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09-27-2008, 10:18 AM   #1
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## Box Fill Calculation

Hi.. I wanted to install a gfci into a one gang outlet box. Cables exiting the box would include a single bx 12/2 , and 2 ea nm12/2. If I understand the box fill calculation, it would be:
2.25 X 6 = 13.5 conductors exiting box
2.25 + 1 = 2.25 grounding conductor
2.25 X 2 = 4.5 gfci
2.25 X 1 = 2.25 clamps

total 22.5

The outlet box installed is 20 in3. and would seem to have plenty of room, but calculates to be smaller than required? Am I doing this correctly? Would this fail an inspection?

Thanks

09-27-2008, 10:39 AM   #2
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i'm using Carlon 22" boxes with no problems. not sure what inspector would say in your case, but others here will. however, are you saying you are putting one live line in, and 5 out, hence 6 wires? i'm not understanding, but what do i know? i'm just a cartoon mouse......

DM

 09-27-2008, 10:54 AM #3 Member   Join Date: Sep 2007 Posts: 61 Rewards Points: 77 Stated another way, one nm12/2 home run into the box. wires exiting the box are one nm12/2 from load side of gfci to down stream protected outlet and another bx12/2 to a lighting circuit.

09-27-2008, 11:05 AM   #4

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Quote:
 Originally Posted by SS396 Hi.. I wanted to install a gfci into a one gang outlet box. Cables exiting the box would include a single bx 12/2 , and 2 ea nm12/2. If I understand the box fill calculation, it would be: 2.25 X 6 = 13.5 conductors exiting box 2.25 + 1 = 2.25 grounding conductor 2.25 X 2 = 4.5 gfci 2.25 X 1 = 2.25 clamps total 22.5 The outlet box installed is 20 in3. and would seem to have plenty of room, but calculates to be smaller than required? Am I doing this correctly? Would this fail an inspection? Thanks
Is the clamp internal to the box?... ie ...part of the box construction? If it is then you are overfilled. If not you just squeak by...well close enough I wouldn't worry about it.

Last edited by Stubbie; 09-27-2008 at 11:07 AM.

 09-27-2008, 11:34 AM #5 Member   Join Date: Sep 2007 Posts: 61 Rewards Points: 77 Thank you both. I guess then that you agree that I am adding up the box fill properly. (Actually, I was hoping that I was doing it wrong.) Seems like the enclosures are made just a little too small for what seems like a reasonable usage. ie If you have a metal box with internal clamps you are limited to one 12/2 in and one 12/2 out. period.
 09-27-2008, 11:40 AM #6 Electrician's Helper     Join Date: Jan 2008 Location: Pennsylvania Posts: 202 Rewards Points: 150 Ok here is how it is done each current carring conductor counts as one wire. So 3 "hots" and 3 neutral is 6. Then you count all grounds as 1. So 3 grounds equals 1 conductor. so we are at 7. plus the device counts as 2 so you are then at 9. And if there is an internal cable clap it counts as one per clamp so it would take you to 12. According to code you would have either 9 or 12 wires depending if you have the internal cable clamps. A B120 meaning a single gang box 20 in3 can hold I think 10 number 12 wires but I am not sure off hand. It should be stamped on the inside of the box most times it will be I will get back to you on how many it can hold though when I find it. Last edited by Pudge565; 09-27-2008 at 08:13 PM.
09-27-2008, 07:53 PM   #7
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Quote:
 Originally Posted by Pudge565 Ok here is how it is done each current carring conductor counts as one wire. So 3 "hots" and 3 neutral is 6. Then you count all grounds as 1. So 3 grounds equals 1 conductor. so we are at 7. plus the device counts as 2 so you are then at 9. And if there is an internal cable clap it counts as one per clamp so it would take you to 12. According to code you would have either 9 or 12 wires depending if you have the internal cable clamps. A B120 box single gang 20 in3 can hold I think 10 number 12 wires but I am not sure off hand. It should be stamped on the inside of the box most times it will be I will get back to you on how many it can hold though when I find it.
What the heck are you talking about?

09-27-2008, 08:10 PM   #8
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Quote:
 Originally Posted by chris75 What the heck are you talking about?
Sorry I did not realize he was talking about calulating out the required volume. However this is a way to get box fill only when using all the same size conductors. Notice how most plastic boxes list how many of certain conductor sizes are allowed in that box this is why.

 09-27-2008, 10:13 PM #9 UAW SKILLED TRADES     Join Date: Jan 2007 Location: Kansas Posts: 5,341 Rewards Points: 2,652 Pudge Your still calculating incorrectly whether same or different size conductors.
 09-27-2008, 10:30 PM #10 Electrician   Join Date: Aug 2007 Location: Connecticut, Litchfield Posts: 2,015 Rewards Points: 1,000 Pudge, check out Art. 314 on box fill.
09-27-2008, 10:56 PM   #11
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The required box fill volume was counted correctly by SS396 if the box used is a plastic box with clamps that are internal, or a metal box with the clamping mechanism inside the box. If a box is used that has the clamping mechanism(s) outside the box, such as a Romex connector with a locknut, the clamps are not counted.

