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?

3 Answers 3


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
    Mar 14, 2017 at 4:27

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.

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? Mar 13, 2017 at 22:07

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