I disagree because it's applying the answer for one problem to another unrelated question.
The '5 threads' rule appears in Article 500 simply because there needs to be a flame path that long to assure that an internal explosion does not ignite the atmosphere outside the conduit. The requirement has nothing to do with grounding.
As for grounding, the UL research study that evaluated the use of conduit as a grounding means included -more by accident than design- a number of poor connections. No matter- the pipe proved to be a fabulous conductor.
Remember the EMT fittings that were once 'crimped' on? Now, there's a loose connection for you! Yet, it somehow managed to work.
Just as important, there are several instances in electrical work where a tapered thread is connected to a straight thread. It's simply not possible to fully engage 'five threads' in that situation; EVERY coupling would be a violation.
I guess Reno was thinking we only care about thread engagement for grounding, supporting or sealing. The 5 thread rule is clearly for sealing. He just wanted to be sure this did not apply to grounding. You would need a rod connector behind a grounding screw to get 5 threads.
I have recently heard this '5 thread' stuff elsewhere, and there the expressed desire was to ensure a good path for the clearing of fault current. In a similar vein I have heard debates over just what constitutes 'wrench tight.'
We're not plumbers. With straight threads, you're never going to 'seal' anything. We rely on the wire insulation for that.
Actually Reno, in 500.8(E), they specify NPT threads and sealing is the object. The only thing they don't require is "dope" but that is really just a lubricant.
(E) Threading. All NPT threaded conduit and fittings referred to herein shall be threaded with a National (American) Standard Pipe Taper (NPT) thread that provides a taper of 1 in 16 (3/4-in. taper per foot). Conduit and fittings shall be made wrenchtight to prevent sparking when fault current flows through the conduit system, and to ensure the explosionproof integrity of the conduit system where applicable. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 500.8(E)(1) or (E)(2). Threaded entries into explosionproof equipment shall be made up with at least five threads fully engaged.
Greg: The term 'sealing' that you use IMHO is incorrect, or mis-used. The five thread requirement in classified use of RGC is to provide a 'controlled escape pathway' for ignited products of combustion that are within the RGC. The 'pathway' is supposed to allow the gasses to cool while venting thru the threads to prevent an 'explosion' in the area.
I have seen that but if you really make pipe threads up "wrench tight" they will pretty much seal. That is particularly true if there is some lube in there and with that oil they put on pipe to keep the factory cut threads from rusting (or the cutting oil itself when field cutting) they can get "gas" tight, particularly over time when you get a little oxidation going. The air system I put in my garage (galvanized, schedule 40, threaded pipe) had a very slow leak for a while. Now it holds 150 PSI for weeks with the compressor off.
The reference cited that describes tapered pipe threads is specific to FIELD MADE threads. It does not apply to our couplings, many conduit bodies, chase nipples, or EMT fittings.
Likewise, even a plumber will have leaks in the best NPT threads if he fails to use pipe dope. There is a slightly different thread -"dry seal"- that is used where it is required to actually seal without the use of any sealant.