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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):

Switch box

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  • The wording of your question seems to be off - you declare you do not have a ground wire or a ground screw/ threaded hole and you state the box is metal and grounded. So lets ask you this - is there metal conduit to that box ? Is it mounted to a metal 2x4. Make a screw hole and attach wire - if the box is truly grounded. (measuring box to line - the box could be connected to a neutral and your reading would be the same.) How do you know the box is grounded and not neutral ?
    – Ken
    Commented Dec 2, 2017 at 9:16
  • A picture of the switch and the ground wire specifically would be required or a switch manufacturer and model
    – Kris
    Commented Dec 2, 2017 at 16:52
  • @Ken You are right, I havn't confirmed that it is not connected to neutral. I read that sometimes the ground wire is wrapped around the box and then plastered in (I have plaster walls) and that I could use a multimeter to test that. Is there any way I can be confident without opening the wall?
    – ChrisV
    Commented Dec 2, 2017 at 20:00
  • With a metal box it is required to be grounded through the conduit or a grounding wire wire connected to the box (required for many code cycles) with metal boxes the switch is not required to have a ground other than the box.
    – Ed Beal
    Commented Dec 3, 2017 at 0:13
  • @ChrisV looking at your picture - when you look at the place where the wires come in - is there a metal conduit that the wires follow through? That is what I would try to get a very good look at to verify. I suspect that you have conduit and therefore the box is grounded. If that is the case - drill and tap the screw hole and attach your ground wire.
    – Ken
    Commented Dec 4, 2017 at 7:13

2 Answers 2

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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.

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  • I thought #6 screws were for the attachment of the device to the box. All the green ground screws I have purchased have been #10 so I am not sure about that they fit the punched holes in the metal boxes.
    – Ed Beal
    Commented Dec 3, 2017 at 0:21
  • @EdBeal i'm saying a #6 or #8 screw is leeeegal... You could hypothetically use a 6-40... But 10-32 is certainly the convention.
    – Harper - Reinstate Monica
    Commented Dec 3, 2017 at 1:25
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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

enter image description here

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.

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    your comment is sound and good, one minor point, there is a clear difference between a self tapping sheet metal and a self tapping machine screw.
    – Paul Logan
    Commented Dec 2, 2017 at 18:26
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    No you can't. No sheet metal screws allowed. The code specifically calls this out, and it's very unusual to prohibit specific methods, which means it happens so often they feel they need to. Also junction boxes are thick...
    – Harper - Reinstate Monica
    Commented Dec 2, 2017 at 19:13
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    A self forming screw is allowed but these are machine screws 32 threads per inch , code requires 2 threads in contact with a machine screw , so it may be a slight difference in terminology a machine screw is allowed but these are called self forming not self tapping. I think Paul pointed this out but used the wrong "term"+
    – Ed Beal
    Commented Dec 3, 2017 at 0:18

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