# Electrical box size

• posted on March 15, 2010, 3:01 pm

Do I have this right?
Three 12/2 NM cables going into a plastic box with an outlet
conductors 6 grounds 1 Oulet 3
Total 10 10 times 2.25 = 22.5 cubic inch box needed.

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<%-name%>
• posted on March 15, 2010, 3:43 pm

Devices are 2 per strap. So for a single gang duplex receptacle, that's just 2.

Total 9, as long as you don't have any cable clamps that extend into the box.

9 * 2.25 = 20.25
Cheers, Wayne

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<%-name%>
• posted on March 15, 2010, 5:26 pm

Thanks Wayne. I read 2 and knew it was 2 but don't know why I wrote 3 for the outlet.
So a dimmer switch or a GFCI outlet count the same as a single pole switch?
It also sounds like you don't have to count wire nuts or pigtails.
I found the table online here: http://ftp.resource.org/bsc.ca.gov/title24_part03_dice/title24_part03_page0219.pdf

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<%-name%>
• posted on March 15, 2010, 3:57 pm

Wayne gave you the math. Personally I just read the numbers printed on the boxes. For the last 20 years or more they tell you right on the box how many and what size wire. Actually I think it is stamped inside the box.
Pigtails that start and end in the box do not count.
I recall from memory that a standard switch box can only handle 2-3 wire cables. I think the deeper box can handle 3.
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Colbyt

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<%-name%>
• posted on March 15, 2010, 4:13 pm

if by "standard" you mean a 2-1/2" deep box, if you have a device in it, you can only use *one* cable.
What this means to me is, I need to replace a lot of boxes in my house, because I am repulling a lot of stuff, and those boxes that met code when they were installed now no longer do.
nate

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<%-name%>
• posted on March 15, 2010, 5:04 pm

if by "standard" you mean a 2-1/2" deep box, if you have a device in it, you can only use *one* cable.
What this means to me is, I need to replace a lot of boxes in my house, because I am repulling a lot of stuff, and those boxes that met code when they were installed now no longer do.
nate
Not sure where you are getting this information. Perhaps your local codes are different. Maybe something changed in the last 12 months.
I had a fully inspected, both rough and final about 12 months ago. Devices did not enter into the equation. Two #12 cables per standard switch box, just like the numbers printed on the box. Three permitted in the deeper box. These are the plastic/composite boxes.
There is no requirement to upgrade anything unless you pull new wire to a box, then you need to do so. Yes there are a lot of old metal boxes that do not meet what I believe to be the current code.
Colbyt

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<%-name%>
• posted on March 15, 2010, 6:17 pm

.
http://www.selfhelpandmore.com/home-wiring-usa/definitions-calculations/box-fill-calculations-2002.php
The Chart Mentioned Says That (NEC Article 314.16.B):
14 Awg. = 2 Cu. In. per conductor, 12 Awg. = 2.25 Cu. In. per conductor 10 Awg. = 2.5 Cu. In. per conductor 8 Awg. = 3 Cu. In. per conductor 6 Awg. = 5 Cu. In. per current carrying conductor counted.
are you counting two conductors for the device yoke, one for the clamps, and one for the ground? A 14/2 WG (3 "conductors",) clamp (1,) and receptacle (2,) requires 12 in^3; another cable would bump that up to 16 in^3. (add 2 "conductors" for the 14/2, ground still is only counted once.) If I were using 12/2WG then that would be 13.5 and 18 in^3 respectively.
http://www.hubbellonline.com/wiring/raco_bell/pdf/a43.pdf
per the above sheet, a 2-1/2" deep Raco metal switch box is 12.5 in^3

Right, that's the issue, I have 2-1/2" switch boxes being used in the middle of a run (e.g. 2x 14/2 romex *without* ground in each box, except at the end of a run) if I replace the old cable with 14/2WG then technically that whole box needs to meet current code, so I have to bust it out and replace it with a deeper box.
nate

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<%-name%>
• posted on March 15, 2010, 8:29 pm

http://www.selfhelpandmore.com/home-wiring-usa/definitions-calculations/box-fill-calculations-2002.php
The Chart Mentioned Says That (NEC Article 314.16.B):
14 Awg. = 2 Cu. In. per conductor, 12 Awg. = 2.25 Cu. In. per conductor 10 Awg. = 2.5 Cu. In. per conductor 8 Awg. = 3 Cu. In. per conductor 6 Awg. = 5 Cu. In. per current carrying conductor counted.
are you counting two conductors for the device yoke, one for the clamps, and one for the ground? A 14/2 WG (3 "conductors",) clamp (1,) and receptacle (2,) requires 12 in^3; another cable would bump that up to 16 in^3. (add 2 "conductors" for the 14/2, ground still is only counted once.) If I were using 12/2WG then that would be 13.5 and 18 in^3 respectively.
http://www.hubbellonline.com/wiring/raco_bell/pdf/a43.pdf
per the above sheet, a 2-1/2" deep Raco metal switch box is 12.5 in^3

Right, that's the issue, I have 2-1/2" switch boxes being used in the middle of a run (e.g. 2x 14/2 romex *without* ground in each box, except at the end of a run) if I replace the old cable with 14/2WG then technically that whole box needs to meet current code, so I have to bust it out and replace it with a deeper box.
*If you have just drywall for your finished wall you can probably pry the old 2.5" box out without too much damage to the wall. To replace it you could take a 3.5" gem box, drill a hole in the side and stick it in the same wall opening. Mount it with some Madison bars and shoot one screw through the hole. The other choice is to install a plastic old work box which will require a bigger opening in the wall. For more info: http://www.mrelectrician.tv/questions/question-InstallOutletBox.html

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<%-name%>
• posted on March 15, 2010, 10:25 pm
On 03/15/2010 04:29 PM, John Grabowski wrote:

http://www.selfhelpandmore.com/home-wiring-usa/definitions-calculations/box-fill-calculations-2002.php
The walls are plaster, but that is what I plan to do. I have already done this with several boxes for other reasons (loose boxes in two instances; another instance I was trying to replace a 2-wire cable with a 3-wire cable so I could have a 3-way switch for the light at teh bottom of the basement stairs, and I couldn't fish the cable without removing the box) so I have the technique down. It just looks like to pass my inspection I need to R&R *every* recep box in the area in which I'm upgrading unless it is at the end of a circuit, whether I need to do so to fish the cables or not. C'est la vie, I was just posting to explain what my (and my local inspector's) interpretation of recent box fill codes was, and why I said that a switch box could only have one cable.
Per the above, it appears that even a 14/3WG switch leg (e.g. for a ceiling fan/ light control in a single gang box) requires a 14 in^3 box; that is possible with a single gang 2.5" deep box, but not the kind with the beveled corners, it has to be a squared-off one. (2x3x2.5 = 15 in^3)
nate
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