That's the 'fine print" I was referring to, Scott... it may say 'conduit,' but it doesn't say 'listed.' The only reason I brought this up was the possibility raised by the OP - that having a coupling might result in a much weaker mast.
ANSI specs for electrical pipe (RMC) and water pipe are identical, with the sole exception that electrical pipe is to have a 'smooth' finish inside. "Smooth" is not further defined; the clean weld seam you often see certainly looks smooth (not likely to snag or catch wires) to my eyes.
Our PoCo service specs are completely without reference to the NEC. Indeed, they are considerably more detailed and specific. Yet, there are no referenced to 'listed' or 'approved.' The install simply has to be acceptable to them - and they have no interest in what anyone else may think.
As for the use of straight threads on some electrical parts .... apart from 'running thread,' I have no idea how that situation came about. I don't think it was about saving money ... perhaps manufacturing processes came into play. Or, perhaps someone tried to over-engineer things. I simply don't know.
Keep in mind that the original 'conduit' was nothing but ordinary gas piping, with ordinary tapered threads. I suspect that the straight threads, and the conduit specs, came late to the party.
