Curious about box fill

jgmillerFebruary 26, 2010

Ok so this is a two part question. First off someone tell me if I'm interpreting the box fill calculation correctly in this scenario.

All of these are #12 THHN conductors in a j-box

1 Hot, 1 Neutral, 1 Ground enter box from a conduit

2 Hot, 2 Neutral, 2 Ground spliced to corresponding feeds and exit into two different conduits

If I'm reading box fill correctly then I've got 6 Hots x 2.25cu in = 13.5 + 6 Neutrals x 2.25cu in = 13.5 + 1 Ground * 2.25cu in = 2.25 for a total of 29.25 cu in.

Is that calculation correct?

If it is correct then here's the second question. I understand the purpose of the box fill limit to control the potential heat in the box. However the calculation doesn't make sense to me. Since all the conductors are on the same circuit the most current they can carry is governed by the breaker they're on so generated heat should also be limited by that which means (to me at least) that the 3 hots should count as 1 hot and the 3 neutrals should count as 1 neutral since they will all carry the same current. I realize this all goes out the window if you have different circuits coming in.

Is my logic all screwed up? The fact that you ignore pigtails in a box calculation also implies that my logic is correct.

This all comes up by the way because I am cleaning up some old wiring a previous owner did and I found a box that seems to violate box fill so while I was looking it up I started thinking it all through.


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Correct me if Im wrong anybody, the way I see your box fill is that you only have a 2 conductors entering the box and 4 conductors leaving the box for a total of 6 and you add 1 volume for the ground. So 7x2.25= 15.75

    Bookmark   February 26, 2010 at 1:13AM
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Sorry you're right, I'm tried and my math was bad. You're correct I should have said 3 hots and 3 neutrals plus the 1 ground. My second question still stands though regarding the heat and current potential.

    Bookmark   February 26, 2010 at 1:39AM
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When determinimg max box fill, engineers must consider many things. In your case, with no more than 20A passing through the J-box, you could probably (improperly, though!) stuff as many wires in there as you could, and not have a problem. However, what if it was a multi-wire circuit? Or buried under insulation in a hot attic? Remember, ampacity of household wiring is limited solely for heat dissipation. In free air, these same wires can carry much more current than when they are confined in walls and ceilings. Whenever I have a box that is at or near the fill limit, I put on an extension ring. For the $1.99 cost, it's a sensible precaution.

    Bookmark   February 26, 2010 at 11:06AM
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Try and fit a GFCI receptacle into a box within fill rules and see how well it works.

The device allowance is 2 of the largest conductor, so often #12.

It is easy to meet the letter of the fill table and struggle to get all the wires in.

Pinching wires between the back of the GFCI and a metal box can cause strange failures that may not even show up for weeks or months until the insulation slowly deforms enough to fail.

I had a call once for a circuit that was tripping the breaker.

The line feed to a GFCI was pinched so hard against the back of the box it split the insulation after about 3 months.
But it was all to code.

    Bookmark   February 26, 2010 at 6:49PM
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you guys are correct, box fill is only the minimum required by the NEC. I very often go to the next size up depending on how well i can fit the wires or if its a GFCI like "brickeyee" mentioned.

    Bookmark   February 27, 2010 at 1:05AM
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