3

I'm installing GFCI outlets in my 1950s era existing kitchen counter wall boxes. The metal boxes in the wall have a number of holes in them that I thought were for standard green grounding screws, but are a bit too big for that. If I tap them to 1/4-20 threading and use a hex/serrated flange 1/4-20 bolt to secure my green grounding pigtail to the box, is that OK with current code? My understanding is that as long as the screw is not a sheet metal screw and has no other function than to secure the grounding wire I should be OK.

Since I have only front receptacle mounting access to inside of box, and clear access to 2 of the holes I can see due to wiring obstruction, it would greatly ease my task if I could mount my grounding screw to one of those 2 locations.

Update 6/2022: Harper was right - after vacuuming out a bunch of debris from the inside of the box, I found what seem to be 2 standard ground screw holes hiding toward the inside sides of the box. It does seem like it is a 2-gang or at least fairly wide box with a 1-gang-opening mud ring on the front. Now I've just got to get a U-joint to be able to use one of the screw holes.

6
  • Are you sure you are the right screw holes? Most boxes will have a few holes for mounting to the structure with nails/screws, but only one or two holes for the grounding screws. The grounding holes might have a small ridge on each side.
    – crip659
    Jun 9, 2022 at 18:09
  • Usually the grounding screws are at the bottom of the box. Assuming the box is grounded at all. What is the meter reading from phase to the box ? Do any cables come with ground wire ?
    – Traveler
    Jun 9, 2022 at 19:00
  • Box reads as grounded via meter (no voltage diff between box and neutral, "idiot tester" shows correctly wired when plugged into original outlet). Box is 40s/50s era and has knockouts for cable and these 4 identical size holes, which I now believe are for attaching box to structure. I am familiar with grounding holes in modern metal boxes and these ain't them :( I have no access to box other than through receptacle mounting opening in front, which is partially obscured by tile.
    – Armand
    Jun 9, 2022 at 19:10
  • Does any of cables come with ground wire ? Also you should measure 120V between phase and the box as well between phase and neutral.
    – Traveler
    Jun 9, 2022 at 19:24
  • If the box is not grounded, adding a ground screw to ground the outlet, to it won't matter. Boxes are grounded by the ground wire in the cable or metal conduit.
    – crip659
    Jun 9, 2022 at 20:23

3 Answers 3

7

No, that won't pass.

You need a fine thread screw - 10-32 is the stock size.

1/4-20 would not be acceptable. 1/4-32 would be (it's number of threads in the not very thick box metal that matters, here.) You need "not less than two threads." NEC 250.8(A) (5) or (6)

(5) does permit "or are secured by a nut"

1/4-32 is not something you are very likely to find at the local hardware store, but specialist metalworking suppliers (such as McMaster-Carr - example, not endorsement) have both fasteners and taps, or your hardware store may be able to order them in.

Simpler would be just to drill an appropriate hole to thread 10-32 (or use a thread-forming 10-32 screw in) if your box lacks the standard ones.

2
  • I thought that 1/8" thick metal box would give 2.5 threads of engagement to a 20 tpi screw, which would be sufficient for code, but I see now that it looks to be more like 1/16", which needs the finer pitch as you point out.
    – Armand
    Jun 9, 2022 at 19:36
  • 5
    1/8" is extremely thick sheet metal--that's more like bar stock. I doubt you'll find any residential boxes that thick. Typical boxes might be 16 ga. at ~1/16" thick.
    – isherwood
    Jun 9, 2022 at 20:04
5

If you need a 10-32 hole in a box, just drill and tap it yourself with your own tap. It's under $10 for a 10-32 tap and tap holder. Google "how to tap".

NEC specifically calls out -32 as the required thread pitch for sufficient engagement into UL-standard junction boxes and enclosures.

On a stock metal box, most holes are NOT tapped to be ground holes, they are simply for nails to mount the box. However, one hole is smaller than the others and will be tapped 10-32.

This tapped hole is typically in the same general pattern as the holes for nails, installers often mess up and put a nail/screw through it. More than once I've had to pry out or unscrew mounting screws or nails to go hunting for the ground hole. Make sure you add one before removing the last one!

