A new ad by AFC Cable brought this issue to the forefront: Just what is the difference between AC and MC? Apart from what's printed on the UL lable, that is.
This matters, because there are a few places in the NEC where one is acceptable, while the other is not. For example, if you ever find one of those mythical AFCI devices, and wish to use it in place of an AFCI breaker, you can use AC, but not MC. Health care locations are another such quagmire.
We have had "health care MC" around for a decade now (MC painted green), and Southwire has been marketing their "Simpull" MC (with a bare wire outside the mylar wrap, rather than the usual green wire) for half that time. Now, it seems, AFC has done away completely with the inner mylar wrapping. It seems that the 'definitions' I have had in my mind for these two products have been obsolete for some time. MC, contrary to what I thought, need not have a mylar wrapper ... and that bare ground they're putting in some MC pretty much duplicates the bond wire in AC.
Maybe it's time to combine these two sections, consolidate the UL standards, and eliminate the code restrictions on MC?
I suspect AC might just go the way of K&T. MC is overlapping it in virtually every regard. Guys like AFC are making MC in just about any configuration you could think of. It is becoming the most versatile wiring method out there.
I hated to use AC cable, I would only use MC once it came out. The old AC cable was useless if you had a lazy installer. They forgot to tighten up the connectors and you would lose your ground. I also saw where the outside case rusted away and the grounding was also lost.
They are making the new MC cable that uses the armor, with an internal aluminum conductor, for the ground path. I am curious how well that will stand the test of time. You do not terminate that aluminum conductor in any way beyond the clamp on the armor and they depend on incidental contact to the armor inside the cable for the ground path. If this is not a gas tight connection it will oxidize, in my opinion. I am curious how well it will perform after 20 years in a real world environment, particularly if that real world was damp, maybe with a little salt air.