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Block construction home. Only about 1.5 inches clearance between back of drywall and block. Existing three gang box cannot accommodate deeper switches and wiring. I can take one switch out of the box -- it switches a half-hot receptacle, but there is another switched outlet in the same room. So I only need two devices in this area, but I need some extra room for existing wiring and to accommodate deeper switches.

I'm thinking about opening up the dry wall, removing the old box, and installing two 4" square boxes side by side, but then with a step down mud-ring on each for just one device. That should leave me plenty of room in the boxes and I'll have a clean look with two switches once the drywall is patched up.

I'm thinking about using a 4 inch square 1.5" deep box with a bracket and attaching it to the stud. And then I would use a knockout on the other side of that box to connect another 4 inch square box using some kind of nipple or connector. The questions:

  1. Is this permitted? So, box 1 is connected to stud and then box 2 is connected only to box 1. Is the stud connection ok to support two boxes connected this way?
  2. What kind of connector do I use between the two boxes? I think I need to have a little bit of distance between the two boxes -- like maybe a half inch or so -- to make sure the mud rings fit nicely. I think I will need to run two pieces of 14-2 wire between the boxes. Do the insides of the connector between the boxes need any kind of insert or something to prevent rubbing, etc.
  3. Does the connector between the two boxes bind the entire set up? In other words, assuming all my grounds are connected in the box can I pig tail a single ground to a ground clip on one box or do I need to ground both boxes?
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    I'm glad we got so much great advice about how to do exactly what you described but I'm curious about 1) why don't you use a 4 gang box with 3 gang mud ring? That will look even cleaner, you can put a blank or night light in the third spot, and 2) Why attach to the stud why not to the block behind? It'll be a lot sturdier especially at the end far from the stud. And even with two boxes you won't need to use extra nipples just for support.
    – jay613
    Sep 20, 2022 at 12:48
  • I have similar construction - Double Brick, no insulation with 1" strapping between the brick and plaster. Whenever you decide to renovate, consider removing the strapping and frame up a 2x3 or 2x4 stud wall against the brick and insulating. It helps heating costs and leaves you with lots of room for electrical. But, only worthwhile if you're doing a full renovation. Sep 20, 2022 at 17:14
  • Thanks Jay. These are good thoughts. Probably time to invest in a hammer drill anyway.
    – Larry
    Sep 20, 2022 at 18:13

3 Answers 3

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(2) What kind of connector do I use between the two boxes? I think I need to have a little bit of distance between the two boxes -- like maybe a half inch or so -- to make sure the mud rings fit nicely.

What you want there is a Rigid (RMC) Box Spacer. It is specifically made for that and gives about 3/8" spacing.

enter image description here

What I do is rotate the boxes so that the knockouts line up. (i.e. the 3/4" KO is opposite the 3/4" KO). Then, I install box spacers in every knockout and bang them down hard. This style of conduit nut is tightened by putting a flat blade screwdriver on the tangs, and then hitting the back of the screwdriver with a hammer or the side of your pliers. I really go to town on these.

Better, if you can find it, is 4x4 boxes with all 3/4" openings. You could probably get acceptable stiffness with 2 box spacers, but I'd still use 3 just to impress the inspector.

Obviously I don't need them all for wire pass-through. They have one job: make the connection stiffer!

Whether that is satisfactory, or whether you'd prefer to add some additional bracing with steel stock, is up to you.

Also beware that drawn steel boxes (curved corners and edges, obviously press-formed from one sheet of metal) sometimes have a slight bevel. So the second box's face may not be entirely on-plane with the first box. The cure for that is welded steel boxes (spot welded from separate sheets of material).


An important note on these multiple pass-throughs: You must put all related wires through the same pass-through per NEC 300.3. Best to just use one. There is no need to use Romex cable, THHN individual wires will suffice.

Also remember with steel boxes, grounds from cables must go to the box first. The box will have at least one screw tapped #10-32 NF for ground screws. Either land the cable on this, or pigtail from this to multiple cables (and optionally: to devices). The cable should go to the box ground that it enters. You must not route the cable grounds to the device and bypass the box, like you would on a plastic box.

The box spacers will then carry ground between the two boxes. There is no reason to route a ground wire between boxes. Switches will automagically pick up ground via their mounting screws and metal yoke, the ground wire can be left unattached. Receptacles will do the same thing if they are labeled "Self-Grounding", which applies to GFCIs and the better $3+ "spec grade" receptacles. Otherwise they can pick up ground via hard flush clean metal contact of yoke to metal box. (so not floating high on drywall ears). Otherwise run a ground wire.

