I am installing a microwave oven, over the stove type. It is rated at 950 watts. I am going to plug it in to a short 15 amp extension cord, as I do not have an outlet close enough to the appliance (the appliance cord is about 12 inches too short to reach the receptacle). The circuit is 12-2 wire with a 20 amp breaker. I know one is advised not to use an extension cord with a microwave, but I am planning to use a heavy-duty cord. Nothing else will be on the circuit except the sparkers for my gas range and the light bulb in the gas range oven. I presently have a 15 amp receptacle but could change the receptacle to 20 amp. Question: Is a very short extension cord likely okay, and should I change the receptacle to 20 amp?

The current will be 950 W/120 V which is no problem for a 15 A receptacle. A heavy duty cord should not be a problem either. The microwave doesn't usually run long enough anyway to be any problem.

You don't have to change the receptacle to 20 A, but if you did I would get a 20 Amp rated extension cord with the corresponding 20 A plug. A 15 A plug will also plug into it.