Threaded hole or through bolt

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pgelectric

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For 30 years in the trade I have always used or made a threaded hole to fasten my ground lugs etc. to whatever surface required, be it a bus bar, metal can whatever........

I am now the electrical inspector for the Navy and I'm getting alot of blank stares when I tell the electricians they need to use a threaded hole and that through bolting is not acceptable.

In searching the '05 NEC I can only come up with Article's 250.8 and 314.40 (D) to back up my interpretation.

Does anyone know where or if there is something more specific to this subject in the NEC? Your help is greatly appreciated.
 
Re: Threaded hole or through bolt

Pete in my opinion you are incorrect.

The only thing that is not allowed is the use of sheet metal screws.

Think about grounding larger equipment.

Say I have a 400 amp feeder passing through a pull can. Now I want to connect my 3 AWG CU EGC to the pull can. Which would you consider a better connection, a 1/4 x 20 threaded hole in the pull can or a 1/4" x 20 nut and bolt?
 
Re: Threaded hole or through bolt

Thanks for your response. In the instance where the material is too thin I've always used a nut & washer to back up the threaded hole.
 
Re: Threaded hole or through bolt

For what it is worth in my opinion 250.12 is more important than if I use threaded holes or nuts and bolts.

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.
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I see this forgotten all the time.
 
Re: Threaded hole or through bolt

Originally posted by pgelectric:
I concur. Even more reason to thread the hole.....
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Well you can feel that way but you can not fail a nut and bolt connection.

I keep a small wire brush in my cordless case that chucks in the drill just for cleaning the paint under a grounding lug. :cool:

I almost never cut threads in a panel enclosure, I truly believe a nut and bolt connection can be tightened much more and result in a better connection.
 
Re: Threaded hole or through bolt

I agree with iwire and like him keep a small wire brush in my cordless case to remove paint and use nut, bolt, washers and lock washers for the ground connection.
I also use No-lox on the back of the lug when I mount them just in case the raw steel rusts.
The belt and suspenders factor. :D

Tapping into bus bars can lead to striping of the threads by overtighting. We all know that all connections should be torqued, but I know for a fact that doesn't happen in the real world. Usually only in switchgear.

I usually go with the nut and bolt method unless it is steel with no access to the back side to install a nut, then I will tap the hole.
 
Re: Threaded hole or through bolt

Originally posted by pgelectric:
I concur. Even more reason to thread the hole.....
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Sounds like you already had your mind made up even before you asked the question.
I hope you are not failing jobs based on your preferred method rather than the NEC.
In my opinion a nut and bolt is superior to a threaded hole.
 
Re: Threaded hole or through bolt

I guess I need to make myself more clear on the application.

The applications involve the bonding of 15kV transformer housings and structural steel I-Beams & columns for lightning protection, grounding & bonding.

I'm appoligize if I appear to have my mind made up. I appreciate everyones input. I just want to do the safest, most code correct thing. After all, the ultimate liability lays in the code enforcement & installation process.........can you spell lawsuit?
 
Re: Threaded hole or through bolt

I think that the point here is that Pgelectric needs to enforce what the code is saying and not which method is better. Both methods are code compliant so regardless of his opinion he would have to allow either installation when enforcing the NEC.
 
Re: Threaded hole or through bolt

You cannot find the requirement of a threaded hole in the NEC, as it does not exist. A bolt with a nut and washer, when properly installed works very well. Like Bob mentioned, cleaning the surface is very important in lowering the contact resistance.
 
Re: Threaded hole or through bolt

How do I reconcile that with Article 314.40 (D) and 250.8?

Article 314.40 paragraph (D) specifically references a tapped hole. As the Authority Having Jurisdiction I interpret this to mean all ground connections require a tapped hole......
 
Re: Threaded hole or through bolt

Originally posted by pgelectric:
How do I reconcile that with Article 314.40 (D) and 250.8?
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You don't have to, they do not conflict. :)

314.40(D) Grounding Provisions. A means shall be provided in each metal box for the connection of an equipment grounding conductor. The means shall be permitted to be a tapped hole or equivalent.
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Permitted means it is allowed not required.

If that section did not have the word permitted in it I could see a conflict.

But than again "tapped hole or equivalent"

What is your interpretation of equivalent?

It's certainly not a tapped hole so it must be something else.

Bottom line is the NEC does not require grounding connections to be made with a tapped hole, it's simply permitted to use a tapped hole.
 
Re: Threaded hole or through bolt

It may be that the reason a tapped hole is mentioned is that they recognize it is not a preferred means, but is the minimum acceptable means. A ring terminal under a bolt with a nut behind provides a far better connection - more bearing surface and more torque.

But UL testing has shown the threaded hole with machine screw approach is adequate.
 
Re: Threaded hole or through bolt

Bob,
A ring terminal under a bolt with a nut behind provides a far better connection - more bearing surface and more torque.
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How is there more bearing surface with the nut and bolt? The contact area of the ring terminal is the same in both cases. The contact pressure can also be the same as long as the thickness of the tapped material is the same or greater than the thickness of the nut. In cases where the tapped material is thicker (very likely when tapping structural steel) than the nut, the tapped connection could provide greater contact pressure. I can see cases where the bolt and nut would be the superior connection and others where the tapped hole would be. Both are code compliant.
Don
 
Re: Threaded hole or through bolt

with a tapped hole there is only the tap itself, and the surface area under the lug as bearing surfaces.

with a nut and bolt, you have the bolt on the back side also.
 
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