Electrical Box Fill Calculation
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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.
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
a) Allowance for proper heat
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.
many wires can be safely put in an electrical box....
Count the numbers....
All wire clamps together
inside the box collectively count as 1
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
Pigtails do not count.
Now add up all of the above
According to the National Electrical Code
(NEC - United States) each item depending on the gage of
14 Awg. requires 2 Cu. In.
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 conductor
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.
event that multiple gages are in use then...
largest wire connected TO A GIVEN DEVICE (switch/receptacle) is used
to determine that device's volume count.
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
actual power conductors [ungrounded conductor (hot) and grounded
conductors (neutrals)] are individually calculated by each size
Now select the electrical box
with cubic inch size required.
Written: June 21, 2013
Read / Released: July 10, 2013
By: Donald Kerr