Electrical – Using receptacle mounting screws for ground connection

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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?

Best Answer

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.

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