Calculating Box Size Requirements - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Calculating box size requirements
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

12-09-2008, 11:49 PM   #1
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

## 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!

12-10-2008, 12:39 AM   #2
Member

Join Date: Dec 2007
Posts: 84
Rewards Points: 75

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.

12-10-2008, 12:54 AM   #3
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

Quote:
 Originally Posted by Matsukaze 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!

 12-10-2008, 08:58 AM #4 Member   Join Date: Oct 2008 Location: New Jersey, USA Posts: 1,362 Rewards Points: 820 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. __________________
 12-10-2008, 09:29 AM #5 Licensed electrician   Join Date: Sep 2007 Location: Maryland Posts: 10,442 Rewards Points: 1,038 I would switch and install the 22 or 23 cubic inch plastic boxes. Less need to fight to get everything into the box.
 12-10-2008, 12:44 PM #6 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,906 Rewards Points: 1,418 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.
12-10-2008, 10:15 PM   #7
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

Quote:
 Originally Posted by Matsukaze 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!

12-11-2008, 01:55 PM   #8
DIYer

Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500

Quote:
 Originally Posted by dc4nomore clamp outside the box, and the smaller part with the lockring on the inside.
No, that's free.

12-11-2008, 03:15 PM   #9
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

Quote:
 Originally Posted by Gigs 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?

12-11-2008, 03:24 PM   #10
DIYer

Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500

Quote:
 Originally Posted by dc4nomore 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.

12-11-2008, 04:14 PM   #11
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

Quote:
 Originally Posted by Gigs Yes.
Great, thanks!

Quote:
 Originally Posted by Gigs Heh, you need at least 5 characters in a message.
What?

12-12-2008, 01:27 AM   #12
Theres more then one way.

Join Date: Jun 2007
Location: N.Y.
Posts: 244
Rewards Points: 150

Quote:
Originally Posted by dc4nomore
Quote:
 Originally Posted by Gigs 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.

_
__________________
.
~ What was once an opinion, became a fact, to be later proven wrong ~

My current learning experience - 22x22 addition - Work shop, music-room, master bedroom above..

12-12-2008, 08:41 AM   #13
Member

Join Date: Sep 2008
Location: Ohio
Posts: 177
Rewards Points: 150

Quote:
 Originally Posted by iMisspell 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. _
Oh, ok. Gotcha. Thanks.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Doug Crf General DIY Discussions 20 02-26-2011 12:29 AM justmefixin HVAC 2 09-20-2008 07:59 PM hhiibel Electrical 14 08-08-2008 01:19 PM slatergrl Flooring 8 07-16-2008 11:08 AM Skydmark1 Electrical 12 12-15-2007 06:00 PM

Top of Page | View New Posts