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    +1 for teaching me about the Box Spacer. It certainly seems like it would be adequately sturdy, but is supporting a regular box this way actually permitted per 314.23(f)? Sep 19, 2022 at 20:58
  • @pericynthion Assuming this link to a bunch of code details on a Utah govt. site is correct, the text of 314.23f is (going piece by piece, my comments in bold): An enclosure that contains a device(s), other than splicing devices, or supports a luminaire(s), lampholder, or other equipment and is supported by entering raceways sure sounds like adding a second box, shall not exceed 1650 cm3 (100 in.3) in size. which is not a problem, we're talking more like 30 cubic inches Sep 19, 2022 at 22:06
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    @Harper-ReinstateMonica agreed they count as short conduits, but where are the required "threaded entries or hubs identified for the purpose"? A typical 4" box does not have threaded entries, unlike e.g. Bell 5341-0 which does. Does the Halex 91641 qualify as a "hub identified for the purpose [of supporting the box]"? I can't find such an identification. In terms of practical robustness, grounding continuity etc I'm sure it's fine, but an inspector might quibble. Sep 20, 2022 at 0:26
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    @pericynthion You've convinced me that the point is enough of a "subject to interpretation"/gray area that the only SURE answer is going to be from your AHJ. The opinion of people on the internet does not matter. Sep 20, 2022 at 0:35
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    @pericynthion Grounding continuity is pretty much a given as these same types of connections are used all over the place. The issue is whether it satisfies the support issue. As part of my recent heavy up, I saw my electrician tightened by putting a flat blade screwdriver on the tangs, and then hitting the back of the screwdriver with a hammer or the side of your pliers. and he really went to town on those. When done right, they are very robust. Sep 20, 2022 at 3:29
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Why not install a 4-gang box. That way, you've got the room, no worries about support or box to box connections. Apparently lots of choices. I found this 2 1/8 inch deep one pretty easily - maybe check with a local electrical supply shop? (thanks for the correction on depth, Harper)

enter image description here

Here's a 1 5/8 deep one, in 4 gang and 3 gang widths:

1 5/8 x 10 7/16 x 4 1/2 inches: enter image description here

1 5/8 x 8 5/8 x 4 1/2 inches: enter image description here

And a giant mud ring for completeness: enter image description here

And a 4-to-2 gang mudring, but the one I found (Garvin GBSD-4250) costs 166 USD!: enter image description here

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  • That's a lot deeper than 1.5". Maybe the listing has the wrong photo. Sep 19, 2022 at 20:03
  • Thanks. I think the issue with a four gang box is finding a two device step down mudring. If it's a four device mudring then I think I need blank face plates with only two switches and I'd like to avoid that, but I may be misunderstanding.
    – Larry
    Sep 19, 2022 at 22:37
  • Now that I said it, I realize the depth is not a problem if the box does not use a mud ring. Sep 20, 2022 at 0:02
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    IMO a three-gang mud ring with a blank in the middle position will look better than two one-gang face plates that will almost certainly not be perfectly aligned, unless you practice this several times on a scrap piece of stud and drywall. I don't think you'll find an affordable 2-on-4 ring.
    – jay613
    Sep 20, 2022 at 12:53
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    @armand Geez whey didn't you do all my research for me a few months ago when I was asking questions like this? :) :) :)
    – jay613
    Sep 20, 2022 at 12:54
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box 1 is connected to stud and then box 2 is connected only to box 1. Is the stud connection ok to support two boxes connected this way?

Generally this is not allowed. NEC 314.23(F) gives the exception but would require that box 2 "shall have threaded entries or have hubs identified for the purpose. It shall be supported by two or more conduits threaded wrenchtight into the enclosure." A standard 4" box does not have threaded entries. I'm not sure if any suitable box exists that would fit in the depth you have available.

Instead of trying to support box 2 indirectly from box 1, you could secure box 2 to the block wall with suitable anchors. Two or three small anchors should be sufficient, doable with a regular cordless drill and masonry bit.

(2) What kind of connector do I use between the two boxes?

Lots of options:

  • RMC or IMC nipple (purchased), passed through box knockouts and secured with a lock nut on each end.
  • EMT nipple (cut yourself), secured with EMT box connectors on each end. This will force a minimum separation.
  • Short pieces of NM cable, secured with clamps at each box. Per code, may need to be secured in between as well. I don't recommend this.

(3) Does the connector between the two boxes bind the entire set up? In other words, assuming all my grounds are connected in the box can I pig tail a single ground to a ground clip on one box or do I need to ground both boxes?

Yes, if you use RMC, IMC or EMT that will serve as a ground (EGC). In fact, if you use self-grounding receptacles you won't need any ground wires except perhaps at box 1.

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    If the boxes butt up against one another then a "chase nipple" is an option. It extends into the boxes less than a minimum-length RMC/IMC nipple will and needs only one lock nut.
    – Greg Hill
    Sep 19, 2022 at 19:09
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    I think the "box spacer" suggested in another answer might allow regular faceplates to fit both boxes. Not sure, I've never seen a box spacer before seeing this answer. A chase nipple definitely will not allow the use of regular faceplates on both boxes.
    – jay613
    Sep 20, 2022 at 14:16
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    I completed this project a couple of days ago. In the end I decided I wanted a clean look with two single gang switches. I went a belt and suspenders approach -- I used three spacers (2x1/2 and 1x3/4) and I also attached the second box to the block using a couple of tapcons. That allowed me to level it off too. The spacers provided plenty of space between the mud rings. Came out great. I decided to go with a pro to do the drywall and get a nice float and texture, instead of trying myself. Thanks all.
    – Larry
    Oct 6, 2022 at 19:10

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