Do self tapping screws have to be UL listed? 250.8 (A) (6)

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lester.cape

Member
Location
Las Vegas
 I am looking for some more information pertaining to Article 250.8 (A) (6).  I have recently purchase the Grounding VS Bonding course offered through this website, and to my understanding a self tapping screw that engages a minimum of two threads is legal. If you do the math this article requires a thread forming machine screw (self tapper) with at least 18 threads per inch. This is about 2.2 threads engaged in the Box. The box I am refering to would be a four square J-box. On anoter note groundclips are not listed in 1 through 6 but actually fall under item 8 that says other listed means. Ground clips are listed therefore legal to use. My question is: Do items 1-6 have to be ul listed? or am I wrong about self tapping screws being thread forming? The way it reads to me is this: If you want to use a permitted method <U>not identified </U>in 1 through 6 then 7 and 8 says it has to be listed. If you reverse this sentance is it still correct? Could I say that if the device is not UL listed it must meet requirments 1 through 6? I have an AHJ telling me self tapping screws have to be listed and I am trying to figure out if this is correct.

Also does anyone know were the two thread engagement came from. Is it derived from a UL standard? Did somebody test a screw in a box and determine that two threads provides adequate continuty for a low impedance path back to the source?
 

cadpoint

Senior Member
Location
Durham, NC
Welcome to the Forum!

I don't know where any of the UL requirements of it came from. :) You can read an overview of what the actual requirements are saying here UL. I just want to throw out this rational;

if your take 18 threads per 1 inch that's 1/18= 0.055;

if one is using this size threading into 10 gauge material it is equivlant to ~ .1345

so 0.1345/0.056= 2.401. So in fact you've made the requirement as set by the listing agent!

I would accept any UL listing as gospel and not try to re-word it or turn it around in any way, There are bigger fish in the NEC that causes way more pain if not followed... :thumbsup:

I used this page for a gauge chart Here
 
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Smart $

Esteemed Member
Location
Ohio
Items #5 and #6 do not have to be listed for the purpose. They only have to meet the mechanical specification. That said, most self-tapping screws are not thread-forming machine screws.

The requirement for listed means is in #'s 1, 3, 7, and 8.

Food for thought: plastic machine screws exist and meet the the requirement of #5.
 

tkb

Senior Member
Location
MA
 I am looking for some more information pertaining to Article 250.8 (A) (6).  I have recently purchase the Grounding VS Bonding course offered through this website, and to my understanding a self tapping screw that engages a minimum of two threads is legal. If you do the math this article requires a thread forming machine screw (self tapper) with at least 18 threads per inch. This is about 2.2 threads engaged in the Box. The box I am refering to would be a four square J-box. On anoter note groundclips are not listed in 1 through 6 but actually fall under item 8 that says other listed means. Ground clips are listed therefore legal to use. My question is: Do items 1-6 have to be ul listed? or am I wrong about self tapping screws being thread forming? The way it reads to me is this: If you want to use a permitted method <U>not identified </U>in 1 through 6 then 7 and 8 says it has to be listed. If you reverse this sentance is it still correct? Could I say that if the device is not UL listed it must meet requirments 1 through 6? I have an AHJ telling me self tapping screws have to be listed and I am trying to figure out if this is correct.

Also does anyone know were the two thread engagement came from. Is it derived from a UL standard? Did somebody test a screw in a box and determine that two threads provides adequate continuty for a low impedance path back to the source?

A 4"sq box is 1/16" thick and would require a screw with 32 threads per inch to have 2 threads engaged. This is why the box comes with a 10-32 tapped hole in the back of the box.

314.40(B) requires the box to be 1/16" thick.

There is nothing in 250.8(A)(6) that would require listing of the screw.
 
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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
...
Food for thought: plastic machine screws exist and meet the the requirement of #5.
Does the screw itself have to be conductive, or does it just have to provide enough pressure between the conductor or lug and the enclosure to provide a low resistance connection? Can you put enough torque on the plastic machine screw to provide the required clamping force?
 

Smart $

Esteemed Member
Location
Ohio
Does the screw itself have to be conductive, or does it just have to provide enough pressure between the conductor or lug and the enclosure to provide a low resistance connection? Can you put enough torque on the plastic machine screw to provide the required clamping force?
Well, if we include other requirements, such as 250.4, a plastic machine screw would not be compliant. But that is not blatantly obvious. :p
 
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