My class have been doing some calculation for single family dwellings. The new Tankless Water heaters take a lot of power, some have two or three circuits. Using the standard calculation (Fixed appliance) we used the 75% demand and the service looks acceptable for the loads. When using the optional calculation for new or existing applications the service is reduced to a point where I would not be confident it would work properly. Does anyone have any experience with using the optional calculation with these water heaters? Just looking for some insight, I teach that most of the code is the Minimum.