Yep, 8 guage. I was going on the requirement they spec'd for running a branch box for 125 amp load center in the garage. There are a number of aspects of what the NEC specs that have mystified me. But sure enough, there it is Voltage 240, Watts 28, 500, Amps AC 120 . . . Wire Size (AWG) 8 for Temp. Rise @ 1.5 GPM 129.77, Temp. Rise @ 2.5 GPM 77.862, Temp. Rise @ 4.0 GPM 48.6637
I would expect that every electric tankless would have very similar specs in terms of watts needed per degree-GPM deliverable
Voltage 120, Watts 3500, Amps AC 29 Now that is not a normal plug-in, but an RV hook-up type plug-in and it can heat Temp. Rise @ 0.5 GPM 48, Temp. Rise @ 1.5 GPM 16 or, not quite handle a shower if one only needs to heat the water 16 degrees