Calculate electrical box fill volume, conductor equivalents, or box size from three inputs and show results in in³, ft³, cm³, L, or m³.
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Electrical Box Fill (Volume) Formula
The basic electrical box fill volume formula is:
- BS = required box volume
- N = total conductor-equivalent count
- D = volume allowance per conductor equivalent
- A = additional volume allowance for devices, fittings, clamps, or other required allowances
If you are solving for the maximum number of conductor equivalents that fit in a known box volume, the formula is:
- floor means the result is rounded down to the nearest whole conductor-equivalent count.
If you are solving for the volume allowance per conductor equivalent, the formula is:
If you are solving for the additional volume allowance, the formula is:
The calculator uses cubic inches as the base unit internally. If you enter cubic feet, cubic centimeters, liters, or cubic meters, the value is converted to cubic inches before the formula is applied, then converted back to your selected output unit.
The calculator can perform four related functions:
- Find required box volume: enter conductor equivalents, volume per conductor equivalent, and any additional allowance.
- Find maximum conductor equivalents: enter box volume, volume per conductor equivalent, and additional allowance. The result is rounded down because you cannot use a fraction of a conductor equivalent.
- Find volume per conductor equivalent: enter box volume, conductor equivalents, and additional allowance.
- Find additional volume allowance: enter box volume, conductor equivalents, and volume per conductor equivalent.
Common Box Fill Volume Allowances by Wire Size
For NEC-style box fill calculations, the volume allowance per conductor equivalent depends on the conductor size. The values below are commonly used for conductors 6 AWG through 18 AWG.
| Conductor Size | Volume Allowance per Conductor Equivalent | Calculator D Value |
|---|---|---|
| 18 AWG | 1.50 in³ | 1.50 |
| 16 AWG | 1.75 in³ | 1.75 |
| 14 AWG | 2.00 in³ | 2.00 |
| 12 AWG | 2.25 in³ | 2.25 |
| 10 AWG | 2.50 in³ | 2.50 |
| 8 AWG | 3.00 in³ | 3.00 |
| 6 AWG | 5.00 in³ | 5.00 |
Typical Conductor-Equivalent Counting Rules
Use the table below to help decide what to enter for N and A. Exact counting can depend on the installation and code edition.
| Item in the Box | Typical Box Fill Treatment | How to Use It in the Calculator |
|---|---|---|
| Insulated conductor that enters and leaves the box | Counts as 1 conductor equivalent | Add 1 to N |
| Pigtail that originates and terminates in the same box | Often does not count as a separate conductor | Usually do not add to N |
| Equipment grounding conductors | Grouped together as one conductor equivalent based on the largest grounding conductor | Add 1 to N, using the proper D value |
| Device yoke, such as a switch or receptacle | Commonly counts as 2 conductor equivalents of the largest conductor connected to it | Either add 2 to N or enter the equivalent volume in A |
| Internal cable clamp or fitting | May require an additional volume allowance | Add the required volume to A |
Example Calculations
Example 1: Find the required box volume
You have 9 conductor equivalents of 12 AWG wire. The volume allowance for 12 AWG is 2.25 in³. There is no extra device or fitting allowance entered separately.
The required box volume is 20.25 in³.
Example 2: Find the maximum conductor equivalents
You have a 22.5 in³ box, 14 AWG conductors, and 4.0 in³ of additional allowance. For 14 AWG, use 2.00 in³ per conductor equivalent.
The box can fit a maximum of 9 conductor equivalents using those inputs.
FAQ
What is a conductor equivalent in box fill?
A conductor equivalent is a counting unit used to convert wires, devices, grounds, clamps, and fittings into required box volume. A simple insulated conductor usually counts as one conductor equivalent. Other items, such as device yokes or grounding conductors, may count differently. After you determine the total equivalent count, multiply it by the required volume allowance for the wire size.
Why does the calculator round down when finding conductor count?
When solving for conductor equivalents, the formula may produce a decimal result. For example, a box might have enough volume for 9.25 conductor equivalents. Since you cannot install 0.25 of a conductor equivalent, the usable maximum is rounded down to 9.
What should you enter for additional volume allowance?
Enter any box-fill volume that is not already included in your conductor-equivalent count. This may include allowances for devices, fittings, clamps, or other items depending on how you are doing the count. If you include those items directly in N, do not also include the same volume in A, or the box fill will be counted twice.
