Box Fill Calculator | Entered Volume and Allowance Worksheet

Inputs

Note

Entered box volume (in^3) Enter the box volume from the product record or project source. Conductor count (conductors) Enter the count represented by the entered conductor allowance. Conductor allowance (in^3) Enter the reviewed allowance per conductor. Device yoke count (yokes) Enter the device-yoke count. Device allowance (in^3) Enter the reviewed allowance per device yoke. Clamp count (clamps) Enter the entered clamp count. Clamp allowance (in^3) Enter the reviewed allowance per clamp. Grounding allowance (in^3) Enter the reviewed grounding allowance. Allowance source reference Enter the source record for the box and allowance values.

Result

Formulas

\(V_{\mathrm{conductors}} = N_{\mathrm{conductors}} \times V_{\mathrm{allowance}}\)
\(V_{\mathrm{fill}} = V_{\mathrm{conductors}} + V_{\mathrm{device}} + V_{\mathrm{clamp}} + V_{\mathrm{grounding}}\)
\(V_{\mathrm{remaining}} = V_{\mathrm{box}} - V_{\mathrm{fill}}\)
\(\mathrm{fill\ percent} = \frac{V_{\mathrm{fill}}}{V_{\mathrm{box}}} \times 100\)

A box fill calculation compares the volume occupied by conductors, device yokes, internal clamps, and grounding conductors with the available volume of an electrical box. The key output is Remaining box volume: the entered box volume minus the total entered fill volume.

This worksheet is used during branch-circuit and feeder layout to document the volume assumptions supporting a junction box, outlet box, switch box, or device box selection. A positive remaining volume indicates that the entered allowances total less than the entered box volume. A negative result indicates that the entered allowances exceed the entered box volume.

The calculation does not select a box, identify conductor volume allowances, determine device-yoke allowances, or establish NEC compliance. Those allowance values must be reviewed from the applicable project record, product information, and code requirements before they are entered.

Entered Allowances

The worksheet uses the following entered values:

Input	Electrical purpose
Entered box volume (in³)	The usable box volume taken from the product record, box schedule, or project source
Conductor count (conductors)	The count represented by the entered conductor allowance
Conductor allowance (in³)	The reviewed volume allowance assigned to each counted conductor
Device yoke count (yokes)	The number of device yokes included in the box-fill review
Device allowance (in³)	The reviewed volume allowance assigned to each device yoke
Clamp count (clamps)	The entered number of applicable clamps
Clamp allowance (in³)	The reviewed volume allowance assigned to each clamp
Grounding allowance (in³)	The reviewed total allowance entered for grounding conductors and bonding conductors
Allowance source reference	A traceable source record for the box and allowance values, such as a box schedule or reviewed worksheet

A conductor count should represent only the conductors covered by the entered Conductor allowance. When a box contains multiple conductor sizes, insulation types, or allowance categories, the entered values must already reflect the reviewed allocation being documented. The worksheet does not classify conductors by AWG, kcmil, insulation temperature rating, current-carrying status, or circuit function.

Device yokes and internal clamps are entered separately because their volume allowances are not part of the basic conductor-count multiplication. The Grounding allowance is entered as a single reviewed volume rather than calculated from a grounding-conductor count.

Calculation Method

The worksheet calculates each entered component volume, then compares their sum with the entered box volume.

\(\displaystyle \text{Conductor volume} = \text{Conductor count} \times \text{Conductor allowance}\)

\(\displaystyle \text{Device volume} = \text{Device yoke count} \times \text{Device allowance}\)

\(\displaystyle \text{Clamp volume} = \text{Clamp count} \times \text{Clamp allowance}\)

\(\displaystyle \text{Total entered fill volume} = \text{Conductor volume} + \text{Device volume} + \text{Clamp volume} + \text{Grounding allowance}\)

\(\displaystyle \text{Remaining box volume} = \text{Entered box volume} - \text{Total entered fill volume}\)

\(\displaystyle \text{Entered fill percentage} = \frac{\text{Total entered fill volume}} {\text{Entered box volume}} \times 100\)

The result is expressed in cubic inches and percent. Entered fill percentage is a quick way to see how much of the entered box capacity has been allocated; it does not independently establish whether the installation is acceptable.

Calculation Example

For an entered box volume of 22.5 in³, the reviewed allowances are:

Item	Entered value	Calculation	Result
Conductors	6 conductors at 2 in³	`6 times 2`	12 in³
Device yokes	1 yoke at 4 in³	`1 times 4`	4 in³
Clamps	1 clamp at 2 in³	`1 times 2`	2 in³
Grounding allowance	2 in³	Entered directly	2 in³

\(\displaystyle \text{Total entered fill volume} = 12 + 4 + 2 + 2 = 20\text{ in}^3\)

\(\displaystyle \text{Remaining box volume} = 22.5 - 20 = \mathbf{2.5\text{ in}^3}\)

\(\displaystyle \text{Entered fill percentage} = \frac{20}{22.5}\times100 = \mathbf{88.8889\%}\)

The results are:

Conductor volume: 12 in³
Device volume: 4 in³
Clamp volume: 2 in³
Total entered fill volume: 20 in³
Remaining box volume: 2.5 in³
Entered fill percentage: 88.8889%
Record comparison: Entered values comparable

The positive remaining volume means the entered allowances are 2.5 in³ below the entered box volume. It does not confirm that the conductor count or each allowance was selected correctly.

Field Verification

Box-fill arithmetic follows conductor routing and box layout decisions. Review the actual installation condition before relying on the entered result:

Verify the box’s marked or documented volume against the Entered box volume.
Verify every conductor included in the enclosure, including pass-through conductors, splices, pigtails, equipment grounding conductors, and bonding conductors, under the applicable counting rules.
Confirm whether device yokes, fittings, or internal clamps require the entered allowances used in the worksheet.
Use the correct reviewed allowance for the conductor size and installation condition; do not assume all conductors receive the same allowance merely because they occupy the same box.
Check that the selected box also works physically for conductor bending, device depth, cable entries, raceway connections, cover clearance, and service access.
Coordinate box selection with raceway fill, conductor ampacity, adjustment factor and correction factor decisions, terminal rating limits, voltage-drop design, and branch-circuit or feeder routing where applicable.
Obtain the required determination from the AHJ or responsible electrical professional for code compliance and project approval.

Calculation Boundary

This is an entered-volume worksheet. It sums entered conductor, device, clamp, and grounding allowances and compares that sum with the entered box volume.

It does not perform NEC table lookups, determine box approval, identify conductor classifications, validate the allowance source reference, or produce a code-compliance result.

For related enclosure and raceway checks, see the Conduit Fill Calculator, Conduit Body Fill Calculator, and Wireway Fill Calculator.