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Discussion Starter #1
Hello,

I have a couple of quick questions for the NEC experts out there. I think they are easy but I would like affirmation. The scenario is wiring of (nominal) 4" x 2-1/8" metal octagon boxes used as "both" junction box and for a porcelain lamp holder in a typical basement application. This box has a volume of 21.5 cu. in., and the box has from the factory internal NM clamps (which I understand to occupy 1 conductor's worth of fill volume) and their associated knockouts, as well as 1/2" and 3/4" knockouts. Based on 14 AWG used in the box, the maximum allowable conductors would be 10.

1. Are the pigtail wires from the Porcelain lamp holder itself counted when computing fill volume for the box? I think the answer to this question is no, as the porcelain lamp holder is in essence a junction box cover, and the pigtails do not enter/leave the box.

2. Does the lamp holder count in fill volume at all? There is no hickey, no stud. Again I think the answer is no.

3. If necessary to meet fill volume requirements, can the factory installed NM clamps be removed and the 1/2" knockouts used in conjunction with an "add-on" clamp such as the 3/8" Halex NM clamp? I understand this type of clamp is not counted when computing fill volume. (I'd post a link to which clamp I'm thinking of but I don't have that privilege based on this being my first post). I am guessing that the answer to 3 is yes.

I understand that any open but unoccupied knockouts must be covered.

Thanks
 

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You're correct on 1 & 2. I'm pretty sure you're correct on 3. Usually if I'm pushing a box fill limit I go up a size. I've learned my lesson. Do a thorough fill count, use the smallest box possible, add another wire to it in a couple days, kick yourself in the explitive.
 

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Discussion Starter #3
You're correct on 1 & 2. I'm pretty sure you're correct on 3. Usually if I'm pushing a box fill limit I go up a size. I've learned my lesson. Do a thorough fill count, use the smallest box possible, add another wire to it in a couple days, kick yourself in the explitive.
As far as I know, a 4" x 2-1/8" octagon box is already a "deep" box as far as the metal ones go. I suppose an extension ring could be added, but I'd rather not. It certainly makes sense to me to use a generous box wherever possible.

I'm looking at alternative ways to route the wire to minimize #conductors in the box, though that's more work.

I know that 4" x 2-1/8" octagon boxes without factory clamps exist, so at a minimum if the wiring layout cannot be altered, then changing from the box with the factory clamps to without is possible. But it seems to me that there should be no harm in simply removing the factory clamps and using different clamps that don't count against fill volume.
 

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Discussion Starter #5
No reason I can think of, as long as I can find the right mud ring to match up with the porcelain lampholder. I'll check. Thanks.
 

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Discussion Starter #7
^ I picked up one of those mud rings and a 4 square. Apparently the mounting screw spacing is for the knock out mounts on the porcelain lampholder. I've never had any luck getting the porcelain to break away cleanly. I have tried multiple ways with no luck.

So I am back to question 3: can I remove the factory clamps from the octagon box and use 3/8" clamp to gain 1 conductor's worth of fill volume?
 

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I'd rather use the 4 square with a 4 0 mud ring rather than a 3 0 which is what you got. Keep in mind that most nm connectors only allow 2 14-2 cables per
 

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Discussion Starter #10
I'd rather use the 4 square with a 4 0 mud ring rather than a 3 0 which is what you got. Keep in mind that most nm connectors only allow 2 14-2 cables per
I haven't found a larger diameter mud ring. My searches on the parts house website turn up nothing, or I am not recognizing the part you are talking about.
 

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A "Handy Husband"
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If you can't deal with the porcelain keyless get a plastic one.
 
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