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):
Best Answer
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.