Residential home - I'm wiring a tankless water heater that calls for 2 - 40 amp 240 volt ckts. It has 2 separate heaters. I was going to run 4-#8's and 1-#8 ground to a junction box on the flat roof (splice there) of the house then go in with 2-#8 NM's into the attic then down to 2 disconnects then to the tankless water heater.
Would it be legal to run 2-#4's instead to the junction box then split off with the 2-#8 NM's to the water heater (2 ckts). Probably be better to keep both cicuits separate all the way.