2

I will soon be adding some outdoor wiring using weatherproof metal boxes and EMT conduit. I noticed when I was playing around with a multimeter that without a ground conductor between a receptacle and the box the receptacle ground terminal still read 0Ω to the box. I realized that the ground screw on the receptacle connects to the receptacle frame, which is fastened to the metal box, providing a ground pathway (as seen here).

If my EMT/metal boxes are grounded, do I need to pigtail ground conductors from the box to the receptacle, or can I use the mounting screws and frame to provide grounding?

4

You cannot use the mounting screws as a grounding path.

However, you can use hard metal-metal contact between the receptacle yoke and the grounded steel box. For this, the yoke mounting screws have to bottom all the way down hard. Here's what doesn't work:

  • the yoke doesn't bottom all the way because its ears are catching the edge of drywall or finish, which is the normal way receptacles are installed
  • there is paint on the box or yoke (make sure to mask it before rolling the walls!)
  • most receptacles come with the mounting screws "held captive" on the yoke by those little squares of paper/plastic. Those are insulators and prevent good grounding.

Boxes or yokes with special spring contacts are only good if listed as such: i.e. their instructions specifically say that's OK.

It doesn't matter if the grounding path "meggers out" OK at the moment. That could change over time, or as someone is pushing a plug in or pulling it out.

  • I understand your comment handles a more general case (ex. interior boxes), but in my case, the outdoor box fully houses the yoke. That said, paint that comes on the box interferes with metal-to-metal contact, and as you said, the mounting screws don't qualify. Ergo, I need to pigtail it. – Hari Ganti Mar 14 '17 at 4:27
2

You have two options:

  1. You can pigtail a ground from the box to the receptacle ground screw
  2. You can use a "self-grounding" receptacle which has spring clips on the yoke to make sure that contact is made between the yoke, the screws, and the box.
0

With a metal box the box needs to be connected to the ground. Make sure to use rain tight fittings and wire rated for wet locations on outside EMT conduit runs.

  • By "ground" you mean "ground screw on the receptacle", right? – ThreePhaseEel Mar 13 '17 at 22:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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