With high current and relatively low voltages (like the cranking circuit in a car or 50w/12vac cabinet lights) the important thing is low contact impedance which means high contact pressure which means larger bolts.
If a #10 screw works for 10kA, anybody have screw sizes for 20, 50 and 100 kA? It probably has to do with the tensile strength of the screw material. And they probably want fine threads so it's less likely to back out with temp. changes.
There is also a difference between a #10 screw carrying 10kA for a few cycles to open an overcurrent device, and continuously carrying 10kA.
NEC 250.8 allows threads into sheet metal for bonding. It requires at least 2 threads into the metal if no nut is used. If the metal is 1/16th of an inch, then an 8-32 or 10-32 screw would work (or #6 or #4 or smaller). To go larger would require either a nut or thicker metal since both #12 and 1/4" fine screws are 28 thread per inch.
Square D seems to have standardized on 8-32 screws for its residential grounding bars. I think most others are 10-32. I tried to put a small square D grounding bar in a 4 square box and its screw didn't fit the 10-32 hole...
When #12 or 1/4" screws are used, they usually have thicker panel wall at the point where the screw is installed. Besides, if they have it listed for use with a particular screw, you do not have to worry if proper number of threads are engaged, the listing says it has been determined adequate.
The Square D grounding bars do use 8-32 screws. they typically also use at least two mounting screws in many cases, which may be better than a single 1/4 inch screw.
