I purchased 2 Carlin 2-gang old work box with max cubic in fill limit of 25. These are being installed in a garage so each receptacle is gfci. I have wired them in parallel so there is a pigtail in each box for ground , neutral, and hot and a respective conductor for each receptacle. I wanted each gfci to be independent of the other since there is a fridge on one and didn’t want that to trip from the other outlets so ended up applying this logic to all of them. However having installing them I realize the gfci are HUGE and take up the majority of the box. In fact having two gfci in one box with the three pigtails using ideal protwist nuts, one in coming nm 14/2 Romex cable supplying the load results in me having to use the screw in force of the outlets to push it in.

So my question does the gfci require any addition volume for box fill (based on nec AND/OR some unofficial electrician rule of thumb adjustment). I ask this because I am worried with the fact I needed to rely on the force of screwing in to actually get the receptacles and wires to go in the outlet box correctly (might sound dumb but this is the first time I’ve encountered that issue)

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    Part of your problem is that you're leaving too much cable sheath in the box. Inspectors I've encountered won't allow more than 1/2". If you trim it back you'll be able to lay the wires into the back of the box much more neatly.
    – isherwood
    Jan 11, 2021 at 14:06
  • @isherwood the cable covering / The sheath. Code requires this to extend 1/4” into the box but there is no maximum, but I could see more than the box width being an issue. This question is on receptacles the link is on lamps.
    – Ed Beal
    Jan 11, 2021 at 14:43
  • The inspectors around here sure think there is! At any rate, my point stands. Cable is much more difficult to organize than individual wires.
    – isherwood
    Jan 11, 2021 at 14:49
  • @isherwood I fully agree two much sheath in a device box will cause problems. I don’t see an issue on the light box and the inspector would have to cite it for me to change it. (But I can push back having a license).
    – Ed Beal
    Jan 11, 2021 at 15:14
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    @isherwood I have to agree with Ed Beal. Inspectors in my neck of the woods want to see at least some sheath in the box and I don't think there is a maximum. Common sense would dictate that you wouldn't want a foot of it in there! If you wanted a "maintenance loop", do it in the wall. Jan 11, 2021 at 15:15

2 Answers 2


A GFCI only requires the same fill as any other device. In your case with 14-2 one cable in one cable out and the device 4 current carrying conductors 1 ground ,x 2 device , clamp. 14 awg =2 so 2 x 8 or 16cu inch. 2 devices ? 20 cu inches?

There should be plenty of room, if you get excessive with your wire length that will quickly gobble up room, if your wire or pig tails are longer than 6” trim them back the next thing would be to make sure to make sure your wires are packed tightly in the box I have had people that try to put a foot long end and long pigtails in boxes and can’t get them in the box so length and arrangement in the box make a difference. Although GFCI devises are bulkier than normal devices they require the same device fill as other devices unless marked by the oem and I have not seen that.

  • Pack your wires first with the wire nuts tightly in the sides of the box. Then connect pigtail ends to outlets. Plenty of room.+1
    – JACK
    Jan 11, 2021 at 16:31
  • Thanks for replies. I have recently checked Home Depot and they offer a slim version of the gfci by Leviton that shaves off about 1/2 inch if that which could help but sounds like I am good to go.
    – Qiuzman
    Jan 11, 2021 at 17:45
  • I have seen the skinny ones but haven’t used them yet.
    – Ed Beal
    Jan 11, 2021 at 18:01
  • foot long... i see it every day
    – DMoore
    Jan 12, 2021 at 7:34

By the rules, no. In practice, definitely!

It's perfectly legal to put a GFCI in a 12 cubic inch "Handy-Box". However in practice good luck.

In my book, you got the wrong box. I use the following:

4-11/16" square deep box, but one with "1/2" (13/16" actual) knockouts on the sides.
Not to be confused with a 4x4 box; that'll be too tight.

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2-gang domed cover (if it's unfinished work) or mud ring (if to be drywalled)

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As you can see from the covers, that box is quite large compared to the GFCI devices, giving at least 3/4" of room behind and to the side of the devices by the time you add a 1/2" domed cover or mud ring.

These boxes are 42 cubic inches + 5-10 for the cover. Is there a plastic equivalent? Not one that's helpful. The plastic "hi-cube" boxes are standard height and put the additional space to the left or right, which is not what you need.

As far as physically mounting a steel box, they do make 'em with mounting flanges for joists, but you can also just drill holes where you need and use them for drywall screws (unlike a plastic box where that's a no-no). Try to buy these boxes at a real electrical supply, they'll have better selection and will charge half the price. Big-box stores really overcharge for these things!

You bring Romex into this box with a readily available cable clamp. Make sure to get 4-11/16" boxes that have 1/2" knockouts as that'll be easier to find the clamps. You fit a #10-32 ground screw on the hole at the little "bump", and pigtail that to the Romex cable grounds. If it's a "mud ring" then give yourself a 6" ground pigtail to nowhere, in case you ever get "non-self-grounding" GFCIs.

Grats, you are done with grounds. Push em into the back of the box and never think about em again! I love metal so much.

If it's a domed cover, you put hot+neutral pigtails onto the GFCIs and assemble them into the domed cover (in that order LOL). That's how I usually do things because it's easier assembling all that at a comfortable workbench. Remember do not add ground wires, the metal box/lid will do that for you.

If it's a drywall mud ring, then you finish the wiring, install the mud ring, drywall, then install the GFCIs like a normal outlet. Leave yourself a ground pigtail in case the GFCIs need it. You do not need ground wires if the GFCI has a "self-grounding" feature, which is a little brush that makes contact with the screw thread.

If it'll be unfinished for now and you'll do drywall someday, fit a domed cover now - a classier finished look, and no cover plates to snag your jacket. Change to a mud ring before you drywall.

  • OP specified an old work box. Are these mountable in old work?
    – The Photon
    Jan 11, 2021 at 23:05
  • @ThePhoton No, but "old work boxes" are not compatible with OP's objective. Old work boxes are narrow in height and width (they get the cubic inches by depth), because being old-work, they must fit in a drywall hole coverable by a standard cover plate. That is incompatible with OP's goal of having a box roomy enough for GFCI receps. Jan 12, 2021 at 0:27

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