how to calculate box fill

ponch37300

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

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

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

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.

__________________
Kyle

Just because you can, doesn't always mean you should

jrclen

Great post Bocolo. Good job.

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.

__________________
John

ponch37300

Thanks guys. One question, what are internal clamps?

Bocolo

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

Bocolo

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

Bocolo

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

Bocolo

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

InPhase277

Only in Canada.

chris75

Here are some examples...

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

ponch37300

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

I come up with 17 cu in.

ponch37300

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.