Do you have to remove paint before bonding an enclosure?

Yes. NEC 250.12 requires nonconductive coatings such as paint, lacquer and enamel to be removed from threads and contact surfaces, unless the connection uses fittings designed to make removal unnecessary.

Do serrated locknuts satisfy NEC 250.12?

Serrated locknuts and listed grounding bushings are examples of fittings designed to cut through coatings, which is the exception 250.12 allows. They only work if they are tightened enough to actually pierce the paint film.

Which other sections cover paint on grounding surfaces?

NEC 250.96(A) repeats the coating removal requirement for bonding, 250.4(A)(5) requires a low impedance fault current path, and 300.10 requires metal raceways and enclosures to be joined into a continuous electrical conductor.

Can a painted panel be grounded only by the threads?

No, not if the paint is still sitting between the metal surfaces that are supposed to be carrying fault current. Section 250.12 of the 2023 NEC is short and specific, and it is one of the easiest rules in Article 250 to fail an inspection on.

What 250.12 requires

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 the connection shall be made by means of fittings designed so as to make such removal unnecessary. That is the whole rule. It applies to the enclosure, to the raceway threads, to the mounting surface of a bonding lug, and to the area under a grounding bushing.

The reason is straightforward. Baked enamel on a panel can be a decent insulator. A ground fault that has to jump through a paint film either does not clear the breaker fast enough or clears it through an unintended path. Section 250.4(A)(5) requires the fault current path to have low impedance and to be capable of carrying the maximum fault current likely to be imposed. A painted joint is the opposite of that.

Fittings that satisfy the exception

The rule gives you an out when the fitting itself cuts through the coating. Locknuts with sharp serrated teeth are made for that, and they bite through the paint on the inside of a knockout as they tighten. Listed grounding bushings, thread forming screws, and paint piercing washers all fall in the same category. If you are relying on one of those, tighten it properly, because a hand snug serrated locknut has not pierced anything.

Where the exception does not help you is a flat contact surface. A bonding lug bolted to a painted backplate, a grounding electrode conductor terminated to a painted cabinet wall, or a subpanel can bonded to a painted enclosure all need the paint scraped or wire wheeled off the footprint of the connection first. Follow up with an antioxidant compound on aluminum enclosures.

Related sections worth knowing

Section 250.96(A) requires metal raceways, cable trays, cable armor, enclosures, frames, fittings and other metal noncurrent carrying parts to be bonded together to establish an effective ground fault current path, and it specifically states that nonconductive coatings shall be removed from threads and other contact surfaces or connected by fittings designed to make removal unnecessary. Section 300.10 requires metal raceways and enclosures to be metallically joined together into a continuous electrical conductor. Section 110.12 requires workmanlike installation, which is what an inspector will lean on if the scraping is sloppy.

Field painted enclosures are the usual offender. If a panel gets sprayed to match a wall after rough in, the paint lands in the knockouts, on the neutral and ground bar mounting pads, and over the bonding screw hole. Anything that was already made up and torqued is fine, but anything landed after the paint dries needs the surface cleaned.

Field takeaway

Scrape or wire wheel the paint under every lug, bar, and bonding jumper footprint. If you are counting on threads alone, use a listed grounding bushing or serrated locknut and tighten it down hard. Never assume a threaded connection into a painted knockout is an effective ground fault current path.