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

[lester.cape]

 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]

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

 

[Smart $]

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]

 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.

 

[don_resqcapt19]

...
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?

 

[ActionDave]

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

And they will never rust.

 

[Smart $]

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.