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If you have a grounded conduit going in to a metal box (no ground wires), do you need to attach a grounding pigtail to the metal box and then to the outlet ground screw? Or is the metal/metal/outlet screws connection enough to provide grounding?

I ask because I have seen both a grounding pigtail from the outlet to the box and no grounding pigtail from the outlet to the box.

  • So the cable inside the conduit is not grounded, but the conduit itself provides a reliable ground? – Martin Bonner supports Monica Apr 25 at 10:46
  • Where are you (pretty sure that using that using the conduit to provide a ground would be against regs in the UK)? – Martin Bonner supports Monica Apr 25 at 10:47
  • Hey @MartinBonner I'm in the US. – Alex Apr 25 at 16:38
  • Do your outlets have "self-grounding" spring clips on their yokes? – ThreePhaseEel Apr 25 at 23:25
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For receptacles, the screws are not enough. That is because you plug in loads, and that load could have a rather significant current draw. That is too much for screws. (and this explains why the screws are enough on switches).

However, there is one thing that is enough. Notice how any common receptacle has a metal "yoke". The ends of the yoke may also have removable drywall ears. Notice also the metal box has metal where the screw goes in. If all these things are true:

  • The metal box is flush to the wall, and the surrounding wall surface is not proud of it
  • The metal parts on both receptacle and box are bare and free of rust, paint or other contaminants
  • you have removed the little paper/plastic "squares" that capture the receptacle's screws on the yoke

Then the hard-flush mating of those bare metal surfaces is an adequate grounding path for the receptacle, and you don't need a ground jumper.

Remember most recpetacles have one hole tapped #10-32 for a ground screw. If you need to wire a ground, you can use that hole, use a grounding clip, or drill and tap your own. If you do, it must be -32 thread of finer; random sheet-metal screws are Right Out. 10-32 is the quasi-standard and cute green 10-32 screws are readily available at any building supply, with or without preattached green pigtails. I'm not a fan of the "with" because they are solid wire, which makes it the only solid wire in my work.

  • If tapping is too big of a pain in the butt, you can get 10-32 self-tappers for this purpose (Garvin GSST or equivalent) – ThreePhaseEel Apr 25 at 23:26
  • What you say makes sense, but I've read that there are certain "self-grounding" receptacles. In any case, I'm a bit confused now because I've got two different answers. Can you point me to the relevant nec section(s)? I've got the 2017 edition on pdf. – Alex Apr 26 at 0:52
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    @Alex -- 250.146(A) and (B) are the governing Code here – ThreePhaseEel Apr 26 at 22:52
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Under current/recent NEC rules I believe the grounding pigtail is required, so that the outlet will still be grounded even if it's not screwed to the box [or because the ground pigtail is regarded as a better connection to the box than the mounting screws are, I'm less sure of the intent than that current rules require the pigtail.]

Consider that if they considered the mounting screws adequate (which they did, for decades, there was no other ground connection on the outlet) you could have a box connected by cable, and tie the ground wire in to the box, yet (I believe we are all pretty sure that) a grounding pigtail to the outlet is required in that case.

  • Can you point me to the relevant nec section(s)? I've got the 2017 edition on pdf – Alex Apr 26 at 0:53

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