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Thread: Box Fill Calc?

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  1. #1
    DIY Senior Member Nate R's Avatar
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    Default Box Fill Calc?

    Planning circuits, and want to make sure I understand box fill calc.

    I want to bring power into a ceiling box, and power a switched light fixture, and distribute power to 2 sets of receptacles as well. (Total of 5 Receptacles, and then the one light fixture.)

    I have a Blue plastic Carlon ceiling box. Box says that it's 20 CU IN.

    2008 NEC says a single 14 AWG conductor is considered 2.00 CU IN of fill.

    So,

    4 CU IN for the 2 wires coming in supplying power
    4 CU IN for the 2 wires going to the switch
    4 CU IN for 2 wires going to one set of Receptacles
    4 CU IN for the other 2 wires going to the other set of Receptacles
    2 CU IN for ground wires
    2 CU IN for clamps in the box? (I've seen no clear answer whether the internal clamps in plastic boxes are considered clamps in box fill. So I assume they are counted to be on the safe side.)

    From what I understand, the 2 wires for the light fixture are not counted because they originate and terminate within the box.

    So, with all that in mind, that adds up to a 20 CU IN fill, which is the max this box is rated for.

    Is this correct?

  2. #2
    Electrician Chris75's Avatar
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    Quote Originally Posted by Nate R View Post
    Planning circuits, and want to make sure I understand box fill calc.

    I want to bring power into a ceiling box, and power a switched light fixture, and distribute power to 2 sets of receptacles as well. (Total of 5 Receptacles, and then the one light fixture.)

    I have a Blue plastic Carlon ceiling box. Box says that it's 20 CU IN.

    2008 NEC says a single 14 AWG conductor is considered 2.00 CU IN of fill.

    So,

    4 CU IN for the 2 wires coming in supplying power
    4 CU IN for the 2 wires going to the switch
    4 CU IN for 2 wires going to one set of Receptacles
    4 CU IN for the other 2 wires going to the other set of Receptacles
    2 CU IN for ground wires
    2 CU IN for clamps in the box? (I've seen no clear answer whether the internal clamps in plastic boxes are considered clamps in box fill. So I assume they are counted to be on the safe side.)

    From what I understand, the 2 wires for the light fixture are not counted because they originate and terminate within the box.

    So, with all that in mind, that adds up to a 20 CU IN fill, which is the max this box is rated for.

    Is this correct?
    You can drop the cu in for the clamp, its not counted in your application.

  3. #3
    DIY Senior Member Nate R's Avatar
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    OK, thanks! Good to know I'm interpreting the section correctly. So, where IS it counted? Metal boxes w/ sizeable internal clamps of some sort?

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    Electrician Chris75's Avatar
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    Quote Originally Posted by Nate R View Post
    OK, thanks! Good to know I'm interpreting the section correctly. So, where IS it counted? Metal boxes w/ sizeable internal clamps of some sort?
    Yes, IMO.... but reading the code as it is, would lead you to believe that it is required to be counted.

  5. #5
    Electrician Johnny C's Avatar
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    Default Box fill Calc.

    I believe the confusion arises because you are not aware of the method of determining the cubic inch capacity by UL, for a device box. The box is filled with a fine sand and the amount is then measured. With metal boxes,the holes are blocked and the screws and clamps are removed and then the box is filled with sand. For plastic boxes, the clamping device is an itegral part of the box, and cannot be removed, so a wire deduction for it is not required.

  6. #6
    IT Consultant / Network Engineer beekerc's Avatar
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    Default conductor count

    this is based on my lay understaning, so (any experts out there) please correct as needed.

    I use the wire conductor count figures stamped into the box. even though I have a minor in mathematics, I find it alot easier to work with the pre-calculated counts than deal with volume calculations. typically you will see figures like (x/10, x/12 and x/14) where x is the conductor capacity of the box for the associated wire gauge. (NOTE: i don't have a box in front of me, so the format may be reversed).

    - all ground wires count as one (1) total
    - each device (switch, receptacle, etc) counts as one (1) each
    - each hot, neutral, common or traveller wire that enters the box counts as one (1) each
    - if a wire is a pass-through (no splicing or break in the sheathing), then each conductor within (not including ground) counts as one (1) each
    - wire nuts, push-in connectors or other splicing devices do not count.

    eg. 1- a 3-way light switch might look like this
    2 for the 14/2 wire in
    3 for the 14/3 wire out
    1 for light switch
    1 for all grounds
    equals 7

    eg. 2- a j-box with three 2-wire NM's coming together, one 3-wire NM splice and one 2-wire NM just running through
    6 for three 14/2's (joins)
    6 for two 14/3's (splice)
    2 for one 14/2 (passing through)
    1 for all grounds
    equals 9

    For the most part I try very hard not to run different wire gauges through the same box, but sometimes it's unavoidable, and if that is the case, i go with the lowest applicable conductor count (ie. if the box states 9/12 and 14/14, even if all but one wire is 14 gauge, if there is a single 12 gauge wire in the box, then i use 9).

    DISCLAIMER: i am not an electrician, so even if the above information is accurate, I lack the authority and certification to say that it is so. However, these are the principles I use for my DIY work and if inaccurate, I invite correction from the experts for the forum's benefit as well as my own.

  7. #7
    Electrician Chris75's Avatar
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    Quote Originally Posted by beekerc View Post
    this is based on my lay understaning, so (any experts out there) please correct as needed.

    I use the wire conductor count figures stamped into the box. even though I have a minor in mathematics, I find it alot easier to work with the pre-calculated counts than deal with volume calculations. typically you will see figures like (x/10, x/12 and x/14) where x is the conductor capacity of the box for the associated wire gauge. (NOTE: i don't have a box in front of me, so the format may be reversed).

    - all ground wires count as one (1) total
    - each device (switch, receptacle, etc) counts as one (1) each
    - each hot, neutral, common or traveller wire that enters the box counts as one (1) each
    - if a wire is a pass-through (no splicing or break in the sheathing), then each conductor within (not including ground) counts as one (1) each
    - wire nuts, push-in connectors or other splicing devices do not count.
    Devices need to be doubled.

  8. #8
    IT Consultant / Network Engineer beekerc's Avatar
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    Default conductor counts

    Quote Originally Posted by Chris75 View Post
    Devices need to be doubled.

    good to know. thanks

  9. #9
    Electrician Chris75's Avatar
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    Quote Originally Posted by beekerc View Post
    good to know. thanks
    Also, it depends on what size wire gauge is going to the device, if # 14's are going to the device then it would count as 4.00 cu. but if it were # 12's going to the device it would count as 4.50 cu. As far as mixed wire sizes in a box, I use cu. in. to find the correct volume allowance.

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