self tapping screws
- Thread starter sjaniga
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bill addiss
Senior Member
Re: self tapping screws
The Tek screws are basically the same thing as sheet metal screws.
Bill
The Tek screws are basically the same thing as sheet metal screws.
Bill
- Location
- Illinois
- Occupation
- retired electrician
Re: self tapping screws
The code wording is poor. It should be changed to require the use of machine screws to eliminate any confusion as opposed to forbidding the use of sheet metal screws. I agree that "tek" have the same type of course threads as sheet metal screws and should not be used for grounding connections.
Don
The code wording is poor. It should be changed to require the use of machine screws to eliminate any confusion as opposed to forbidding the use of sheet metal screws. I agree that "tek" have the same type of course threads as sheet metal screws and should not be used for grounding connections.
Don
flightline
Senior Member
Re: self tapping screws
If the box manufactrers have gone to all of the trouble of determining that a fine thread, 10-32 is what is required, and UL has universally listed these boses this way, universally, and, since grounding pigtail manufacturers make these grounding pigtails, a version supplied with screws for connection to the box side, and then only manufacture the scre type with 10/32 screws, then why are we to accept any installation otherwise as maintaining the listed installation??
P.S. Sorry for run on sentence.
If the box manufactrers have gone to all of the trouble of determining that a fine thread, 10-32 is what is required, and UL has universally listed these boses this way, universally, and, since grounding pigtail manufacturers make these grounding pigtails, a version supplied with screws for connection to the box side, and then only manufacture the scre type with 10/32 screws, then why are we to accept any installation otherwise as maintaining the listed installation??
P.S. Sorry for run on sentence.
pierre
Senior Member
- Location
- Westchester County, New York
Re: self tapping screws
Bill
I am 'thinking outloud' here, not questioning you.
250.126 mandates 'Device Terminals'. I do not see (that doesn't mean it does not exist) where it mandates 'box termination' screw color and type other than stating, sheetmetal screws cannot be used.
In retrofit work (there is plenty in NY, as I am sure you know), would a tek screw not be better than a groundclip for bonding an EGC to the 'old' box that does not have a 10/32 tapped hole? In testing and manufacturing of the groundclips, did they design for the thickness of new and old boxes or just new boxes? It seems every time I try to install the clips on old boxes they deform or break.
Pierre
Bill
I am 'thinking outloud' here, not questioning you.
250.126 mandates 'Device Terminals'. I do not see (that doesn't mean it does not exist) where it mandates 'box termination' screw color and type other than stating, sheetmetal screws cannot be used.
In retrofit work (there is plenty in NY, as I am sure you know), would a tek screw not be better than a groundclip for bonding an EGC to the 'old' box that does not have a 10/32 tapped hole? In testing and manufacturing of the groundclips, did they design for the thickness of new and old boxes or just new boxes? It seems every time I try to install the clips on old boxes they deform or break.
Pierre
charlie
Senior Member
- Location
- Indianapolis
Re: self tapping screws
I seem to remember that the reason for not using sheet metal screws is that they can't get three treads in contact with the metal. Where a screw is inserted in a predrilled hole in a panelboard, the panelboard is punched so the sheet metal is rolled up and thicker at the point of cutting in the grounding screw. The listing depends on the three threads. Don't forget, I am trusting my memory which seems to be as long as my . . . UH . . . well, it is short.
I seem to remember that the reason for not using sheet metal screws is that they can't get three treads in contact with the metal. Where a screw is inserted in a predrilled hole in a panelboard, the panelboard is punched so the sheet metal is rolled up and thicker at the point of cutting in the grounding screw. The listing depends on the three threads. Don't forget, I am trusting my memory which seems to be as long as my . . . UH . . . well, it is short.
bill addiss
Senior Member
Re: self tapping screws
Pierre,
The Sheet Metal screw Q has been hashed out here before, I don't recall details though.
IMO I would take "terminal screw" as something having threads like a machine screw.
I could never get the hang of using those Ground clips.
Bill
[ July 03, 2003, 01:14 AM: Message edited by: bill addiss ]
Pierre,
The Sheet Metal screw Q has been hashed out here before, I don't recall details though.
IMO I would take "terminal screw" as something having threads like a machine screw.
I could never get the hang of using those Ground clips.
Bill
[ July 03, 2003, 01:14 AM: Message edited by: bill addiss ]
- Location
- Illinois
- Occupation
- retired electrician
Re: self tapping screws
Peter,
It is not even close to being a clear statement as there is no standard definition of a sheet metal screw. The section should call for a minimum number of threads to be engaged with the enclosure or call for a standard machine screw. The minimum number of threads that are engaged would be the best way because as the size of a machine screw gets larger the pitch is reduced. In a 1/8" thick enclosure the common 10-32 machine (the tapped hole in electrical boxes is a 10-32 thread) screw would have 4 threads engaged, a 10-24 would have 3 and a 1/4"-20 machine screw would have 2.5 threads.
