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Thread: box fill for metal 42 cu in box

  1. #1
    DIY Junior Member Antonio4791's Avatar
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    Default box fill for metal 42 cu in box

    Hi all,

    I'm adding 4 new circuits to my main panel for a kitchen remodel. To get the new wires from the panel into my house, I added a new conduit from the panel which enters into my crawl space. The new conduit goes to a junction box. I will have 4 hots, 4 neutrals and 4 grounds coming into this box, via the conduit (1 inch EMT). Going out of the junction box I will have 4 romex cables (going to the 4 new circuits).

    I have a 42 cubic inch metal box. There is nothing in the box other than spliced wire and wire nuts. I think this should be OK. Here is my calculation:

    I have 8 hots, 8 neutrals and 8 grounds in the box. The grounds as a whole count as one conductor. The 8 hots and 8 neutrals add to 16 conductors. That's a total of 17 conductors. To my knowledge each 12 awg wire counts as 2.25 cu inch. So I have 17 x 2.25 = 38.25. So I think I should be fine. Can someone confirm this.

  2. #2
    DIY Senior Member BobL43's Avatar
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    Lightbulb

    This one interests me as well
    I am definitely not a pro plumber, but I am a pro crastinator

  3. #3
    Electrical Contractor/Instructor jwelectric's Avatar
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    Default

    Quote Originally Posted by Antonio4791 View Post
    I have 8 hots, 8 neutrals and 8 grounds in the box. The grounds as a whole count as one conductor. The 8 hots and 8 neutrals add to 16 conductors. That's a total of 17 conductors. To my knowledge each 12 awg wire counts as 2.25 cu inch. So I have 17 x 2.25 = 38.25. So I think I should be fine. Can someone confirm this.
    Your math is correct.

  4. #4
    Moderator & Master Plumber hj's Avatar
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    My chart calls a # 12 = 2.62 cu.in. which would make you a bit undersized, but in the real world it probably does not make a big difference.

  5. #5
    Electrical Contractor jbfan74's Avatar
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    You could have used one ground wire from the panel, but the math is correct, just like Mike said.
    Yes I am A Pirate-Jimmy Buffett

  6. #6
    DIY Junior Member Antonio4791's Avatar
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    Default thanks

    Thanks guys. I'll continue with my plan.

    Now I will have everything on dedicated circuits.

  7. #7
    Electrical Contractor Jim Port's Avatar
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    Quote Originally Posted by hj View Post
    My chart calls a # 12 = 2.62 cu.in. which would make you a bit undersized, but in the real world it probably does not make a big difference.
    The NEC allowance for #14 is 2 CI, for #12 2.25 CI.

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