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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.

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  • So the cable inside the conduit is not grounded, but the conduit itself provides a reliable ground? Apr 25, 2019 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)? Apr 25, 2019 at 10:47
  • Hey @MartinBonner I'm in the US.
    – Alex
    Apr 25, 2019 at 16:38
  • Do your outlets have "self-grounding" spring clips on their yokes? Apr 25, 2019 at 23:25

2 Answers 2

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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.

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  • 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) Apr 25, 2019 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, 2019 at 0:52
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    @Alex -- 250.146(A) and (B) are the governing Code here Apr 26, 2019 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.

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  • Can you point me to the relevant nec section(s)? I've got the 2017 edition on pdf
    – Alex
    Apr 26, 2019 at 0:53
  • I have a similar issue. The box and conduit are metal within a concrete wall. It's an old home and only the live and neutral wires come out. Is connecting a 10 AWG cable from the grounding receptacle to some point on the box enough? What machine screw size should I use? Don't worry much about codes.
    – Rick
    Jun 12, 2021 at 13:38
  • No need for 10AWG unless it's a 30A outlet, but you can use it if you want to. The standard grounding screw (which there may be a pre-tapped hole for - examine the smallest holes in the box closely) is 10-32 - you need the fine thread so there are enough threads in the thickness of the box wall.
    – Ecnerwal
    Jun 12, 2021 at 13:57
  • If the box is embedded in concrete, you're not gonna have much luck installing a ground screw anywhere, I think. If you can get a ground clip on the edge of the box, that might be good? If you can't, pull any paper/plastic tabs off the mounting screws for the outlet and make sure the outlet's ears are firmly seated against the metal box.
    – Nate
    Feb 15, 2022 at 15:49

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