Here's why:

* Amperage: A 1000 watt microwave draws a significant amount of current. The actual amperage will depend on the voltage (120V or 240V), but it's likely to be around 8-10 amps.

* Wire Gauge and Ampacity: 14-gauge wire is rated for a maximum of 15 amps, while 12-gauge wire is rated for 20 amps. Using 14-gauge wire could lead to overheating and potential fire hazards.

* NEC Code: The National Electrical Code (NEC) sets standards for wiring, and for a 1000 watt appliance, 12-gauge wire is the minimum recommended.

Always consult a qualified electrician for any electrical work. They can determine the correct wire size based on the specific appliance and your home's electrical system.