I'm planning on running 2-2-4 URD service cable to a tankless hot water heater. It's a long story, but I have to run from an outside load center underground and through the subfloor of a building built on piers. The tankless water heater calls for #6 wire since it draws 55 amps. I have some 2-2-4 already, so was just going to use that since it's rated to (at least) 75 amps. My question is, I'll have to sleeve the cable as it exits the ground and runs up through the bottom plate of the wall. How do I figure the conduit fill for service entrance cable? Treat it like THHN/THWN/THWN-2?