a box that measures 4 by 4 by 1.5 inches?: The box has a volume of 21 cubic inches and one single 12 AWG cable requires 2.25 cubic inches so the box can hold nine cables without other contents. If you add lugs, devices, and ground wires then it reduces to five cables.
Do cardboard rings or extension rings increase the volume of a box?: Yes, if the volume of the ring is given. Before comparing the total fill amount with the box volume, add the volume indicated in the ring or cover frame to the box volume.

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Disclaimer: This calculator is provided for general informational and educational purposes only. Our calculators are under active development, and results may be inaccurate, incomplete, or unsuitable for your situation. Always verify the figures independently and seek advice from a qualified professional before relying on them. We make no warranties and accept no liability for any loss or decision arising from use of this tool.

References

NFPA 70: National Electrical Code
Article 314.16 is the governing text for box volume and box fill calculations.
Wikipedia: National Electrical Code
Background on the code, its three-year revision cycle and how states adopt it.