A machine screw or bolt with a nut and lock washer should also be acceptable.
Don
Peter,
It is not even close to being a clear statement as there is no standard definition of a sheet metal screw. The section should call for a minimum number of threads to be engaged with the enclosure or call for a standard machine screw. The minimum number of threads that are engaged would be the best way because as the size of a machine screw gets larger the pitch is reduced. In a 1/8" thick enclosure the common 10-32 machine (the tapped hole in electrical boxes is a 10-32 thread) screw would have 4 threads engaged, a 10-24 would have 3 and a 1/4"-20 machine screw would have 2.5 threads.
A machine screw or bolt with a nut and lock washer should also be acceptable.
Don
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: self tapping screws
Since the code specifically mentions a sheet metal screw, then anything else would be allowed, such as a wood screw or dry wall screw. We know what the intent is, and Dons comment on the minium number of threads to be engaged is the way it should read.
Since the code specifically mentions a sheet metal screw, then anything else would be allowed, such as a wood screw or dry wall screw. We know what the intent is, and Dons comment on the minium number of threads to be engaged is the way it should read.
Re: self tapping screws
I agree that contact area is important, however the mating pressure of metal to metal is more important. A loose thread is no good no matter what the count. A tek screw presents a tight fit, properly installed.
I've seen some ground terminals painted green, that were almost insulated.
[ July 04, 2003, 05:15 PM: Message edited by: bennie ]
I agree that contact area is important, however the mating pressure of metal to metal is more important. A loose thread is no good no matter what the count. A tek screw presents a tight fit, properly installed.
I've seen some ground terminals painted green, that were almost insulated.
[ July 04, 2003, 05:15 PM: Message edited by: bennie ]
- Location
- Massachusetts
Re: self tapping screws
If the code changes to threads per inch we could use Tek screws.
These ones come in 12-24 thread sizes.
Heavy Duty Teks
[ July 04, 2003, 07:12 PM: Message edited by: iwire ]
If the code changes to threads per inch we could use Tek screws.
These ones come in 12-24 thread sizes.
Heavy Duty Teks
[ July 04, 2003, 07:12 PM: Message edited by: iwire ]
Re: self tapping screws
I don't have the UL standard, but as I remember, it is two (2) full threads of a machine screw in the thicknes of the metal.
Many thin metal enclosures have a 'bossed' punched-in the thin metal so that the extra metal of punched hole allows for the two full threads of an 10/32 to be in the metal of the enclosure.
I don't have the UL standard, but as I remember, it is two (2) full threads of a machine screw in the thicknes of the metal.
Many thin metal enclosures have a 'bossed' punched-in the thin metal so that the extra metal of punched hole allows for the two full threads of an 10/32 to be in the metal of the enclosure.
Re: self tapping screws
Tom Baker,
Drywall screws are basically sheet metal screws. I assume they we invented when metal studs were invented because drywall nails didn't seem to work very well in metal studs.
There are two types of drywall screws -- coarse thread and fine thread. The fine thread is for metal. And there are two types of those. The sharp point is for thin guage metal studs and the "self-drilling" [with the drill point tip] is for the thicker metal studs.
The course thread drywall screws are for stuff like wood. I doubt if they or wood screws would work at all in the sheet metal of an electrical box.
To get a little bit lawyerly, wouldn't anything screwed into sheet metal be a sheet metal screw?
In any event, it is possible to get "self-tapping" machine screws. These have little grooves or slits on the end much like a machinist's tap. These will self tap into the holes on the sides of a 4" square box.
~Peter
Tom Baker,
Drywall screws are basically sheet metal screws. I assume they we invented when metal studs were invented because drywall nails didn't seem to work very well in metal studs.
There are two types of drywall screws -- coarse thread and fine thread. The fine thread is for metal. And there are two types of those. The sharp point is for thin guage metal studs and the "self-drilling" [with the drill point tip] is for the thicker metal studs.
The course thread drywall screws are for stuff like wood. I doubt if they or wood screws would work at all in the sheet metal of an electrical box.
To get a little bit lawyerly, wouldn't anything screwed into sheet metal be a sheet metal screw?
In any event, it is possible to get "self-tapping" machine screws. These have little grooves or slits on the end much like a machinist's tap. These will self tap into the holes on the sides of a 4" square box.
~Peter
hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
- Occupation
- EC
Re: self tapping screws
I don't think it has anything to do with threads per inch but rather "installer error" and inconsistancy.
Anybody know off hand what the correct hole size should be for a #10 sheet metal screw in 12ga steel? I doubt it. We usually grab a drill bit that looks about right or find a screw that fits in an existing hole. If the hole is too loose the screw can't be tightened properly and will loosen over time.
