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# Conductor Fill

#1
01-22-01, 03:24 PM
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What size (in Cubic Inches) nonmetallic box do I need for wiring a 20 amp circuit with 12-2 wire with ground? Do I need a bigger box than if I wire that same circuit as 15 amp with 14-2 with ground? Can I use the same blue plastic boxes? I think they are 8 Cubic inches.

#2
01-22-01, 04:47 PM
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Join Date: Sep 2000
Location: United States
Posts: 18,497
Life is a bit more complicated than your question implies. The fill calculation requires more than just the size of the wire. You also have to take into account the number of splices, the number of internal clamps, the type of fixture, etc.

First of all, 8 cubic inches isn't big enough for anything. Did you mean 18 cubic inches?

Q: Do I need a bigger box than if I wire that same circuit as 15 amp with 14-2 with ground?
A: Yes.

I recommend you just buy the biggest boxes you can find. Then you won't have to do all those complex computations. Get yourself some nice 22.5 cubic inch single-gang boxes.

If you have only two 12/3 cables, one in and one out, along with a single duplex outlet, you can get by with an 18 cubic inch box. I recommend you get 22.5 anyway -- it's just easier to get all the stuff in without accidental shorts.

Q: Can I use the same blue plastic boxes?
A: Yes, use the plastic boxes (I don't much care whether they are blue or not).

#3
01-22-01, 05:24 PM
Wgoodrich
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box fill=

2-12/2 Romexes and one switch or receptacle in a single plastic box would require 15.75 cubic inches.

3-12/2 Romexes and one switch or receptacle in a single gang plastic box would required 20.25 cubic inches.

1-12/2 Romex and 1-12/3 Romex and one switch or recepatcle in a single gang plastic box would require 18 cubic inch.

2-12/2 Romex and 1-12/3 Romex and one switch or receptacle in a single gang plastic box would require 22.5 cubic inch.

2-12/3 Romex and one switch or recepatcle in a single gang plastic box would require 20.25 cubic inch

A single gang plastic box does not require clamps.

A single gang steel box does require clamps

A multi-gang any style box does require clamps

If you change to steel boxes or a multi-gang style box then add 2.25 cubic inch to the above calculations for the clamps.

Look in your blue plastic box. You should find the maximum allowed conductors in that box.

Remember that a switch or receptacle counts as two conductors. All insulated conductors count as one conductor each, a clamp counts as one conductor, all the bares in the box count as only one conductor for your conductor count.

12 ga wiring requires 2.25 cubic inch per current carrying conductor

14 ga wiring requires 2 cubic inch per current carrying conductor.

Good Luck

Wg

#4
01-23-01, 06:49 AM
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Yes I did mean 18 C.I.
Thanks for the help as I,m trying to wire my new cabin.

#5
01-24-01, 03:53 PM
Wgoodrich
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If this cabin is a log home style construction with solid logs. A couple of suggestions to make your installation easier are as follows;

You can mount a wire mold box surface close to the floor level and use a 3/4" wire mold section to protect the Romex coming through the floor, just remember to protect the Romex from sharp metal at the end of the wire mold after that wire mold enters into the crawl.

YOu also can use floor receptacles within 18" of the wall to fulfill the 6'/12' rule. If you use floor receptacles be sure to use a box listed and labled for use as a floor receptacle box.

Good Luck

Wg

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