If the microwave has a 15 AMP cord cap (plug) then a short 15 AMP rated extension cord will be fine. If the cord cap had one blade turned 90° then you'd need the 20 AMP cord. The circuit needs to have a 20 AMP breaker (and #12 wire) because of the microwave's rating and the fact it can be a continuous load (defined as 3 or more hours). The rules change at the receptacle. The microwave wouldn't have a 15 AMP cord cap on it if a #14 cord wasn't acceptable to use on it for at least a short distance.

Just a note on the reasoning. If you have a "normal" branch circuit with multiple outlets, you can use the 15A receptacle. This is the majority of the wiring you'll find in a home. Basically, when you have multiple receptacles with a total load of less than 20A, no single receptacle is going to require more than 15A. If you have a dedicated circuit where there is only going to be 1 receptacle, then you need to use 20A. The appliance (or whatever 1 item) needs 20A available, so it all has to be able to flow through that receptacle. No matter what, the wires need to be able to carry the full 20A safely.

...SAFETY SAFETY SAEFTY 10 THINGS...12 THINGS..irrelevant home in on the no continous loads over 80% . .and of seldom do we turn on everything at one time . .but modern era homes DO run a much higher loads everything you can buy has an rating on it that will tell you either amps or wattage . . .Google for a Ohms chart for the conversions if you're experience level is low . .arm your self with one of the Color..residential wiring books at the Big Box Store they are a wealth of knowledge good luck . .let us know what you work out ?

I am getting ready to wire up the garage in my existing home. THey only had 14/2 wire on a 20 amp and only 2 plugs, thus when I used my saw with spot lights and the the pncake compressor would go off, so would the power. I am going to run 12/3 from the main panel to the garage and the first 2-gang outlet then use 12/2 wire, one will be on 1 ciruit and the plug on another then I will go from there and onto the workbench with the three other sets of plugs. Plus I will be running a cable out there and cat5. I'm just a DIY woth woodworking, thus I will not be setting up a big shop, I just want the right power to operate the new Tablesaw without having to worry. QUESTIONS: 1) should I use 2 GFCI outlets in the first box or do a GFCI breaker instead? any preferences on brand? 2)should I use 20amp plugs in a different color such as grey so as not to have confusion with the 2 other 15 amp plugs (for future homeowners) 3) where online is a good electrical supply shop?