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  1. #1
    Join Date
    May 2008
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    1

    Default 2 circuits in an electrical box?

    Is it permitted to have more than one electrical circuit in an outlet box? I'm installing three lights each controlled by two or three switches and it will be more convenient to feed them from separate circuits (since those are the wiring diagrams that I can find and understand). But I'm worried that this might not be considered proper. I'm thinking maybe stray electrical fields, or the possibility of a wire becoming loose and touching another circuit, etc. Is this something I should avoid.

  2. #2
    Join Date
    Apr 2008
    Location
    Houston, TX
    Posts
    62

    Default Re: 2 circuits in an electrical box?

    Yes, it is basically acceptable to have more than one circuit in one box. A few things to keep in mind though...you must keep each circuit separate...keep each circuit's wires seperated from other circuits (except for the grounds). In other words, keep the hot wires (typically black) and the neutral wires (typically white) from circuit A separate from those of circuit B, C, etc. Also, there is a limit to how many cables/wires you can bring into a box. If you are bringing a lot of wires into one box, it can get overcrowded and heat up, causing a fire hazard. The maximum wire limit usually will be stamped on the back of the box. If you are unsure, consult with an electrician.

  3. #3
    Join Date
    May 2008
    Location
    Baltimore, MD
    Posts
    76

    Default Re: 2 circuits in an electrical box?

    I have done this (carefully) with no worries.

    But I've heard of someone who did it and decided to take a shortcut by sharing the same neutral on more than one circuit.
    Please resist that temptation as this could get someone hurt very badly later if they have to work on something and don't know to turn off ALL circuits coming into that box.

  4. #4
    Join Date
    Jan 2008
    Location
    Lakeland ,MN
    Posts
    356

    Default Re: 2 circuits in an electrical box?

    [QUOTE=JEWEATHER;22676]Yes, it is basically acceptable to have more than one circuit in one box. A few things to keep in mind though...you must keep each circuit separate...keep each circuit's wires seperated from other circuits (except for the grounds). In other words, keep the hot wires (typically black) and the neutral wires (typically white) from circuit A separate from those of circuit B, C, etc. Also, there is a limit to how many cables/wires you can bring into a box. If you are bringing a lot of wires into one box, it can get overcrowded and heat up, causing a fire hazard. The maximum wire limit usually will be stamped on the back of the box. If you are unsure, consult with an electrician.[/QUOTE

    If it is a older box and no marking is present on the amount of wires this is how it is calculated.
    Calculate the cubic inches of the box (LxWxDepth).
    A 14 wire requires 2 square inches
    A 12 wire requires 2.25 square inches
    a switch or outlet counts as two wires
    a clamp counts as one wire

    So if you have a two gang box that is as a example
    3" X 4" x 3" the box is 36CU inches
    You are using # 14 wire (typically used for residential lighting)
    There is on two clamps in the box to hold wires
    and you have 2 switches.

    Two clamps require one wire calculation (2 CU inches)
    and each switches requires two wire reduction(4 CU inches)
    All the ground wires count as one (2 Cu Inches)
    2+4+2= 8 cu Inch reduction on the box

    now the box has 28 cu inches of usable space.
    that equates to 14 #14 wires
    so in theroy you can have 7 whites and 7 black wires and 7 grounds
    most new boxes are marked with the square inches of the box and the amount of wires you can put in the box. But you have to deduct two wires for each wiring device you use. (switch or outlet)
    I hope this was understandable to you
    Last edited by Ravens53; 05-18-2008 at 09:22 AM.
    Harry

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