device wiring/box fill

This topic contains 23 replies, has 12 voices, and was last updated by  theamcguy 1 year, 11 months ago.

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    Ajax, ON

    Like this but not rockers

    Missed that part.

    Nope, never seen any. Sorry for the earlier confusion.


    Wannabee pro.


    elmwood park, NJ

    Is that the end of my tutorial? LOL I need more info cleared up in laymen’s terms. 🙂 Purely educational I have 2 electricians.

    Working Pro since 1993

    Tom M


    there are a couple of things to keep in mind with box fill. 1)it is not a science but more of a rule of thumb thing. Yes there are hard and fast rules and calculations, but I think the original background information was pretty much guess and by golly, and also hasn’t changed for decades even though insulation thicknesses, device dimensions, and clamping device designs have. This is way different than conduit fill calculations- those are based on hard geometry and actual measured dimensions of the conductor and raceway used. Box fill is far more just based on some old time field experience that got put in the code and hasn’t changed much.
    2) you have two things, box volume and conductor ALLOWANCES the volume of the box is the maximum fill and is an actual measurement. The things you put in the box are given an allowance, and the allowances have to add up to less than the volume. THe allowances are not a measured dimension but just a bare minimum amount of space needed for bending to avoid damage.
    There are some real quirky things. Like a single device “strap or yoke” takes up two allowances based on the conductor size attached. so the exact same receptacle will be either 4 cubic inches if you attach #14 or 4.5 cubic inches if you attach #12. it also doesn’t matter if it is a cheesy Eagle single pole switch or a big ole GFCI, you get the same two conductor allowance. if you are installing gfci or afci receptacles or big dimmers you shouldn’t push the limit of fill in my opinion.

    all equipment grounding conductors present are given one allowance based on the largest equipment grounding conductor in the box. in the old days all but one ground was cut short, one was left long and a crimp was used to put them together. so they didn’t take up as much box space as a circuit wire, and they weren’t typically insulated in cable wiring methods. things can be way different today but the rules are still based on those old assumptions.

    all internal cable clamps present are assigned a single allowance, whether there are one, two or more present. again its kind of a rule of thumb thing and has worked out pretty much ok over time as a minimum.

    another thing to keep in mind is that the code is not a design specification or training manual for untrained individuals. Code compliant installations might not assure an efficient operation, convenient installation or operation or room for future expansion but will provide an installation “essentially free from hazard”. It’s all about fire safety but not double redundant failsafe never fail safety, just pretty darn safe. In fact you might not even be able to hammer that GFCI in to the box if it is crammed to the limit but yet minimum size”legal”.
    general rule is that “Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors” people rush past that statement to the minimum sizing rules but that general rule still applies so even a minimum size box that is “legal” may not be sufficient.

    hope that helps explain some of the illustrations posted.


    Fayetteville, NC

    hope that helps explain some of the illustrations posted.

    It sure does, Thank you for your input.

    Automotive Pro
    Fayetteville, NC

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