# of connections permitted in junction boxes


Old 04-19-07, 08:06 AM
Thread Starter
Join Date: Apr 2007
Posts: 5
# of connections permitted in junction boxes

Last post for the day. Does anyone know where I can find the limits on the number of connections allowed in various junction boxes as well as the wire guages (# of connections allowed varies depending on guage). For examle

a 4" x 4" x 2" deep (square box)
a standard two gang box for two switches or outlets

Maybe a website or reference sheet that would list these?
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Old 04-19-07, 08:10 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Go and buy the book "Wiring Simplified." It is available at all the big box stores and available on-line. It will, answer this question and most or perhaps even all the others you have.

You should not be doing electrical work without a good reference.

There are other books available, and you might want one or two of of those depending on the scope of your projects, but I suggest that you start with this one.

I'll give you a hint. There is no such thing as a standard two gang box. They are all different sizes. The answer to your question depends on the size of the box and the size and type of the wires and the devices being used. The size 4x4x2 is wrong (or at least not accurate enough). Look inside the box. it will give you the cubic inches.
Old 04-19-07, 08:16 AM
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Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,342
What you're looking for is called "box fill"; it doesn't limit the number of connections inside the box, but limits the number of wires entering the box.

The basic rules are that you need 2 cubic inches for every #14 wire entering the box and 2.25 cubic inches for every #12 wire. All of the ground wires together count as one wire. Devices, such as switches and receptacles, which are installed in the box count as (2) wires toward the cubic inches fill. Cable clamps, if not part of the box, count as (1) wire.

Many boxes have the max fill count stamped on them somewhere. If not, then you've gotta do the volume calculation.
Old 04-19-07, 08:49 AM
Thread Starter
Join Date: Apr 2007
Posts: 5

Thanks for the help. I do have some reference manuals but they don't list these items they are how to books. I will go look for that specific book.
Old 04-19-07, 09:20 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
... or just google "box fill calculation".
Old 04-19-07, 09:58 AM
Join Date: Feb 2006
Location: USA
Posts: 6,312
Box Fill

Simply multiplying the dimensions of a box to get cubic inches is not the correct way according to the NEC.

All references below are from NEC 2005:

Table 314.16(A) lists popular "trade size" boxes and their volumes. Most boxes will have the volume stamped on/in the box.

NEC 314.16(B) lists the 5 factors which must be used in box fill calculations.

Table 314.16(B) lists the volume allowance required for various sized conductors.
Old 04-19-07, 02:52 PM
Join Date: Apr 2006
Location: New Bern, NC
Posts: 1,623
I agree with wirepuller.

If the box is marked use that for box fill. If the box is not marked use the table from the code.

For boxes larger than those shown in the code you are allowed to calculate the demensions. However, if you are using a box larger than listed in the code you are in the wrong forum. This is the DIY forum.

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