Field Grounding vs. Factory Grounding

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
With sheet metal enclosures there simply isn't an easy way in the field to ensure that you have engaged two full threads. The gauge of metal and the size of the screw and the thread pitch of the screw make for a crazy field calculation.
It is not hard at all to make this calculation. If you use a fine thread there are 32 threads per inch so two threads is 1/16 of an inch. Guess how thick 16 gauge steel is? It is conveniently 1/16 of an inch thick. I don't believe that any listed boxes are allowed to be made of sheet metal less than 16 gauge so pretty much by default All listed boxes meet this criteria.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
In my opinion, a nut-and-bolt attachment of a lug or bus requires paint removal, while a thread-forming bolt in a formed hole does not.

Isn't the threaded hole in a 4" sq box merely a drilled-and-tapped hole, or is it formed and then threaded?
 

Nuber

State Certified Practitioner of Electrical Arts
Location
Colorado
Occupation
Master Electrician
How do you take the paint off?
Do you put something on to prevent corrosion, to replace the function of the paint?
Wire brushes and the like with a drill. I have a set for just such occasions in my tool kit. I have modified my process over the years and add some no-lox to the bottom of the lug for corrosion prevention (even though it is not rated for that application). Seems to work in the few installations that I have had to return to. Keep in mind that I work in a relatively dry state here in Colorado - corrosion is not a very large factor here.

Then again the inside of a NEMA enclosure is a dry location so perhaps such concerns are not warranted.
 

Nuber

State Certified Practitioner of Electrical Arts
Location
Colorado
Occupation
Master Electrician
It is not hard at all to make this calculation. If you use a fine thread there are 32 threads per inch so two threads is 1/16 of an inch. Guess how thick 16 gauge steel is? It is conveniently 1/16 of an inch thick. I don't believe that any listed boxes are allowed to be made of sheet metal less than 16 gauge so pretty much by default All listed boxes meet this criteria.
Not that hard for some. Then again I see other electricians stabbing residential receptacles in the back side with 12 guage. Not everyone can see the light at all times. My theory on the code making panel's reasoning for paint removal is just a guess. Take it or leave it as you see fit.

I still stand by my code reasoning for paint removal - and in this case the best argument to support your presentation of this in my opinion is the fact that a factory guy in a UL listing facility can do exactly what you describe but I can't in the field. Just because I can see your argument doesn't mean that I agree with it by code.

Paint is non-conductive and would be a barrier in the fault current path - so I remove it per code.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Not that hard for some. Then again I see other electricians stabbing residential receptacles in the back side with 12 guage. Not everyone can see the light at all times. My theory on the code making panel's reasoning for paint removal is just a guess. Take it or leave it as you see fit.

I still stand by my code reasoning for paint removal - and in this case the best argument to support your presentation of this in my opinion is the fact that a factory guy in a UL listing facility can do exactly what you describe but I can't in the field. Just because I can see your argument doesn't mean that I agree with it by code.

Paint is non-conductive and would be a barrier in the fault current path - so I remove it per code.
Then you better take the paint off the whole cabinet! If its that non-conductive, how will a screw in a small area with paint removed, help if the fault is on a part of the cabinet with paint?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
In my opinion, a nut-and-bolt attachment of a lug or bus requires paint removal, while a thread-forming bolt in a formed hole does not.

Isn't the threaded hole in a 4" sq box merely a drilled-and-tapped hole, or is it formed and then threaded?
I think it is formed so that the hole is actually not plush with the rest of the back of the box but is indented somewhat on some boxes. I think that is so you can flush mounted and still drive the ground screw in so the screw has some place to go other than the wall that it is mounted on.
 

Nuber

State Certified Practitioner of Electrical Arts
Location
Colorado
Occupation
Master Electrician
Then you better take the paint off the whole cabinet! If its that non-conductive, how will a screw in a small area with paint removed, help if the fault is on a part of the cabinet with paint?
Of course not - unless you can show me a code section that requires that I take all paint off of all cabinets. I doubt you can. Highly doubt it.

I can show you a code reference that requires that I take the paint off of a bonding component that I install in the field - 250.12

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

I am on very solid ground. Pun intended.
 

Nuber

State Certified Practitioner of Electrical Arts
Location
Colorado
Occupation
Master Electrician
Really? Receptacles that will accept 12 gauge in the back stab ports haven’t been made in something like 30 years.
Yes you can, and yes you can't - depending on the device in question. I made a blanket statement in passing that I should not have made as a blanket statement. My statement was geared toward some people aren't making quality enough judgements in the field to understand the limits of what they are installing, let alone make the TPI calculation Petersonra made. I should have come up with a better example.

Point to you.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
"Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded or bonded shall be removed from threads and other contact surfaces to ensure good electrical continuity or shall be connected by means of fittings designed so as to make such removal unnecessary."
In my opinion, the thread-forming screws that come with grounding buses qualify, like the 1/4-20 bonding screw in a 200a panel.
 

Nuber

State Certified Practitioner of Electrical Arts
Location
Colorado
Occupation
Master Electrician
In my opinion, the thread-forming screws that come with grounding buses qualify, like the 1/4-20 bonding screw in a 200a panel.
If you are talking about 250.8(A)(7) or 250.8(A)(8) conditions then I agree with you (listed items used for a designated purpose). No paint removal necessary by code although I still do it for field added ground bars even when sold/designed for use in the specific panel I am installing. I don't like having arguments with inspectors in the field. Such things cost me money. I do my arguing here on occasion when it is free.;)

All other sections of 250.8(A) trigger 250.12 in my opinion.
 
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