how to calculate box fill

Plumbvoltage

7 current carrying 7x2(14ga)=14
All grounds=1
switch=2

14+1+2=17 (providing the box doesn't have internal clamps)

Plumbvoltage

You were right with 20cu in

All grounds count as 1 conductor which counts as 2
The switch counts as 2 conductors which counts as 4
If you are using 14 ga.

7x2=14
1x2=2
2x2=4

14+2+4=20

Scuba_Dave

I always go oversize on boxes since I use 2x6 walls
After trying to insert what fits under code its a good idea to go slightly larger if you acn

Bocolo

2 14/2 = 4 wires at 1 volume = 4 x 2.0 = 8
1 14/3 = 3 wires at 1 volume = 3 x 2.0 = 6
all grounds = 1 volume = 1 x 2.0 = 2
One switch = 2 volumes = 2 x 2.0 = 4

Total is 20 cubic inches. I am still new at this but I believe this is right.

SD515

I came up with 20 cu in.

ponch37300

Thanks everyone. I understand it now. Guess I had it right the first time and when I went back to double check I only counted the switch as one volume. Thanks again everyone for sharing your knowledge!

jrclen

I think it's great that this thread will be found during searches and give accurate and good advice to many people in the future.

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John

Bocolo

It surprises me that someone does not have a "calculator" for this. I made an excel spreadsheet which I am still testing but so far it has worked great. Just plug in numbers and you get a result. I did see some software for all types of calculations but it is a bit expensive.

jrclen

People that do it everyday barely think about it. For instance we know that a 2 gang switch box (32 CI) is ok with 2 14-3 and 2 14-2 along with 2 three way switches. That would be two 3 way circuits at the 2nd set of switches, or at the 1st set of switches with the line set continuing to another box. Or a single gang receptacle box (20.3) is ok with 3 12-2 and a receptacle outlet. And those are the most common filled up situations. Most boxes would have less wires that that in a typical circuit layout. Those are the ones which approach the fill limit.

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John

Bocolo

makes sense. The more I work with this the more "automatic" it becomes. It makes perfect sense that if you work with this on a daily basis it would just be second nature to you.