It kind of depends on how you get the lower amount of heat. Some heaters you can wire up either in wire or Delta configuration and you get a different amount of heat. If that was the case, then there's no reason why you can't provide conductors and circuit protection for the way the heater is actually wired.
Where are you installing something that runs on 415Y240V that would need to comply with the US NEC? That's not a normal voltage here.
Be that as it may, the NEC is not concerned with what you SAY you will be doing, it is concerned for what a piece of equipment is RATED for. If it is rated for 40kW, it is what it is.
What about modifying the unit to disable the 42 kW setting, relabeling it as a 21 kW unit, and then sizing the circuit based on 21 kW? A viable strategy, or a problem?
In the end this is exactly what we ended up doing. As you pop the top front open on the heater, the higher setting (42kW) had a breaker that we just left open. So if anyone were to accidentally switch to the higher setting from 21kW on accident, nothing happens. Easy Peasy. When we want to return to normal, open the front and close the breaker.
