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Old 12-22-2010, 09:19 AM   #1
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Counting wires in a box?


Interestingly enough, I couldn't find the right combination of words to put into the search function to get the answer to this question although I am sure it has been asked and answered numerous times.

I just spoke to my city electrical inspector and he reminded me to make sure I don't try to put too many wires in the boxes. This has me worried because some of my switch boxes are already installed and the finish surfaces have been repaired and painted. The largest switch box that would fit into the interior walls of my house (without punching through the finish surface of the other side of the wall) was the 2.5 inch deep metal boxes as shown here:



http://www.lowes.com/pd_165713-427-5...3Dswitch%2Bbox

They have 12.5 cubic inches of space and I am running 12-2 Romex. The box that I am using does not have the internal cable clamps. I am using these:



So I have one 12-2 Romex coming into the switch box and one 12-2 Romex going out which should be 2 neutral, 2 Hot, and 2 ground wires plus 1 ground wire to the switch and 1 ground wire to the box. So how do I count all of these up and what is the max that can go into this box? Also if I am overloaded, what options do I have?

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Old 12-22-2010, 09:29 AM   #2
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Counting wires in a box?


You need 15.75 cu. in. for what you described. No solution without a bigger box.

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Old 12-22-2010, 09:43 AM   #3
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Counting wires in a box?


This exerp is out of a guide and is based on NEC but things may have changed since this was written, but here is the guide line.

Based on the following chart each #12 conductor that enters a box needs 2.25 cubic inches with the exception of the grounding conductor which requires one 2.25 cubic inch for all of the grounds. Also, each strap containing one or more devices is counted as the equivalent of two conductors.

VOLUME REQUIRED PER CONDUCTOR
  • #14 - 2 cubic inches
  • #12 -2.25 cubic inches
  • #10 -2.5 cubic inches
  • #8 - 3 cubic inches
  • #6 - 5 cubic inches
Plus the recepticles, switches and yokes

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Last edited by Jackofall1; 12-22-2010 at 10:19 AM.
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Old 12-22-2010, 10:02 AM   #4
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Counting wires in a box?


Well rats. So looking at boxes online, I can't go deeper because the wall is not deep enough to accomodate a box that is more than 2 1/2 inches deep and if I go too high the wall plate won't cover the opening. How do the pros handle a situation like this? And a follow up question to that would be, what good is it to sell a switch box that doesn't have enough room to accommodate the minimum number of wires required to operate a switch?

Last edited by jerseyguy1996; 12-22-2010 at 10:06 AM.
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Old 12-22-2010, 10:26 AM   #5
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Counting wires in a box?


That switch box is perfectly adequate for what you have described.
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Old 12-22-2010, 10:33 AM   #6
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Counting wires in a box?


Uh oh......conflicting messages. May have to call the electrical inspector back. I was hoping not to because he is impossible to get a hold of. Am I just counting the wires wrong and that may be the confusion. I know that I can do an alternate wiring where the switch is at the end of the circuit rather than between the feed and the fixture but I kind of like the idea of being able to flip the switch and completely kill all power to the fixture (in case I need to work on it). The alternate wiring would still leave a live feed wire up in the fixture.
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Old 12-22-2010, 10:41 AM   #7
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Counting wires in a box?


You have 2 black wires=4.5
You have 2 white wires=4.5
You have 1 ground wire=2.25
you have 1 switch =4.5
total=15.75


The box is too small.
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Old 12-22-2010, 10:52 AM   #8
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Counting wires in a box?


Quote:
Originally Posted by jbfan View Post
You have 2 black wires=4.5
You have 2 white wires=4.5
You have 1 ground wire=2.25
you have 1 switch =4.5
total=15.75


The box is too small.
Exactly.....
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Old 12-22-2010, 10:56 AM   #9
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Counting wires in a box?


Are you using romex?
You can get an old work plastic box that has 18 cu capacity.
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Old 12-22-2010, 10:58 AM   #10
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Counting wires in a box?


Okay so given the constraints that the depth of the box can't be more than 2 1/2 inches (unless I want the back of the box poking through the other side of the wall) what box would you guys use or have used in a similar situation?

P.S. I didn't build this wall and would like to put my boot up the rear end of whoever did build it. The house is around 100 years old.
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Old 12-22-2010, 11:00 AM   #11
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Counting wires in a box?


Quote:
Originally Posted by jbfan View Post
Are you using romex?
You can get an old work plastic box that has 18 cu capacity.
Yes it is Romex. I will look into plastic boxes but I was hoping to go with metal because I like to be able to clamp the wires to the box. I will not be able to get into the wall to staple the wire to a stud although I do staple them in the attic at the point where they drop down into the wall.
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Old 12-22-2010, 11:02 AM   #12
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Counting wires in a box?


I'm with jbfan. Why would you use a metal gang box when you can get the plastic at less than half the cost, and the size you need? Use the plastic one and you won't have a problem.
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Old 12-22-2010, 11:02 AM   #13
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Counting wires in a box?


I also like the wings on the side of the metal boxes that I am using to secure them to the plaster walls. The finish surfaces are about an inch thick and those wings when tightened down do a great job of securing the box.
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Old 12-22-2010, 11:03 AM   #14
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Counting wires in a box?


Quote:
Originally Posted by gdberry01 View Post
I'm with jbfan. Why would you use a metal gang box when you can get the plastic at less than half the cost, and the size you need? Use the plastic one and you won't have a problem.
All of the plastic boxes that I found were too deep for the wall in that even when pressed all the way in they would still stick out about an inch.
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Old 12-22-2010, 11:06 AM   #15
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Counting wires in a box?


For instance this box is labeled as an "old work" box but it is 4.3 inches deep. That will not fit into the wall cavity as the wall cavity is only about 3 inches deep (maybe less).


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