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Electrical Box Fill Calculation

WHILE EXTREME CARE HAS BEEN IMPLEMENTED IN THE PREPARATION OF THIS SELF-HELP DOCUMENT, THE AUTHOR AND/OR PROVIDERS OF THIS DOCUMENT ASSUMES NO RESPONSIBILITY FOR ERRORS OR OMISSIONS, NOR IS ANY LIABILITY ASSUMED FROM THE USE OF THE INFORMATION, CONTAINED IN THIS DOCUMENT, BY THE AUTHOR and / OR PROVIDER.

Volkswagon crowd At least some of us may remember back in our high school days trying to see how many people can be crammed into a Volkswagen Beatle.
wire overload box

wire overload box
Although the pictures here are by far the most extreme box fill code violations, some common sense along with electrical code requirements limits the number of wires that can be put into an electrical box.

There are a number of reasons for this such as...

a) Allowance for proper heat dissipation

b) Ability to trace wire cables.

c) Damage to connections as well as the outlet or switch from overcrowding and pressure from wires pushed against it.

So how many wires can be safely put in an electrical box....

1st....

Count the numbers....

All wire clamps together inside the box collectively count as 1

Each power conductor counts as 1.
(all wires count even if unused)

All equipment grounding wires (usually bare) collectively count as 1

Each Switch or outlet count as 2

Pigtails do not count.

Now add up all of the above
2nd...

According to the National Electrical Code (NEC - United States) each item depending on the gage of wire


14 Awg. requires 2 Cu. In. per conductor

12 Awg. requires 2.25 Cu. In. per conductor

10 Awg. requires 2.5 Cu. In. per conductor

8 Awg. requires 3 Cu. In. per conductor

6 Awg. requires 5 Cu. In. per conducto

Now take the number you came up with in the 1st column and multiply it by the cubic inch required [listed in 2nd column] for the gage wire you are using.


In the event that multiple gages are in use then...

a) The largest wire connected TO A GIVEN DEVICE (switch/receptacle) is used to determine that device's volume count.

b) For clamp fill, it is counted based on the largest conductor in the box (power or ground).

c) The fill for the equipment grounding conductors (bare or green) is based on the one largest GROUNDING conductor, not POWER conductor.

d) The actual power conductors [ungrounded conductor (hot) and grounded conductors (neutrals)] are individually calculated by each size wire.

Now select the electrical box with cubic inch size required.


By: Donald Kerr
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