I'm guessing these days even when you get a built-in microwave in a modern kitchen (such as above stove with ventilation hood unit) it's still a 1500W max load

I'm guessing these days even when you get a built-in microwave in a modern kitchen (such as above stove with ventilation hood unit) it's still a 1500W max load thing designed to go on a normal household 120V 15A circuit. You'd need to go to some expensive high end stuff before you have like a 240V 30A capable microwave.