Hi Mike - Great Electrical Repair Question!
Mike, 30 feet is not a distance to be concerned about as it applies to voltage drop, what you need to pay attention to is what the circuit is to be used for - or what is the application? For example: general purpose lighting and/or outlets can be wired with 12-2 Romex placed on a 20 amp circuit breaker, or 14-2 Romex placed on a 15 amp circuit breaker. This page has a chart showing wire sizes and amperages which should be helpful as well:
Be Careful and Be Safe - Never Work on Energized Circuits! From: Mike C Dave I'm wiring in a over the stove microwave approximately 12 amp required. Thanks Mike
Mike, Install a 20 amp circuit for the microwave. The wire should be a 12/2 romex with ground. Dave
Dave Rongey, Licensed Electrical Contractor, Assisting with Home Electrical Troubleshooting and Electrical Repairs