# how to calculate box fill



## ponch37300 (Nov 27, 2007)

How do I calculate box fill? How do you turn wires into cubic inches? Is a 12-2 wire a certain amount of cubic inches, or a 14-2, or 14-3? Do you have to count for the switch or recep? Just wondering how to count all the wires up so I don't exceed the box fill. Thanks


----------



## Bob Mariani (Dec 1, 2008)

Use the table 314.16(A) for conductor fill in boxes. Each wire is added, not each cable then compared to the fill allowed in the type and size box you are using. Some cables are counted and some like the grounds are added and counted as one... Read all of 314.16


----------



## Bocolo (Jan 24, 2009)

Ponch,
I am not an electrician but this is what I have read:

Each #14 wire 2.00 CI
Each #12 wire 2.25 CI
All Ground wires count as 1 volume
All Receptacles and Switches count as 2 volumes
All internal clamps count as 1 volume

Example:

2 12/2 with ground cables entering a box through internal clamps for a duplex receptacle. 

4 12 awg circuit wires 4 x 2.25 = 9.0 Cubic inches
2 12 awg ground wires (count as one) 1 x 2.25 = 2.25
2 internal clamps (count as one) 1 x 2.25 = 2.25
Duplex receptacle (count as two) 2 x 2.25 = 4.50

Add them all up = 18.00 cubic inches.

If you were using 14awg you would multiply times 2.00. You need to calculate based on the largest wire and you would also need to calculate for each fixture wire. If the fixture has 4 or fewer fixture wires with one ground and are entering the box from a domed fixture canopy you do not need to count them.

Again I am not an electrician. This is what I read on the 2008 code. Just trying to contribute. I hope I explained this correctly. Some of the other guys in here will most likely do a better job at explaining than I did.


----------



## SD515 (Aug 17, 2008)

Bocolo said:


> ...
> All Receptacles and Switches count as 2 volumes
> ...


This should be worded as _each_ device (receptacle, switch, etc.) counts as 2 volumes. Sorry, I don't have my code book handy to be able to word it specifically.


----------



## jrclen (Feb 20, 2008)

Bocolo said:


> Again I am not an electrician. This is what I read on the 2008 code. Just trying to contribute. I hope I explained this correctly. Some of the other guys in here will most likely do a better job at explaining than I did.


Great post Bocolo. Good job. :thumbsup:

Lets add that #10 is 2.5 CI and that wires that don't enter or leave the box such as pig tails, are not counted.


----------



## ponch37300 (Nov 27, 2007)

Thanks guys. One question, what are internal clamps?


----------



## Bocolo (Jan 24, 2009)

Some metal boxes have internal clamps to attach wire to a box.


----------



## Bocolo (Jan 24, 2009)

SD515 said:


> This should be worded as _each_ device (receptacle, switch, etc.) counts as 2 volumes. Sorry, I don't have my code book handy to be able to word it specifically.


You are correct. I think it is worded "for each yoke or strap containing devices". Thanks.


----------



## Bocolo (Jan 24, 2009)

jrclen said:


> Great post Bocolo. Good job. :thumbsup:
> 
> Lets add that #10 is 2.5 CI and that wires that don't enter or leave the box such as pig tails, are not counted.


Thanks. It took me a while to understand this but I finally get it.:thumbup:


----------



## Bocolo (Jan 24, 2009)

One question I still have regarding this. Should not "wirenuts" be included in the calculation? Thanks.


----------



## InPhase277 (Feb 9, 2008)

Bocolo said:


> One question I still have regarding this. Should not "wirenuts" be included in the calculation? Thanks.


Only in Canada.


----------



## chris75 (Aug 25, 2007)

Here are some examples...

Read this as well....http://ecmweb.com/nec/electric_box_fill_calculations/index.html


----------



## ponch37300 (Nov 27, 2007)

Thanks for the pics Chris. So plastic boxes don't have internal clamps right? 

So if you have 2 14-2 wires and one 14-3 wire in a single gang plastic box with a switch you would need a 20 cubic inch box right?


----------



## Plumbvoltage (Dec 2, 2008)

ponch37300 said:


> So if you have 2 14-2 wires and one 14-3 wire in a single gang plastic box with a switch you would need a 20 cubic inch box right?


I come up with 17 cu in.


----------



## ponch37300 (Nov 27, 2007)

Wouldn't (2) 14-2 wires and (1) 14-3 be 7 wires plus one ground plus one switch times 2 for 14 gauge wire be 18 cubic inches? Guess I was wrong with 20.


----------



## Plumbvoltage (Dec 2, 2008)

ponch37300 said:


> Wouldn't (2) 14-2 wires and (1) 14-3 be 7 wires plus one ground plus one switch times 2 for 14 gauge wire be 18 cubic inches? Guess I was wrong with 20.


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

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


----------



## Plumbvoltage (Dec 2, 2008)

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 (Jan 16, 2009)

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 (Jan 24, 2009)

ponch37300 said:


> Wouldn't (2) 14-2 wires and (1) 14-3 be 7 wires plus one ground plus one switch times 2 for 14 gauge wire be 18 cubic inches? Guess I was wrong with 20.


 
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 (Aug 17, 2008)

ponch37300 said:


> Wouldn't (2) 14-2 wires and (1) 14-3 be 7 wires plus one ground plus one switch times 2 for 14 gauge wire be 18 cubic inches? Guess I was wrong with 20.


I came up with 20 cu in.


----------



## ponch37300 (Nov 27, 2007)

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 (Feb 20, 2008)

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.


----------



## Bocolo (Jan 24, 2009)

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 (Feb 20, 2008)

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.


----------



## Bocolo (Jan 24, 2009)

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.


----------

