i have a 3 phase 480/277 panel i wanted to bond to the building steel. which metal roof rafters that support the roof. is it ok to drill and 1/4x20 a lug or ive heard of a lug that the head of the bolt snaps off at the right torqe? which one would be legal
Are you bonding the Building Steel to the Electrical system via a connection at the Sub Panel, or are you planning to ground the Sub Panel by bonding it to the grounded Building Steel?
If Bonding the Steel to the Grounded Electrical System, then do so via approved lugs and bolt sizes according to the lug. Torque all to the specifications only ONCE!!! Do not re-torque the lug. If the bolt used to land the lug snaps off when torqued, find a better bolt! Maybe someone can suggest what to do here. I have broke bolts during torquing, but they were few and far between [and sometimes the "Cheap'os" from the Home Depot's open bins].
Lugs have been known to strip out during torquing. Can't really elaborate much on this one except that "Doo-Doo Occurs"
Now, if you are planning to ground bond a panel by connecting it to a grounded Steel frame, this would be something that falls under 250-32. Could you expand a little more on your topic?
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
2002 NEC, 250.53(C) Shall be connected in a manner specified in 250.70. (exothermic welding,listed lugs,listed pressure connectors,listed clamps,or other listed means,Connections depending on solder shall not be used. If the lug is listed for the wire property on the connection to the steal, than it can be connected to the building by the 1/4*20 screw, locknuted and torqed to the bolts manufacture standards. as long as any paint or primer is removed from the building steal first. To insure a good connection than it would be recommended to install a min. (hex head bolt). and torqued. A illegal connection would be with a self drilling screw... As far as the other lug, again, as long as it is listed... (You are using for the purpose it is intended for.)
My interpatation: A sheet metal screw has threads that tear the metal than set their self by force. A self tapping screw would tap the threads in to the metal and could be screwed to a specific toque. The tapping screw would be legal. You are correct.....
Most enclosures that may be used where an EGC is installed will have a termination point such as one with a lug, or "a means provided" and this may be a simple "hump with a thread" to be used with a machine screws, and not a sheet metal screw.
Any kind of a "sheet metal screw" is not permitted.
[This message has been edited by Joe Tedesco (edited 12-24-2001).]
Joe, What is the definition of a "sheet maetal screw" and where do we find it. I know that the intent is to provide a secure electrical connection, but when you look in a hardware catalog you will find that the ones in your pictures are not shown as "sheet metal screws". Look at: http://www.mcmaster.com/param/asp/psearch.asp?FAM=selfdrill&desc=Self%2DDrilling+Screws
Here are "sheet metal screws": http://www.mcmaster.com/param/asp/psearch.asp?FAM=selfdrill&desc=Self%2DDrilling+Screws
I would not like to see either one of the above used to terminate an EGC, but if I was an inspector, I would have no grounds to turn down the self drilling screws. The code needs to be changed to require the use of a machine screw or a machine bolt to make the section clear and enforceable. Don(resqcapt19)
My picture of a self drilling screw. It looks very much so like a mach. bolt, but the bottom of the bolt does not drill. A pilot hole must first be drilled,(smaller than the bolt) then the mach bolt is inserted into the hole when at the same time it acts like a tap. The bottom looks like a tap, but only about 1/2" up. The threads it makes will accept any normal mach. bolt of the same thread.
[This message has been edited by aphares (edited 12-20-2001).]