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.
