I have a Rheem rte 13 http://www.rheem.com/product/point-of-use-tankless-electric-rte-13
Looking @ the table it says use # 6 with a 60 amp breaker but the whip that came pre wired to it is labeled Romex 10, it's 2 conductor with a ground.
I've had other small cheap electric tankless heaters...