Quote:
 Originally Posted by Pudge565 Ok here is how it is done each current carring conductor counts as one wire.
Sortof misleading. Each conductor that originates outside the box and is spliced or terminates inside the box, and any conductor which goes through the box without splice, is counted once. However, a conductor which has no part which leaves the box is not counted. For example, a jumper wire from one receptacle to a second receptacle in the same box (like a hot conductor) is not counted. Neither is a pigtail from a switch or receptacle to a splice. I believe these should be counted, but the code specifically excludes them from the count.

Quote:
 So 3 "hots" and 3 neutral is 6. Then you count all grounds as 1. So 3 grounds equals 1 conductor. so we are at 7. plus the device counts as 2 so you are then at 9.
Correct.

Quote:
 And if there is an internal cable clap it counts as one per clamp so it would take you to 12.
All clamps together count as one, not one each. Total is 10 so far.

Quote:
 According to code you would have either 9 or 12 wires depending if you have the internal cable clamps.
So we now have 9 or 10, depending on the clamps.

Quote:
 A B120 meaning a single gang box 20 in3 can hold I think 10 number 12 wires but I am not sure off hand. It should be stamped on the inside of the box most times it will be I will get back to you on how many it can hold though when I find it.
The Carlon B120 has an internal volume of 20 cu in. It is molded with allowable fill numbers of 9 for #12, 10 for #14. Nine #12 allowances would add up to 20.25 cu in required, ten #12 allowances would add up to 22.5 cu in required.

If using the info molded into a Carlon blue box, Carlon's position on the clamps is that the UL box fill calculations are with the clamps removed. Therefore, presently the clamp allowance should be deducted from the number of conductors listed on their boxes when using the box So... if a B120 is used, w/ nine #12 conductors marked on it, we have:
9-1 clamp allowance leaves 8 allowances left.
but...we need 6 (hots & neutrals) + 1 (grounds) + 2 (single device yoke) = 9 allowances more

So there we have it, folks. By the required volume calculation method (which is really the only correct one per the NEC), the volume required is 22.5 cu in. unless the clamps are outside the box. By the "use the numbers on the Carlon boxes" method, we are still 1 allowance overfilled.

Either way, the box is too small, but like Stubby said, it's just a little bit. At least you're looking at box fill requirements - some of the posts have demonstrated box fills way overfilled, like 30 cu in required but stuffed into a 20 or 22.5 cu in box!
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 09-28-2008, 04:46 AM #12 Member   Join Date: May 2008 Location: Apple Valley, MN, USA Posts: 1,002 Rewards Points: 506 Personally I prefer using 4" junction boxes with a 1 or 2 gang mudring. Generally have more than enough space in the box to do all the wiring needed. If its going to be a 2 device switch box, I usually go with the deep j-box in the event a dimmer needs to be installed; plus the metal box acts as a decent heat sink for the dimmers (better than a plastic box anyway).
09-28-2008, 10:04 AM   #13
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Quote:
 Originally Posted by theatretch85 plus the metal box acts as a decent heat sink for the dimmers (better than a plastic box anyway).
Actually it does nothing as far as improvment, the heat is displaced towards the front so the box really does nothing. Besides that, if you dont overload the switches its never a problem anyhow.

09-28-2008, 09:14 PM   #14
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Quote:
 Originally Posted by williswires The required box fill volume was counted correctly by SS396 if the box used is a plastic box with clamps that are internal, or a metal box with the clamping mechanism inside the box. If a box is used that has the clamping mechanism(s) outside the box, such as a Romex connector with a locknut, the clamps are not counted. Sortof misleading. Each conductor that originates outside the box and is spliced or terminates inside the box, and any conductor which goes through the box without splice, is counted once. However, a conductor which has no part which leaves the box is not counted. For example, a jumper wire from one receptacle to a second receptacle in the same box (like a hot conductor) is not counted. Neither is a pigtail from a switch or receptacle to a splice. I believe these should be counted, but the code specifically excludes them from the count. Correct. All clamps together count as one, not one each. Total is 10 so far. So we now have 9 or 10, depending on the clamps. The Carlon B120 has an internal volume of 20 cu in. It is molded with allowable fill numbers of 9 for #12, 10 for #14. Nine #12 allowances would add up to 20.25 cu in required, ten #12 allowances would add up to 22.5 cu in required. If using the info molded into a Carlon blue box, Carlon's position on the clamps is that the UL box fill calculations are with the clamps removed. Therefore, presently the clamp allowance should be deducted from the number of conductors listed on their boxes when using the box So... if a B120 is used, w/ nine #12 conductors marked on it, we have: 9-1 clamp allowance leaves 8 allowances left. but...we need 6 (hots & neutrals) + 1 (grounds) + 2 (single device yoke) = 9 allowances more So there we have it, folks. By the required volume calculation method (which is really the only correct one per the NEC), the volume required is 22.5 cu in. unless the clamps are outside the box. By the "use the numbers on the Carlon boxes" method, we are still 1 allowance overfilled. Either way, the box is too small, but like Stubby said, it's just a little bit. At least you're looking at box fill requirements - some of the posts have demonstrated box fills way overfilled, like 30 cu in required but stuffed into a 20 or 22.5 cu in box!
Thank you chris left me hanging as to why I was counting wrong this was very informative.

09-29-2008, 11:09 AM   #15
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Chris probably just wanted you to be familiar with looking it up in the code book for yourself instead of explaining the concern right away. That's always the best way...

Hey, at least I gave you 30 mins before I spoiled the fun!

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