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Old 12-09-2008, 10:49 PM   #1
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Calculating box size requirements


I've got a few questions regarding this:

First, a normal outlet or switch is counted as "2" when calculating box size requirements, but what about a GFCI? It is much larger than a standard outlet so should it be counted as more?

Second, is it common to use a double-gang size box for a single outlet or GFCI? Without doing that, I don't see how I'm going to get enough cubic inches.

I'm currently remodeling my kitchen and have 4" by 2" by 2 1/8" boxes installed in most locations. They are 16.5 cubic inches, and are the largest single-gang metal boxes I could find at Home Depot or Lowe's.

If I'm figuring this correctly, for the outlets in the middle of a run, there is:

4 (one for each hot and neutral entering the box)
2 (for the outlet itself)
2 (one for each cable clamp)
1 (one for ALL ground wires - even though there are 4, plus a wirenut)

For a total of 9, multiplied by 2.25 (for 12 gauge wire), means I need a box of at least 20.25 cubic inches for this example?

Any help with figuring this out would be appreciated. Thanks!

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Old 12-09-2008, 11:39 PM   #2
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Calculating box size requirements


For code purposes the GFCI is treated the same as a plain receptacle. It is larger and may be a very tight fit even though the code's box fill requirements are satisfied. You only need one allowance for cable clamps, no matter how many there are; if the clamps are installed on the outside of the box, they don't need to be counted at all. A single-gang deep box measuring 3"x2"x3-1/2" has a volume of 18 cu. in. and will meet your needs, although you'll have to be careful to neatly fold your wires into the box to make everything fit.

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Old 12-09-2008, 11:54 PM   #3
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Calculating box size requirements


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Originally Posted by Matsukaze View Post
For code purposes the GFCI is treated the same as a plain receptacle. It is larger and may be a very tight fit even though the code's box fill requirements are satisfied. You only need one allowance for cable clamps, no matter how many there are; if the clamps are installed on the outside of the box, they don't need to be counted at all. A single-gang deep box measuring 3"x2"x3-1/2" has a volume of 18 cu. in. and will meet your needs, although you'll have to be careful to neatly fold your wires into the box to make everything fit.
So for a standard NM type clamp for a metal box (the kind with the two screws on the outside and just the small threaded part and lock ring that extends to the inside of the box) -- those clamps do not need to be counted at all? If that is the case then my most crowded boxes will be ok since that only adds up to 15.75, and my boxes are 16.5 cu. in.

I saw the 18 cu. in. boxes, but they were plastic. I don't think it's against code to use plastic in my area, but all the existing boxes are metal and I was just trying to keep them all the same. Do they make 18 cu. in. single gangs in metal?

Thanks a lot!
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Old 12-10-2008, 07:58 AM   #4
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Calculating box size requirements


IMO, if you're concerned about box fill, you are better off using plastic boxes. I think they may actually be a little larger than their metal counterparts, and you don't have to worry about conductors getting cut on the clamps or anything in the metal box.
Of course with the plastic ones, the cable clamping for NM isn't as secure in the box as for metal ones, so you need to make sure you staple the cable per code before it enters the box.
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Old 12-10-2008, 08:29 AM   #5
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Calculating box size requirements


I would switch and install the 22 or 23 cubic inch plastic boxes. Less need to fight to get everything into the box.
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Old 12-10-2008, 11:44 AM   #6
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Calculating box size requirements


To meet (US) code the GFCI receptacle counts as two points and all of the internal clamps together count one point, all of the ground wires together count one point, and pigtails don't count, but you should still be thinking in a practical sense. If it looks like it will be difficult stuffing the stiff solid Romex conductors inside, you should have a bigger box.
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Old 12-10-2008, 09:15 PM   #7
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Calculating box size requirements


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Originally Posted by Matsukaze View Post
if the clamps are installed on the outside of the box, they don't need to be counted at all.

Does this type of clamp need to be counted for calculations?

http://shopping.msn.com/specs/nm-cab...-connector-3-8

This being installed with the larger part of the clamp outside the box, and the smaller part with the lockring on the inside.

Thanks!
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Old 12-11-2008, 12:55 PM   #8
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Calculating box size requirements


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Originally Posted by dc4nomore View Post
clamp outside the box, and the smaller part with the lockring on the inside.
No, that's free.
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Old 12-11-2008, 02:15 PM   #9
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Calculating box size requirements


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No, that's free.
Sorry but I'm not 100% sure what you mean. By free do you mean it doesn't have to be counted?
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Old 12-11-2008, 02:24 PM   #10
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Originally Posted by dc4nomore View Post
Sorry but I'm not 100% sure what you mean. By free do you mean it doesn't have to be counted?
Yes.

Heh, you need at least 5 characters in a message.
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Old 12-11-2008, 03:14 PM   #11
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Calculating box size requirements


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Yes.
Great, thanks!


Quote:
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Heh, you need at least 5 characters in a message.
What?
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Old 12-12-2008, 12:27 AM   #12
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Calculating box size requirements


Quote:
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Quote:
Originally Posted by Gigs View Post
Heh, you need at least 5 characters in a message.
What?
He's referring to a setting in the forum/message-board.
You need to have five characters in a post in order for the forum to process and display a post.

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Old 12-12-2008, 07:41 AM   #13
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Originally Posted by iMisspell View Post
He's referring to a setting in the forum/message-board.
You need to have five characters in a post in order for the forum to process and display a post.

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Oh, ok. Gotcha. Thanks.

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