What size wire does a dedicated microwave circuit need?

For a built-in or over the range microwave, run a 20 amp individual branch circuit with 12 AWG copper conductors. That is the answer in almost every dwelling. The NEC does not have an article that says "microwave circuit" by name, so the requirement is assembled from three sections: 422.10(A) for the branch circuit rating, 210.23(B) for how much of a circuit a fixed appliance may take, and 210.52(B)(2) for what the kitchen small appliance circuits are allowed to serve.

Why 12 AWG and 20 amps

Section 422.10(A) requires the branch circuit for an individual appliance to be rated not less than the marked rating of the appliance, and not less than 125 percent of the load if the appliance is a continuous load. A typical over the range microwave draws 1,000 to 1,800 watts, which is 8.3 to 15 amps at 120 volts. A 15 amp circuit on 14 AWG will carry a 1,000 watt unit, but the larger 1,800 watt models push a 15 amp breaker close to its limit every time the unit runs on high. Manufacturers almost universally call for a dedicated 20 amp circuit in their installation instructions, and 110.3(B) makes those instructions enforceable. Run 12 AWG copper on a 20 amp breaker and the question does not come up at inspection.

Why it usually cannot share the countertop circuits

Section 210.52(B)(2) says the two or more 20 amp small appliance branch circuits required for the kitchen shall have no other outlets, with limited exceptions for receptacles in the same room and for a receptacle supplying an electric clock or gas range ignition. A cord and plug connected microwave plugged into a receptacle in the cabinet above the range is fed from a receptacle outlet, so it can technically sit on a small appliance circuit. The problem is 210.23(B): where a circuit supplies both fixed in place appliances and other loads, the fixed in place appliance may not take more than 50 percent of the circuit rating. Fifty percent of a 20 amp circuit is 10 amps, and a 1,500 watt microwave is 12.5 amps. That alone forces the individual circuit.

Countertop and built in variations

A small countertop microwave that simply plugs into a normal kitchen receptacle needs nothing special. It is a cord and plug connected load on the small appliance circuits, and 210.23(B) does not apply because it is not fixed in place. A microwave drawer, a built in wall microwave, or a microwave and convection combination unit is a different story: check the nameplate and the instructions, because some combination ovens exceed 20 amps and call for a 30 amp or 240 volt circuit.

GFCI and receptacle details

Section 210.8(A)(6) requires GFCI protection for 125 volt through 250 volt receptacles serving kitchen countertop surfaces, and 210.8(D) requires GFCI protection for specific appliances including microwave ovens rated 150 volts or less to ground and 60 amps or less. A dedicated microwave receptacle in the cabinet therefore needs GFCI protection under current code, which is easiest to provide with a GFCI breaker so a tripped device is not hidden behind the appliance. Do not forget 422.16(B)(4) if a flexible cord is used, and keep the receptacle accessible so the microwave can be unplugged as its disconnecting means under 422.33(A).

Field takeaway

Dedicated 20 amp circuit, 12 AWG copper, GFCI protected, receptacle accessible. Read the appliance instructions for anything larger than a standard over the range unit.