Is my non-standard outlet box grounding screw code-compliant? (US in 2022)

Question

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.

Answer 1

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.

Answer 2

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!

Answer 3

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.