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Joined: Nov 2001
Posts: 75
B
Bill39 Offline OP
Member
Section 250.8 prohibits the use of sheetmetal screws for grounding. Would self-drilling screws be permissable?

The threads on sheet metal screws and self-drilled ones are about the same, so I'd think they would not be allowed either.

What do you think?

2017 / 2014 NEC & Related Books and Study Guides
Joined: Nov 2003
Posts: 269
E
Member
I think the self-drilling type would still be a sheet metal screw and thus not be allowed. What is required is a machine screw threaded into a tapped hole.

John


John
Joined: Nov 2000
Posts: 2,148
R
Member
There are self drilling/tapping screws that have a machine screw thread. This a code section that needs to be changed because there is always a questions as to what a sheet metal screw is. The code should require the use of machine screws in place of prohibiting sheet metal screws.
Don


Don(resqcapt19)
Joined: Aug 2002
Posts: 1,691
S
Member
Why does the code prohibit sheet metal screws?

Is it because the screw threads do not make good contact with the hole and there's always the possibility of screws stripping out the hole in the sheet metal?

If it's a thin sheet of metal where you can't really tap a machine thread for a proper screw, is it OK to use a nut and lockwashers behind the panel for the machine screw?

Joined: Aug 2003
Posts: 1,374
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Moderator
Bill, I agree with John and Don and that one, especially with Don's idea for a code change. With the way it is written, you could use a wood screw. Don: Did you submit anything on this for the 2005?

Sven: I THINK you have the right idea as far as intent goes. To answer your question, I would say it is perfectly fine to use a nut and lock-washers.


Ryan Jackson,
Salt Lake City
Joined: Nov 2000
Posts: 2,148
R
Member
Ryan,
I did not. There was a proposal along those lines that was submitted for the 02 code and rejected. I don't remember the details.
Don


Don(resqcapt19)
Joined: Apr 2001
Posts: 518
J
Member
This is a common topic. My opinion is clearly in the minority- but, here it is...

Any type of screw will hold securely if it is used in the right material and has the right size pilot hole. There generally isn't a very large range of hole sizes that a given thread will grip securely.

Therefore, I have no problem with self-drilling screws; by design, they make the right size hole for the threads. {I would have a problem with a self-drilling screw being used in a pre-existing or deformed hole).
While drywall screws are not sheet metal screws, I have a problem with them anchoring anything, unless they are "pulling" into the wood they were designed to pull into (or light steel framing, as the case may be!)

As for the size of the screw as it relates to the ability of the screw to carry current, I suspect that head size is more important than screw diameter or material thickness- what we need to do is connect as much wire area to as much metal area as we can. (We also need to cut through the paint).

Joined: Apr 2002
Posts: 197
G
Gwz Offline
Member
It is my understanding that two full machine threads must be in contact between the screw threads and the box wall thickness whne threaded holes are used. That is why the 32 threads per inch instead of 24 thread or 20 thread for most metal thickness.

Gwz2


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