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  1. #1
    Join Date
    Jun 2012
    Posts
    81

    Default box fill calculations?

    holy smokes does this have me confused. i have a couple of filled boxes and i want to make sure they are ok but the charts i have found confused the ---- out of me. this is for a residential addition

    box 1= 2 switches. 1 is a 3way and 1 is a 4 way. each switch has 2 wire power coming in and 3 wire going out. so that is 14 wires (counting grounds) and 2 switches. can this go in a deep 2 gang box?

    box 2= 3 switches. 1 is 3 way, 1 is 4 way, 1 is single pole. the 3 way has 3 wire in and 2 wire out. the 4 way has 3 wire in and 3 wire out. the single pole doesnt have any in (gets power from the other switch) but has 2 wire out. can this go in a deep 3 gang?

    box 3 = 2 switches. 1 is a 4 way, the other is a single pole. the 4 way has 3 wire coming in and 2 wire going out and the siungle pole gets power from the 4 way but has 2 wire going out. deep 2 gang work?

    walls are 2x4 so that limits the depth and i pulled 12 gauge wire
    Last edited by gwilkrrs; 04-29-2013 at 09:14 AM.

  2. #2
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
    Posts
    623

    Default Re: box fill calculations?

    Quote Originally Posted by gwilkrrs View Post
    holy smokes does this have me confused. i have a couple of filled boxes and i want to make sure they are ok but the charts i have found confused the ---- out of me. this is for a residential addition

    Well GW, for starters: you must use the box manufacturers cubic inch number, not one you measure; you must count the number of yokes (they hold the devices) not the switches or receptacles; the number of wires allowed in a box is determined by the actual conductor sizes. In your case the “Volume Allowance” for #14 is 2.0 and 2.25 for #12. Basically if you have a 20 CI box you’re allowed (10) #14 conductors.; don’t count fixture wires.
    But,
    you have to deduct (1) for one or more ground wires
    Deduct (2) for each yoke
    Deduct (1) for one or more clamps (based on the largest wire in the box)
    Deduct (1) for one or more support fittings (based on the largest wire in the box)
    Deduct (1) for one or more isolated grounds (rare) (based on the largest wire in the box)
    Deduct (1) for one for each un-spliced conductor that passes thru the box. Rare in residential using cables.
    Deduct (1) for one for each conductor that enters or leaves a box.


    box 1= 2 switches. 1 is a 3way and 1 is a 4 way. each switch has 2 wire power coming in and 3 wire going out. so that is 14 wires (counting grounds) and 2 switches. can this go in a deep 2 gang box?

    Assuming the two gang box has an internal volume of 36 CI and there are (11) #14 wires (by my count) 4 and there are (2) switch yokes: wires=22, gnd=2, yokes=4 romex clamps=2 Total: 30 in a 36 CI box (so it’s OK)

    box 2= 3 switches. 1 is 3 way, 1 is 4 way, 1 is single pole. the 3 way has 3 wire in and 2 wire out. the 4 way has 3 wire in and 3 wire out. the single pole doesnt have any in (gets power from the other switch) but has 2 wire out. can this go in a deep 3 gang?

    Assume three boxes have Vol of 54 CI: wires=26, gnd=2, yokes=6 romex clamps=2 Total: 35 in a 54 CI box (so it’s OK)

    box 3 = 2 switches. 1 is a 4 way, the other is a single pole. the 4 way has 3 wire coming in and 2 wire going out and the siungle pole gets power from the 4 way but has 2 wire going out. deep 2 gang work?
    Assume box has 36 CI vol. wires=14, gnd = 2, yokes=4, Romex clamps=2 Total: 22 in a 36 CI Box, so it’s OK
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  3. #3
    Join Date
    Feb 2013
    Location
    The thumb, MI
    Posts
    54

    Default Re: box fill calculations?

    ***--now I'm confused
    Pardon my ignorant questions but why aren't devices counted? Obviously a dimmer takes up much more space than a switch & can really jam the box. I don't understand the logic of the code.

  4. #4
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
    Posts
    623

    Default Re: box fill calculations?

    Quote Originally Posted by guy48065 View Post
    ***--now I'm confused
    Pardon my ignorant questions but why aren't devices counted? Obviously a dimmer takes up much more space than a switch & can really jam the box. I don't understand the logic of the code.
    Guy, I've been asking that question for years! Also, conduit fill. If you try to jam as many wires as the Code allows into conduit, you'll be sorry.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

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