Residential Wiring - The Right Way!
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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 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....
Count the numbers....
All wire clamps together inside the box collectively count as 1
conductor counts as 1.
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
According to the National Electrical Code
(NEC - United States) each item depending on the gage of
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.
clamp fill, it is counted based on the largest conductor in the box
(power or ground).
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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