12/2 simply isn't enough. Look at the nameplate; I think you'll find you need at least #10, and you'll want to have that red wire as well as a neutral. After all, some of the new ones have motion sensors, etc in them - so they might need that neutral.
I don't think the NEC specifically addresses the issue; the usual ampacity rules apply.
Since you're using MC, you really want to get it right the first time.
ALSO .. pay special attention to mounting height. Some folks place them so thay can do double duty as hair dryers.
