Installing a surface electrical box over an existing wall box?

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I've been trying to install GFCI receptacles wherever an existing receptacle wasn't grounded.

I put this aside because I wasn't sure what to do with it. I won't get into what was there to begin with other than it wasn't good, other than a pretty heavy-duty Bryant receptacle. This was from an existing ungrounded setup and tearing up the walls to add ground is a nonstarter. It's within a cabinet for a microwave and an oven (which is on a separate 240V circuit), and I believe it was an existing wall outlet (over a countertop) before a kitchen remodel.

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This is clearly a solder job underneath electrical tape. I've seen another like it in another part of the house, but with a larger junction box. If there was enough wire to work with, I'd cut it off at the point it's soldered, as well as bridge/pigtail with a wire nut. I did something like that previously with a 3-port WAGO 221, but this has even less wire to work with, so I'm inclined to keep it. And that's the smallest junction box I've ever seen. Even with a small receptacle, it was kind of crammed in there with about two inches of twisted wire soldered together for each wire pair. I'm also inclined to remove the electrical tape and cover it with rubber tape. Something like this:

https://www.3m.com/3M/en_US/p/d/b00017540/

However, there's plenty of room in this cabinet (the microwave just sits on a shelf and isn't very deep) and there's no physical reason why a surface mounted box couldn't mounted over it since it's usually hidden. Is there anything that could accomplish that? Preferably something that could just mount right on the wall box screw holes. I'd like something that is easily big enough for a GFCI receptacle where just the wire (and not that soldered wire) could be extended with a wire nut into a surface mounted box.. It's also a huge pain dealing with 12 AWG solid wire in a box this small, as well as that soldered assembly.
 
Go to the big box store and peruse the electrical aisle . You'll see plenty of options .
 
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you could use one or the other.

https://www.lowes.com/pd/RACO-1-Gang-Steel-New-Work-Old-Work-Box-Extender-Electrical-Box/1082899

https://www.lowes.com/pd/MonoSystems-SnapMark-1-Piece-White-Raceway-Electrical-Box/1000059291

I’m not normally a fan of push style connectors with 12awg wire or 20amp circuits. I wouldn’t disturb the solder connection use the existing box as a splice box and extend the existing wires and mount the new GFCI in the surface mount box.

I ended up getting this. I didn’t see anything else that looked like it would work. Didn’t see the one you posted. It has hanger holes that can mount/hang on an existing box with screws. One isn’t really a hole, and the other is a keyhole. Looks kind of odd but it’s going to be hidden anyways. My only concern is possibly shorting the side terminals on the metal, but it’s pretty wide. Maybe I’ll tape up the sides with electrical tape. It will look really odd with a regular wallplate. They had a galvanized GFCI plate but I’m not sure how that would work since the holes didn’t seem to line up.

https://www.lowes.com/pd/RACO-1-Gan...dard-Box-Extender-Wall-Electrical-Box/1005005
https://www.hubbell.com/raco/en/products/p/142884

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OP, are you sure the box isn't grounded? Not sure how old your house is, but my 1953 house has the ground wire tied to the back of almost every box. Yes, the back, as in inaccessible. The boxes are metal and have a threaded stud protruding from the back with a nut. I only figured this out because some of the boxes in the basement are exposed and you can see how they wired them. Totally weird and not serviceable after drywall/plaster, but it works. Use a multimeter to check yours.
 
Why don't you just remove the old metal shallow box and replace it with a deeper (20 cu. in) plastic box designed for old repair work like the one below? That would allow a standard cover plate to be installed flush with the wall and allow plenty of room to wire nut or Wago connect pigtails in the box with the GFCI.

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I ended up getting this. I didn’t see anything else that looked like it would work. Didn’t see the one you posted. It has hanger holes that can mount/hang on an existing box with screws. One isn’t really a hole, and the other is a keyhole. Looks kind of odd but it’s going to be hidden anyways. My only concern is possibly shorting the side terminals on the metal, but it’s pretty wide. Maybe I’ll tape up the sides with electrical tape. It will look really odd with a regular wallplate. They had a galvanized GFCI plate but I’m not sure how that would work since the holes didn’t seem to line up.

https://www.lowes.com/pd/RACO-1-Gan...dard-Box-Extender-Wall-Electrical-Box/1005005
https://www.hubbell.com/raco/en/products/p/142884

665_515.jpg
That extension ring will work drops over the top screw and keyhole slot slips over bottom screw. if it’s a decora/gfci trim plate screw holes should line up no problem decora trim screws are shorter. Yes the trim will look odd cause it’s floating from the wall instead of sitting flush against. Just wrap 3-4x with electrical tape.
 
