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Just wondering if I need to pull out and replace an old plastic electrical wall box that sits a good 3/4" behind the drywall? Seems like you would want a tight fit for heat sealing, but is there an electrical code reason? Box has three 10-2 romex cables with grounds for a 240v thermostat. It is on an interior wall. Part of me wants to get a deeper plastic box and part of me wants to just install the thermostat the way the box sits now...

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  • While you do want to consider insulation (assuming that's what you mean by "heat sealing") for electrical boxes, you typically will use spray foam to seal them and slide some kind of insulation behind the box before putting drywall or plaster board up. Even a tiny gap will allow airflow and heat/conditioned air loss. Otherwise, yes, you want the drywall hole to be as small as possible, especially at the top and bottom, since many old work boxes have claws/hooks/lips to "catch" the drywall there (for positioning).
    – TylerH
    Feb 18, 2021 at 2:50
  • Is the inside of the box marked with the cubic inch volume?
    – NoSparksPlease
    Feb 18, 2021 at 4:19
  • The NEC has a minimum legal size allowance, if you count a single device strap (which counts as two wires) three 10/2 NM cables would require at least 22.5 in³. You could measure the box and consult table 314.16
    – NoSparksPlease
    Feb 18, 2021 at 16:01

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You need an extender

Assuming that your 3/4" "behind" the drywall means that the box is recessed into the drywall, you will need a box extender to fix the situation, as per NEC 314.20:

314.20 Flush-Mounted Installations. Installations within or behind a surface of concrete, tile, gypsum, plaster, or other noncombustible material, including boxes employing a flush-type cover or faceplate, shall be made so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (¼ in.).

Installations within a surface of wood or other combustible surface material, boxes, plaster rings, extension rings, or listed extenders shall extend to the finished surface or project therefrom.

Fortunately for you, plastic box extenders are cheap as chips at any home-improvement store and should serve you just fine in your application.

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  • Thank you so much! I was so focused on bringing the box out I didn't think of the extenders....:p
    – DAS
    Feb 18, 2021 at 6:42
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    Interesting that the NEC lists the dimension in mm, then in imperial in parens "()" afterwards. Almost as though we used metric here in the US instead of the imperial system. Wonder if that came about in the late 70s when there was a big, but brief, push to transition to metric.
    – FreeMan
    Feb 18, 2021 at 13:50
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    @FreeMan 1981 was the first edition to include SI units secondary to inches, 2002 first to put inches secondary to SI units. 2002 was also the year when the changed dashes to dots (ie 310-16 to 310.16).
    – NoSparksPlease
    Feb 18, 2021 at 18:49

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