A microwave should be on a separate circuit!
Microwave Oven Circuit
The microwave oven needs a dedicated 20-amp, the 120/125-volt circuit to feed it. This will require 12/2 NM wire with a ground. Microwave ovens come in different varieties and sizes. Some are countertop models, and other microwaves mount above the stove.
Although it's not uncommon to see microwave ovens plugged into standard appliance outlets, larger microwave ovens can draw as much as 1500 watts, and these need their own dedicated circuits.