I have run into this on occasion over the years that AC, MC and Flexible Metal Conduit (FMC aka Greenfield) type cables get confused by many people including myself. MC has conductors preinstalled while Greenfield does not. For someone like me who usually gets supplies at Home Depot for a specific project, its easier to keep the empty conduit in stock at home and then buy the wires of the size I need to length. In this case it was a Solar DC home run. It has been awhile but think it was a 40 foot run of a couple of #10s and grounding conductor. FMC is cheap so I bought a 50 foot box of 3/4" (maybe 1") which is still kicking around and then bought wire to length adn pulled it in. Its also handy for when I need to run additional conductors.