When Romex comes down a basement wall, it needs some sort of physical protection. That could be:
- A piece of 2x4 with a slot dado'd into it
- A piece of bent steel
- A chunk of gas pipe
- A three-headed dog charmed to guard the Romex
It doesn't really matter what it is, as long as it confers physical protection. Simple enough!
Here's where people make it complicated. Sometimes, they roll right past the lumber, bent steel, gas pipe and Lovecraftian pet aisles, and go into the electrical aisle and grab a piece of electrical conduit particularly. An unfortunate choice because they have confused themselves between "random piece of thing to physically protect wire" and "electrical conduit as a wiring method, with all the Chapter 3 rules which pertain to conduits as wiring methods".
Which it's not.
So I say get unconfused, get out of that aisle, and go hit the gas pipe aisle :)
I jest, but seriously, the water pipe aisle will be fine. :)
My concern in this case is would the coupling between the EMT and FMC need to be "accessible?" I see some debate about this
If we're talking "conduit as a wiring method", couplings between conduit systems can be buried if they are pullable. If you can reasonably pull wires through it.
That's what they mean by "straight". They mean the coupler/joiner of dissimilar conduit types must be straight. What you can't do is come up out of EMT, have a hard 90 plumbing elbow and then into FMC, because you wouldn't be able to pull wires through it.
NEC requires that in conduit as a wiring method, and I hope we're clear on that distinction by now, the conduit system must be consructed empty until it is complete and anchored down. Then, wires must be pulled in, using only pulling points which will remain accessible. That means every other bend in the system must be easily pulled.
Wires aren't magic and elbows aren't pullable.