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If metal pipe is attached to a painted metal box with a metal locknut how is the ground path achieved between the box and the pipe? Is the paint conductive? Or do we rely on the locknut to pierce the paint? Surely the latter would be hit and miss.

3 Answers 3

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To answer the question, the paint is not conductive.

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Surely the latter would be hit and miss.

Not at all. If properly tightened, it's guaranteed.

Since data can be fun: My multimeter reads open circuit/infinite resistance (more than 2 Mega-Ohms, I forget what the top of its range is, could be more than 20MΩ) for any distance between probes on the painted breaker box surface. It reads less than 1 Ohm between a galvanized cover screw on the painted breaker box and a junction box cover connected to a (galvanized) junction box connected to a fitting connected to EMT connected to a fitting connected to the main panel. The fitting in the main panel painted box was simply installed and tightened, no paint-scraping or the like.

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    The paint is not conductive.
    – JACK
    Jan 29, 2021 at 22:04
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    Yep. This is also why those locknuts have ridges on the box side, to scrape off any paint or rust as it tightens, ensuring a good electrical connection.
    – Nate S.
    Jan 29, 2021 at 22:18
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    "properly tightened" : with a hammer and a slotted screwdriver. They make an actual tool for that but it still isn't going to be as good as properly tightened, +1. - I've never seen a lock nut that wasn't "designed so as to make such removal unnecessary."
    – Mazura
    Jan 30, 2021 at 3:51
  • Hey @JACK if you make your comment an answer I'll accept it since it's the closest thing here to a definitive answer to my question. All the rest kind of skirt around what I've concluded: grounding is not an automatic feature of box connectors over paint, you have to be intentional about it and a pro will develop a rehearsed technique that will work reliably.
    – jay613
    Feb 13, 2021 at 14:07
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You're supposed to scrape the paint around the KO. The National Electrical Code is pretty clear:

250.12 Clean Surfaces. Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to ensure good electrical continuity or be connected by means of fittings designed so as to make such removal unnecessary.

See also

https://www.nfpa.org/NEC/About-the-NEC/Grounding-and-bonding

In grounded systems, it is important to bond the equipment grounding conductors to the system grounded conductor to complete the EGFCP back to the source of electricity. The conductivity of the EGFCP is critical for protective devices to work properly. This speaks to why we scrape the paint from contact surfaces of metallic enclosures to make our electrical system bonding connections. Removing the paint, as required in Section 250.12, provides for a better connection and conductivity path.

In my experience, most locknuts do not reliably remove enough paint to ensure bonding. If the fitting (which is unpainted) is touching the edge of the knockout when the locknut is tightened, you still get electrical continuity, but it's best to scrape the paint and get really solid reliable contact.

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    I'm guessing the "or be connected by means of fittings designed so as to make such removal unnecessary" part is the main difference between this answer and the other -- in my experience, the locknuts generally have ridges that do a good job scraping away enough paint to make good electrical contact, but it's possible that not all of them do, and in that case, scraping the paint manually would be required.
    – Nate S.
    Jan 30, 2021 at 0:16
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    I have tested it pretty thoroughly and although some styles of locknuts are better than others, none of the commonly available types reliably get through a coat of paint. Jan 30, 2021 at 5:22
  • I came back here to select a best answer and I'm a bit distraught by @ NateS. comment "generally .... but it's possible that not all" along with @ mazura comment above re "properly tightened". In my unprofessional opinion this is basically what I said in the question ... it's hit and miss. Yikes. I think the best answer was from @JACK who commented "The paint is not conductive". The rest comes down to technique. Scrape, tighten, test with a meter .... and I suppose a pro ends up using consistent brands along with practiced technique that becomes reliable.
    – jay613
    Feb 5, 2021 at 14:54

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