If a light is the only thing on that circuit, that is not a problem. There is no code issue here.

Code requires that equipment that draws more than 50% of the circuit rating be on a dedicated circuit.

I think that most over the range microwaves would fall into that category.

There isn't just one light fixture, it's all the lights on the main level. This includes the outlets. If I were to run the microwave and a space heater in one of the outlets, it trips the circuit.

They only way to "fix this" is to run a new circuit so that the microwave is is on a different circuit from the other items.

Depending on how the wiring is run and the house construction will determine how and where to run this new circuit. You might run the new circuit to the mircwave or you might disconnect the other items from the microwave circuit and connect them to the new circuit.

Or it might be some combination such as running the new circuit to place where the microwave taps off the other circuit and make the connections there.

And while technically not to code it would not be that bad if you end up with one or two ceilng fixtures on the MW circuit, but you definitely don't want any receptacles on it.

Thanks so much for the advice. I will definitely use it.