Hi, I have some confusion about wiring tankless electric water heaters.

#1 When does one require a sub panel, and if so why?

#2 Is there a distinction made between tankless heaters for space
heating and tankless heaters for potable water? There's a tankless
company out there with a separate model for each situation. The
potable model is not UL listed, should I worry about that?

I have the 2002 NEC. If you could site chapter and verse it would be
appreciated.