I'm running some cables for an electric tankless hot water heater. I haven't decided which one yet. It'll either need 2 x 40A, 2 x 50A, or 3 x 40A, all 240V (there are some that need 60A, but I'll pass on them). I plan on running two sets of 6/2 NM and one 8/2 NM from my breaker panel to some 4x4 metal boxes in the wall where I'll mount the water heater. That way, I can pick the breakers, and use the right two or three runs I need. Once I decide which water heater I'm using, I'll run FMC from the box(es) to the water heater.
I'm wondering what size cable clamp connector (and hence, knockout) I need for each 6/2+G NM cable and for the 8/2+G NM cable (I'm assuming no doubling of cables in a clamp). Also, can I have more than one (maybe even all three) in a single 4x4 box? Or would it be better to use two or three boxes?
Also, what kind of box(es) would I use? I'm only seeing 3/4 and 1" KOs, and I suspect I need something larger...