FW: Grounding screw requirement [RE-wrenches]

[Brian Wiley]

Hi Aaron,

I do not think there is any such specification for electrical 
connections.  The 3 thread rule of thumb is for mechanical engineering 
and relates to mechanical strength, not current carrying capacity.

This is why UL requires the manufacturer to provide a specific screw 
with their product.

Tests in my laboratory have established that a 10-32 stainless steel 
screw with 3 threads (as is commonly used on PV modules) is capable of 
carrying enough current to be used with a 10 AWG wire maximum.

Brian Wiley
Wiley Electronics LLC



Aaron Wellendorf wrote:
> 
> Fellow wrenches,
> 
> I am resending this as it did not seem to post.
> 
> I am trying to come up with documentation regarding the requirements for
> ground screws, such as those used to mount a mechanical lug inside of a
> disconnect switch or a junction box. I have seen installations with a
> Tek screw holding ground lugs to electrical boxes. I have always
> used/required the typical 10-32 ground screw that is self tapping, or a
> properly sized pre-threaded hole to put in the 10-32 screw. I was taught
> that proper thread engagement was 3 threads touching the metal to ensure
> long term contact and continuity. I am searching for the requirement for
> the 3 threads or any other specification relevant. I have searched my
> electronic copy of the NEC and can't find a reference to this. Can
> anyone tell me where a written specification can be found?
> 
> Square D tech support says that to maintain their UL listings, their
> safety switches require the use of their grounding kit, part #GTK0610,
> which has a 12-24 hex head self-tapping screw 0.62" long with a washer.
> The screw must be installed in the location marked with the ground
> symbol. 
> 
> 	"Aaron,   All of our  safety switches have a UL label inside the
> switch that lists the UL listed accessories that can be used.  The
> ground kits for these switches are listed on this label.
> These UL listed kits are the only accessories that should be used in
> these switches to retain the UL listing, and warranty.  Using anything
> not listed on this label will not be UL listed as an accessory, and void
> any warranty.
> 	Tim Milbert  Product Support Specialist"
> 
> Thank you,
> 
> Aaron Wellendorf


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