The Canadian Electrical code would allow #12 feeding base board heater to be loaded to the full 20 amps or 4800 watts on a 25 amp or 30 amp breaker depending on the availability from a manufacturer for a 25 amp breaker. #14 can be loaded to 3600 watts on a 20 amp breaker.
This is one of the very few places where a wire can be loaded to its full rating and only applies to baseboard heaters.
I know this is an NEC question except that unification of our respective codes is supposed to be a mutual objective.