On the other hand, with a 10-32 tapped hole the only screw that is going to fit is a 10-32 machine screw and there is little doubt that it will tighten properly.
For old work, I see nothing wrong with "Teks" type self drilling screws. They drill their own hole so it's got to be right. I think the NEC needs to recognize this and encourage the manufacture of self drilling and tapping ground screws. Certainly more reliable than those clips...
I don't think it has anything to do with threads per inch but rather "installer error" and inconsistancy.
Anybody know off hand what the correct hole size should be for a #10 sheet metal screw in 12ga steel? I doubt it. We usually grab a drill bit that looks about right or find a screw that fits in an existing hole. If the hole is too loose the screw can't be tightened properly and will loosen over time.
On the other hand, with a 10-32 tapped hole the only screw that is going to fit is a 10-32 machine screw and there is little doubt that it will tighten properly.
For old work, I see nothing wrong with "Teks" type self drilling screws. They drill their own hole so it's got to be right. I think the NEC needs to recognize this and encourage the manufacture of self drilling and tapping ground screws. Certainly more reliable than those clips...
Re: self tapping screws
In the July/August 1995 IAEI NEWS on page 34, "The UL Question Corner" discusses the use of outlet extension boxes grounding. Some of the text is " - - -. A box extension is secured by to the flush-mounted box by the cover screws in the corners. these are size No. 8/32 screws providing two full threads of engagement each. UL listed box extensions have been evaluated for secureness of the assembly and effectiveness of grounding. - - -."
I believe there was a comment in the UL Question Corner about the Sheetmetal Screws also discussing the two threads of an 10/32 machine screw. I just have not found that text yet.
In the July/August 1995 IAEI NEWS on page 34, "The UL Question Corner" discusses the use of outlet extension boxes grounding. Some of the text is " - - -. A box extension is secured by to the flush-mounted box by the cover screws in the corners. these are size No. 8/32 screws providing two full threads of engagement each. UL listed box extensions have been evaluated for secureness of the assembly and effectiveness of grounding. - - -."
I believe there was a comment in the UL Question Corner about the Sheetmetal Screws also discussing the two threads of an 10/32 machine screw. I just have not found that text yet.
V8SonomaGT
New member
- Location
- Colorado Springs, Colorado
So was a 10/32 ever required by the NEC? or is it just widely practiced. See my Electric Residential Wiring book (version 15) had a question about the size of the screw holes for equipment grounding conductors, being 10/32.
here is the problem, the teacher wants us to back up all our answers with NEC code, but not even he can find the answer. I referenced multiple sections of the NEC but found nothing requiring 10/32.
So here is what i came up with after referencing UL, is that if the box has a 10/32 machine screw holes pre-drilled, that they must be used, and using a 8/32 is not allowed. ref. http://www.ul.com/regulators/ode/0904.pdf
so any additional help on the subject i have a class full of other students, and the instructor who are all looking for a little advice,
thanks
here is the problem, the teacher wants us to back up all our answers with NEC code, but not even he can find the answer. I referenced multiple sections of the NEC but found nothing requiring 10/32.
So here is what i came up with after referencing UL, is that if the box has a 10/32 machine screw holes pre-drilled, that they must be used, and using a 8/32 is not allowed. ref. http://www.ul.com/regulators/ode/0904.pdf
so any additional help on the subject i have a class full of other students, and the instructor who are all looking for a little advice,
thanks
- Location
- Massachusetts
From the UL document by Mark Ode
This to me is either stretching or ignoring the rules depending on your view point.
If we must use listed grounding devices than grounding screws are not allowed as there is no UL listing for a grounding screw.
If the NEC allows either a 'grounding screw or some other listed grounding device' then we can use any screw other than sheet metal screws as a grounding screw.
If they feel the need for us to use 'green grounding screws' than the screws should be listed as a grounding device. But IMO that would be over kill.
This to me is either stretching or ignoring the rules depending on your view point.
If we must use listed grounding devices than grounding screws are not allowed as there is no UL listing for a grounding screw.
If the NEC allows either a 'grounding screw or some other listed grounding device' then we can use any screw other than sheet metal screws as a grounding screw.
If they feel the need for us to use 'green grounding screws' than the screws should be listed as a grounding device. But IMO that would be over kill.
FNCnca
Member
- Location
- Portola, CA / Reno, NV
hbiss said:
#21 drill bit.
Edit : I use this size all the time, not trying to be a smart*** here. : )
#21 is about half way between 5/32 and 11/64 bits.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Also from the Mark Ode article:
Does this mean self tapping screws are "acceptable" to UL?
Does this mean self tapping screws are "acceptable" to UL?
And his reference to 8/32 screws is very specific, not a general condemnation.
edited to fix font size.
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