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Box Fill


If the box only contains wires, and all wires are the same size, then you can use the table at 314.16(A).

Step 1

Figure out the cubic inches (ci) of your box using314.16(A) if it's not marked on the box itself. You can add the cubic inches of the mudring also.


Step 2


Calculate the cubic inches for the conductors themselves.
Remember, you only count the ground once.  
You count 1 for the largest equiptment ground conductor.
You also count 1 for the largest isolated grounding conductor .
If a conductor runs unbroken in the box and then back out, it counts as only 1 unless it is a loop longer then 12 inches, in such case it counts as 2.
If a conductor doesn't leave the box, it doesn't count.
Multiply the number of conductors for any gives size by the values in 314.16(B)
Size of Conductor (AWG) Free space within
box for each conductor
18 1.5
16 1.75
14 2
12 2.25
10 2.5
8 3
6 5

Step 3


Add for other adjustments such as devices, studs, and internal cable clamps.
Studs: count as 1 for the largest conductor entering box
One or more cable clamps: Count 1 for largest conductor
Devices: count 2 conductors for the largest conductors that connect to the device per gang.  If a devices needs a 2 gang box, then it's 4 conductors.  If it fits a 1 gange box/mudring, then it counts as 2 conductors.



Example:


Quantity Size Wire Vol / Conductor Total CU
Pipe 1




4 12 2.25 9

2 10 2.5 5
Pipe 2




3 12 2.25 6.75
Ground 1 10 2.5 2.5
IsoGround 1 12 2.25 2.25
Switch 2 12 2.25 4.5








Total
30

This example would ft into a 4 square deep box with 30.3 cubic inches.


Table 314.16(a) can be found at:
http://www.appletonelec.com/pdf/1999.pdf

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