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Old 04-14-2010, 11:18 AM   #1
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Wiring Junction Box From Scratch


Let's say you want to install a brand new junction box with four cables(all 12/2), one entering the box on each side.

First of all, how much wire should you pull through into the box? Do you need to pull in the same amount for all four cables, or can/should some be shorter.

Also, when connecting wires together, do you just pull them straigt down to line them up or is there another method to that.

Finally, in this case we have of 12 wires in the junction box. How do you know if you exceeding the box fill?

Thanks.
dw1256
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Old 04-14-2010, 11:43 AM   #2
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Here's a good article on calculating box fill:
http://www.selfhelpandmore.com/home-...tions-2002.php

I usually have each cable extend about 6-8" out of the box. If the box is metal it must be grounded with a screw or clip. Also, if the box is metal, you need cable clamps at each knockout.

If you have four 12-2 cables entering the box, each current-carrying cable (black & white in your case) counts as 1 wire. All the gounds together count as 1 wire. Clamps & fittings all together count as 1 wire.

I think this is the proper calculation for your box. Each 12AWG wire takes 2.25 cubic inches.

Current-carrying wires: 8 x 2.25 = 18
Ground wires: 1 x 2.25 = 2.25
Clamps: 1 x 2.25 = 2.25
Total Required 22.5
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Last edited by secutanudu; 04-14-2010 at 11:49 AM.
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Old 04-14-2010, 05:08 PM   #3
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Quote:
Originally Posted by dw1256 View Post
Let's say you want to install a brand new junction box with four cables(all 12/2), one entering the box on each side.

First of all, how much wire should you pull through into the box? Do you need to pull in the same amount for all four cables, or can/should some be shorter.

Also, when connecting wires together, do you just pull them straigt down to line them up or is there another method to that.

Finally, in this case we have of 12 wires in the junction box. How do you know if you exceeding the box fill?

Thanks.
dw1256
we would hypothetically look at the code for your area and that would tell you the length of wires needed inside the box and the box fill numbers(of which 8 conductors will fit in most JB's). Hypothetically of course
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Old 04-14-2010, 05:53 PM   #4
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For me I oversize junction boxes so I don't have to worry about box fill
I have some very large junction boxes in the basement that have been there a long time -since house was upgraded to 200a years ago

Min size of box will depend upon the wire size
I like to leave about 8" of wire for each cable with the larger boxes
I pull them to the center & twist them together



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