Box Fill Calculation

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

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.

Originally Posted by chris75

What the heck are you talking about?

Originally Posted by Pudge565

Ok here is how it is done each current carring conductor counts as one wire.

Originally Posted by theatretch85

plus the metal box acts as a decent heat sink for the dimmers (better than a plastic box anyway).

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!