I'm installing a smart light switch into a box with no ground wire, screw, or threaded hole for a screw. The switch box itself it metal and grounded (live wire to box was 120V on my multimeter).
What's the proper way to attach the ground wire from the switch to the box when the box doesn't have a wire or screw?
Edit: Switch is a Wemo Dimmer. Added picture of box (with old switch):
First, is the box actually metal? If not, forget about it.
Most boxes are tapped for a #10-32 screw for a ground screw because as a matter of convention and industry practice, that is what everyone uses. I believe the minimum screw size is #6, however -32 is the coarsest allowed thread pitch. That is because UL standardizes box thicknesses, and this assures enough thread engagement to be effective. -40 is also allowed. A sheet metal screw is out of the question. A self-tapper is ok if it is -32 or finer.
If you do not see a ground screw hole tapped, either the box does not have one (some don't), you're not seeing it because you expect it to be on a dimple and it's flush, or the installer happened to use that hole for a mounting screw/nail.
You are welcome to either use a ground clip, or drill and tap a 10-32 hole yourself. Use the correct drill listed with the 10-32 tap.
If you are positive the box is grounded (and not connected to neutral as @Ken questions), you can drill a self tapping sheet metal screw directly into the box. I find it easier to drill a small pilot hole before using a self tapping screw, but it generally isn't required to drill a pilot hole when using a self tapper.
So I would take an 8" length of green wire and make a pigtail out of it
and then screw that into the box with the self tapping machine screw. Then wire nut your switch's ground to the new ground wire you installed.
You can purchase pre-made pigtails at Home Depot or other hardware stores, but you can certainly make your own if you have the extra wire.
