NEC Microwave Dedicated Circuit
The code doesn't address microwaves as a specific appliance, but NEC 210.23(A)(2) Utilization Equipment Fastened in Place limits equipment to 50% of a circuit when shared with lighting or other equipment not fastened in place.
So if it's fastened in place, which would likely place it on a designated kitchen circuit, a shared kitchen circuit would have to be 20A, so a microwave would not be able to exceed 1200W without requiring a dedicated circuit.