How many circuits in one splice box?

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Old 02-10-05, 01:44 PM
Retripp
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How many circuits in one splice box?

I planning to install a hidden stair case and as luck would have it there are four circuits running in the attic right where I want to put the stairs. Can I put one junction box on each side and run 4 wires around the opening, putting all the splices in the same box or do I need to put four boxes on each side and keep my circuits seperate? That's a lot of splices in one box but if I could get by with two on each side it would help.
 
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Old 02-10-05, 02:16 PM
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Originally Posted by Retripp
I planning to install a hidden stair case and as luck would have it there are four circuits running in the attic right where I want to put the stairs. Can I put one junction box on each side and run 4 wires around the opening, putting all the splices in the same box or do I need to put four boxes on each side and keep my circuits seperate? That's a lot of splices in one box but if I could get by with two on each side it would help.
Do you mean 4 cables (more than one wire)? Or is knob & tube wiring (single conductor)?

If it's cable, we need to know how many individual wires are in each cable and the wire gauge (#14. #12).
 
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Old 02-10-05, 02:18 PM
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The maximum allowable number of wires and devices in a junction box is the 'box fill', determined by the volume of the box. Tables for this are found in NEC guides, also I found this one online.

Basically you add the number of conductors entering the box. You count only one ground wire, the largest gauge. Then multiply the number of wires by its required box volume:
2.0 cubic inches per #14 wire
2.25 cubic inches per #12 wire
2.5 cubic inches per #10 wire.
2 cubic inches per device (switch or receptacle).

In your case, if all 4 circuits were 12/3, you would have 16 conductors (plus grounds) entering the box (4 wires each with 2 conductors, entering and leaving). Add the ground gives 17 conductors times 2.25 ci per conductor = 38.25 cubic inches, too big for a regular 4 in. square junction box. Easier to go with two junction boxes of 20 ci or more on each side. Remember that junction boxes must remain permanently accessible (can't be buried), also cables in an attic near the stairs have additional physical protection requirements.
 
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Old 02-10-05, 05:34 PM
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If the box is big enough (according to all the rules already given here), one box on each side is sufficient.
 
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Old 02-11-05, 06:43 AM
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I guess the 'hidden staircase' won't be hidden any more, since we know all about it now
 
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Old 02-14-05, 05:52 AM
Retripp
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Thanks

Thanks for the good information. My circuits are 12/2 wg and I think I will go with two boxes on each side, keeping them accessible. The present wires run on top of my rafters so when I splice them I plan on drilling holes and running the wires below future flooring around the steps.
 
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Old 02-14-05, 11:48 AM
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I assume you meant that the wires run on top of the joists. Be sure to observe the structural rules regarding drilling joists and the electrical rules regarding distance from edge (so as to avoid driving a nail or screw through a cable).
 
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Old 02-15-05, 01:16 PM
Retripp
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Thanks again. "Joists" it is. I was thinking of an old Blue Notes song about swinging from the rafters. My joists are 2X8 so I'm thinking I'm okay with 1" hole in center.
 
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Old 02-16-05, 07:33 AM
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Guess you could always use one box 4 11/16 x 2 1/8 with a box extention on it if you REALLY wanted only one box per side..... Increase that FILL SPACE baby...

Also I think 3/4" Auger Hole would do fine....but hey....you are doing the drilling...
 
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