Originally Posted by Scuba_Dave
Read section 200 of NEC for Load calculations
Without looking at my code book - Art. 200 is "Use and Identification of Grounded Conductors" and what would load calculations have to do with microwaves. The only place load calculations and branch circuits really overlap is the controversial requirement that limits receptacle outlets on commercial circuits due to the load calculation. That and the small appliance and laundry requirements.
If you are saying that a microwave is a continuous load (greater than 3 hours) and needs to apply the 80% rule, that's a stretch. Please cite a specific code section.