8
  • This metal box was likely made in the early 1950s or even earlier (many re-used items found in this house). I'm quite sure there is no 10-32 tapped hole in it, but will leave open a 1% chance that I just didn't spot it. I haven't used my new endoscope yet; maybe that will reveal one. I don't see any nails or fasteners inside the box, but I can't see any of the outside of the box or any mounting flanges there might be there. It is mounted securely somehow!
    – Armand
    Jun 9, 2022 at 21:33
  • @Armand Endoscope? Sounds like "making it harder than it is" lol. I don't know what an endoscope costs but it sounds like more than $12 which is the cost of a 10-32 tap and tap holder and appropriate size drill. Jun 9, 2022 at 21:36
  • I have no visibility of what is behind the metal where I would drill a hole - there is a modest chance there are live wires there, so I don't want to drill at this point.
    – Armand
    Jun 9, 2022 at 21:36
  • This endoscope was strangely enough around $12 :) ; your advice is correct and appreciated but I am just not in a position to take it at this point.
    – Armand
    Jun 9, 2022 at 21:37
  • 1
    @Armand The "lip" sounds like a mud ring. Real common to have a 1-gang mud ring on a 4x4 box, in which case there's more box than you can see (upside: plenty of room for GFCI). Yeah, GFCIs with self-grounding clips are readily available, I would just swap this GFCI either at the store or with a GFCI elsewhere in the house. Does it not have the words "Self-grounding"? Jun 9, 2022 at 21:46
0

There are three separate issues with grounding a receptacle in a box. Two are for metal boxes, one for plastic boxes:

  • Panel to metal box

This can be metal conduit (of various types) or a ground wire in a cable or a separate ground wire added later. A ground wire (whether in cable or separate) needs to connect to the box, normally with a -32 screw.

  • Receptacle to metal box

This can be a wire from a -32 screw in the box to a grounding screw on the receptacle. Or it can be magic. Specifically, a high-quality receptacle - and that should include most, if not all, GFCI receptacles - can ground directly from the yoke (the metal frame) to a metal box. No ground wire or special screws needed! That alone is a reason to go up from the bottom-of-the-line receptacles when using non-GFCI receptacles (e.g., where GFCI protection is not needed or is provided earlier in the circuit).

  • Panel to receptacle in non-metal box

In this case, you don't screw any wires to the box (that would be pointless) and either connect the ground wire from the panel directly to the screw on the receptacle or use a wire nut to connect all ground wires (including a short one to the receptacle) together.

Since you have metal boxes and GFCI, and the question seems to focus on grounding the receptacle, not dealing with the wire from the panel to the box, you don't need to do anything. You can really on magic.

Actually, to be sure, check for continuity/low resistance between the metal box and the neutral wire before connecting the receptacle. If that doesn't work then you don't have a good ground connected to the box and you have to decide whether to fix that (ideal) or to use GFCI and add a label to indicate that there is no ground.

Once you have confirmed that your box is grounded:

Make sure the yoke is clean (should be on a new GFCI), make sure the metal on the box where the yoke screws in to it is clean, and make sure there are no paper spacers involved (throw them away). Test when done with a 3-light tester to make sure. Or if you want to be 100% sure, test with a multimeter - after attaching hot & neutral but before screwing the receptacle into the box, the ground pin should be open and after screwing in the receptacle it should show continuity/very low resistance to the neutral pin.

4
  • I should have mentioned that there is no direct yoke to box contact because the tile between box surface and wall surface is not cut out with a big enough opening for the GFCI, although the basic 50+ years old outlet did fit in there. No magic I'm afraid :(
    – Armand
    Jun 9, 2022 at 21:27
  • Then you may have other code compliance issues. Pictures? Jun 9, 2022 at 21:31
  • 2
    I've got probably zillions of code compliance issues in this house. I'm just trying to make sure I don't add new ones as I work on upgrading the existing setup.
    – Armand
    Jun 9, 2022 at 21:35
  • Re: "after attaching hot & neutral but before screwing the receptacle into the box, the ground pin should be open and after screwing in the receptacle it should show continuity/very low resistance to the neutral pin." This is an excellent idea and I will do so when I finish up the install.
    – Armand
    Jun 9, 2022 at 21:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.