Enter the required parameters to precisely calculate “Box Fill" requirements for an electrical wiring box.
This box fill calculator precisely estimates the total box fill volumes for electrical utility boxes, prioritizing safety and electrical system reliability in various installations.
By using this tool, you can easily determine the perfect dimensions for your electrical boxes, ensuring they meet the safety standards of the National Electrical Code®.
It is the calculation of the combined volume occupied by conducting wires, grounding wires, and other components in an electrical utility box.
The main objective of this box fill calculation is to prevent overcrowding in electrical boxes because an overloaded box can result in faults, arcing, or even pose a risk of fire.
Follow the below-mentioned steps to calculate box fill:
Now, let's calculate the volume allowances for each component within an electrical box
\(\ A_w =\ n_w\)
And
\(\ V_w =\ A_w\times \ V_{largest\ conductor}\)
Where
In the following table we have provided the conductor wire size and their allowance volumes. For ease, use our wire size calculator and optimize your wire size assessments.
| Size of Conductor (AWG) | Volume Allowance Required Per Conductor (in.³) |
| 18 | 1.5 |
| 16 | 1.75 |
| 14 | 2 |
| 12 | 2.25 |
| 10 | 2.5 |
| 8 | 3 |
| 6 | 5 |
Source Better Homes & Gardens
If you are using an electrical box with internal clamps, then:
\(\ A_c=\ 1\)
Otherwise (no clamps):
\(\ A_c=\ 0\) \(\ V_c =\ A_c\times \ V_{largest\ conductor}\)
Where
If there are one or more luminaire studs or hickey inside the box:
\(\ A_s=\ 1\)If
there are no luminaire studs or hickey inside the box:
\(\ A_s=\ 0\)\(\ V_s =\ A_s\times \ V_{largest\ conductor}\)
Where
\(\ A_d = 2 * n_d\)
Where
\(\ V_d =\ A_d\times \ V_{largest\ conductor}\)
Where
If 1–4 grounding conductors that are entering in the electrical box,
\(\ A_g=\ 1\)
If 5 or more:
\(\ A_g=\ 1+\dfrac{n_g−4}{4}\)
Or
\(\ A_g=\dfrac{n_g}{4}\)
Where
In equation form, it can be expressed as:
\(\ V_g =\ A_g\times \ V_{largest\ ground\ Wire}\)
Where
\(\ V_{total} =\ V_w+V_c+V_s+V_d+V_g\)
If the largest conductor is equal to the largest grounding wire size then find the total volume allowance and multiply it by the free space required for the largest conductor.
\(\ A_{total} =\ A_w+A_c+A_s+A_d+A_g\)
\(\ V_{total} =\ A_{total}\times \ V_{largest\ conductor}\)
Where
Solution:
Given that:
Largest conducting wire size = 12 AWG = 2.25 cu inches
Number of conducting wires = 10
Devices = 5
Grounding conductors = 10
Largest Grounding conductor wire size = 12 AWG = 2.25 cu inches
\(\ A_w =\ n_w\)
\(\ A_w =\ 10\)
Conductor Fill Volume:
\(\ V_w =\ A_w\times \ V_{largest\ conductor}\)
\(\ V_w =\ 10\times \ 2.25\) \(\ V_w =\ 22.5\)
Device Fill Volume allowance:
\(\ A_d = 2 * n_d\)
\(\ A_d =\ 2\times \ 5\)
\(\ A_d =\ 10\)
Device Fill Volume:
\(\ V_d =\ A_d\times \ V_{largest\ conductor}\)
\(\ V_d =\ 10\times \ 2.25\) \(\ V_d =\ 22.5\)
Equipment Grounding Fill Volume Allowance:
\(\ A_g=\dfrac{n_g}{4}\)
\(\ A_g=\dfrac{10}{4}\)
\(\ A_g=\ 2.5\)
Grounding Wire Volume:
\(\ Largest\ Grounding\ Wire\ Size =\ 12\ AWG\)
\(\ Grounding\ Wire\ Volume =\ 12\ AWG =\ 2.25\ cu\ inches\)
Equipment Grounding Fill Volume:
\(\ V_g =\ A_g\times \ V_{largest\ ground\ Wire}\)
\(\ V_g = 2.5*2.25\) \(\ V_g =\ 5.625\ cu\ inches\)
Total Volume Allowance Needed:
\(\ A_{total} =\ A_w+A_c+A_s+A_d+A_g\) \(\ A_{total} =\ 10+0+0+10+2.5\)
\(\ A_{total} =\ 22.5\)
Total Box Fill Volume:
\(\ V_{total} =\ V_w+V_c+V_s+V_d+V_g\)
\(\ V_{total} =\ 22.5+0+0+22.5+5.625\)
\(\ V_{total} =\ 50.625\ cu\ inches\)
Ensure precision by cross-checking your manual calculations with our electrical box fill calculator and optimize your results for a safe electrical system.
The box fill calculations are very important in electrical installations for many reasons, including:
A standard single-gang box has 18 cubic inches of space, which can contain:
These figures apply to boxes that house wires. If you have to install a box with devices, deduct two wires from the specified counts.
Yes, the grounding wires are also included in calculating box fill volumes. According to the NFPA 70: National Electrical Code® 2020, a single volume allowance is specified for one to four equipment grounding conductors or equipment jumping conductors. Every additional grounding conductor needs an extra 1/4 volume allowance.
The box fill code, outlined in NEC 314.16 states that the boxes and conduit bodies must have the appropriate approved size to provide free space to all the enclosed conductors. The volume of the box calculated as per 314.16(A) should never be less than the fill calculation as determined in 314.16(B) in any case.
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