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NEC question

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Paul

07-23-2001 12:26:50




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I've seen some sharp answers to electrical questions on this board, so I hope one of you guys can help me. I'm confused about the number of conductors permitted in a junction box. The boxes are metal, 4" octagonal, 1 1/2" deep, with two "built in" cable clamps. I'm using 12-2 WG cable and splicing with wire nuts. Can I make a three cable (six wires including the bare grounds) splice in a box this size without violating the code? This question pertains to residential and machine shed/farm shop wiring.

The book I have says six number 12's are allowed, but deduct one for each internal clamp and one for any amount of ground wires entering the box. This makes me think that a three cable splice would not be allowed and every box on the farm would be overcrowded. I might need to install some box extensions.

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Bill Mc

07-24-2001 14:37:56




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 Re: NEC question in reply to Paul, 07-23-2001 12:26:50  
You should be ok. The six #12's = 6x2.25= 13.5, all three grounds only count as one conductor 2.25 Now the two internal cable clamps only count as one single deduction per 370-16 (b) 2 = 2.25 add it all up 13.5+2.25+2.25=18 ci. A 4 square by 1-1/2" deep box has a area of 21ci. note if you add a plug you must reduce 2 conductors for the plug.



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Thanks Noel and Dean

07-24-2001 07:16:52




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 Re: NEC question in reply to Paul, 07-23-2001 12:26:50  
Thanks for your thoughts - I think I'll get some box extensions - they're easy to install and aesthetics is not a problem. I know that there are millions of 4" boxes with three cable splices, and they don't appear overcrowded to me, but I'm concerned about dealing with the insurance company in the aftermath of a fire. I try to keep everything "right", and these boxes have been bugging me for a long time. I guess maybe I don't have much to worry about, eh? Thanks again fellas. From now on I'm going to use larger junction boxes around here.

Paul

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Noel

07-23-2001 14:07:08




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 Re: NEC question in reply to Paul, 07-23-2001 12:26:50  
The all ground wires in that box count as only 1 conductor. Each #12 conductor requires 2.5cu.in. of capacity, plus the allowances you stated I don't remember the capacity stamped 1 and 1/2" deep octboxes right off the top of my head, so I can't make the calculation, but what about using an extension ring? That would solve the capacity problem. So would the use of a 4" round 1 1/2" box, but barely, if available in your area. I'll dig up chapter and verse in the NEC shortly.

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Noel

07-23-2001 14:31:34




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 Re: Re: NEC question in reply to Noel, 07-23-2001 14:07:08  
Boy, did I stick MY foot in it! # 12 requires 2.25 cu.in. per conductor. Look up Section 370-16 (page 70-192 in the 1999 NEC). Round or octbox minimum capacities are the SAME (15.5 cu. in.) but the extension ring idea is still valid, if aesthetics permit. If the 1 1/2" depth is the critical dimension, just sub in a 4 square box x 1 1/2". Might have to use a flat plaster ring, oct or single gang if a fixture is to be strap mounted.

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Dean

07-23-2001 14:01:52




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 Re: NEC question in reply to Paul, 07-23-2001 12:26:50  
If you have 3 12-2 w/grd cables and you are using the box as a work box (no fixture) you have 3 three-wire splices, right? Far be it from me to suggest that anyone violate the NEC but I'd bet there are a couple of million boxes in the country wired just that way.

If your state has wiring inspections, ask a local inspector, usually an electrician, what is right for your locality. Or you can ask the place where you are buying your electric supplies.

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Old Sparky

07-24-2001 17:00:04




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 Re: Re: NEC question in reply to Dean, 07-23-2001 14:01:52  
Be sure that no wire insulation touches the edges of the box extension. If there is pressure, over time the insulation may compress and be cut enough to cause a ground fault. Not so well known is the 4 11/16 square box. Usually do not have cable clamps and require separately purchased wire connectors. These are great for attic work in additions or back when we wired houses that did not originally have electricity.

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Noel

07-24-2001 17:57:07




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 Re: Re: Re: NEC question in reply to Old Sparky, 07-24-2001 17:00:04  
Excellent suggestion! Most folks will think that just because Code says the application is acceptable at a minimum, that better solutions aren't out there and you show an excellent one.



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