Why don't you just remove the old metal shallow box and replace it with a deeper (20 cu. in) plastic box designed for old repair work like the one below? That would allow a standard cover plate to be installed flush with the wall and allow plenty of room to wire nut or Wago connect pigtails in the box with the GFCI.

View attachment 219240
That’s what I would do to bring it up to spec.
 
OP, are you sure the box isn't grounded? Not sure how old your house is, but my 1953 house has the ground wire tied to the back of almost every box. Yes, the back, as in inaccessible. The boxes are metal and have a threaded stud protruding from the back with a nut. I only figured this out because some of the boxes in the basement are exposed and you can see how they wired them. Totally weird and not serviceable after drywall/plaster, but it works. Use a multimeter to check yours.
I’ve seen that in older homes it’s not always a good ground sometimes one would get hot to box 90v.
 
Why don't you just remove the old metal shallow box and replace it with a deeper (20 cu. in) plastic box designed for old repair work like the one below? That would allow a standard cover plate to be installed flush with the wall and allow plenty of room to wire nut or Wago connect pigtails in the box with the GFCI.

View attachment 219240

Have no idea how to extract the old box, or if there would even be room in the wall for that.
 
OP, are you sure the box isn't grounded? Not sure how old your house is, but my 1953 house has the ground wire tied to the back of almost every box. Yes, the back, as in inaccessible. The boxes are metal and have a threaded stud protruding from the back with a nut. I only figured this out because some of the boxes in the basement are exposed and you can see how they wired them. Totally weird and not serviceable after drywall/plaster, but it works. Use a multimeter to check yours.

I replaced several 2-prong receptacles with GFCI in an another house and they tested as grounded. The metal boxes must have been grounded and they obviously connected to the ground pin via the mounting screws. I’ll find out when it’s done.

I’ve seen ground wires inside boxes in an older part of this house but the outlets tested as ungrounded.
 
I replaced several 2-prong receptacles with GFCI in an another house and they tested as grounded. The metal boxes must have been grounded and they obviously connected to the ground pin via the mounting screws. I’ll find out when it’s done.

I’ve seen ground wires inside boxes in an older part of this house but the outlets tested as ungrounded.
there’s still no equipment ground with older two wire that’s why the GFCi comes with the labels. Theoretically it would test that a ground is present
 
That extension ring will work drops over the top screw and keyhole slot slips over bottom screw. if it’s a decora/gfci trim plate screw holes should line up no problem decora trim screws are shorter. Yes the trim will look odd cause it’s floating from the wall instead of sitting flush against. Just wrap 3-4x with electrical tape.

Sure. It’s going to be hidden inside a cabinet behind a microwave oven, so appearances aren’t important. It might be wide enough that I won’t have to worry about the terminals touching the metal. I did have concerns about cutting my hands on the sharp edges though. Might make sense to sand down some of the edges, if only to protect any wire.
 
Have no idea how to extract the old box, or if there would even be room in the wall for that.
If that wall is framed with 2' x 4" studs, the plastic deep box will fit since it is 3.65" deep. You can also stick a thin screwdriver or ruler into the wall to verify the depth.

Most steel outlet boxes in older homes are side-nailed through the box into the adjacent wooden stud. If you can see both nails inside the box, there is an easy trick to dismount the box without cutting out any drywall. First, use a stud finder to determine whether the stud is on the left or right side of the box. Then, take needle nose Vise-Grip pliers and tightly clamp it onto one of the nails inside the box on the side of the stud. Insert a long screwdriver between the inside of the metal box and pry the Vise-Grip pliers/nail laterally away from the stud. Repeat with the second nail and the box can be pulled directly out of the wall through the existing opening.

Some metal boxes are nailed to the stud with a side bracket. The video below shows how to cut it out with a hacksaw or SawZall.

 
I brought it back and checked out how it all fits with an 18 cubic inch plastic box (cost maybe 32 cents) that I keep around for practicing installation and for checking size. I installed a salvaged, working GFCI receptacle that's on the big side. I don't think there's risk of the terminals touching the side because of "bumps" on the side. I'm not sure how to cover it. Typical shallow Decora wallplate screws are still too long but it does secure a plate. I remember the galvanized "GFCI cover" that was on the same shelf had holes/screws that lined up with the holes where a switch or receptacle would be mounted.

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