250.8

[Gordon Stevenson]

How do we accurately identify what are not sheetmetal screws. Is any self tapping screw not compliant? Do the screws used need to be 10/32 like green ground screws? Does 10/32 assure true metal to metal continuity? Any thoughts, field approaches, tips appreciated.

 

[charlie b]

Re: 250.8

All I know about sheet metal screws is that the threads extend from the point all the way to the head. This is as compared to a wood screw that has some amount of smooth round space (no threads) near the head. The difference is there because sheet metal is thin, and you need all the threads you can get, in order to tighten one sheet of metal against another. I think the reason they are not allowed is that the threads near the head could damage the conductor. But that's just a guess.

 

[electricman2]

Re: 250.8

Sheet Metal screws are identified by No. and length as "No. 8X2 and by head type as flat, hex or binding. Machine screws and cap screws are identified by No., thread pitch and length as 10-32X2. Machine screws used for grounding also have threads full length so that may not be the reason for prohibiting sheet metal screws. Maybe its because the typical thickness of a box or enclosure requires tapped threads to hold well. See also this thread.