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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.
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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. |

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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.
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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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