Copper 12-gauge wire can handle 20 amperes of current, and aluminum 12-gauge wire can handle 15 amperes. Temperature doesn't affect the amount of current 12-gauge wire can carry, though it can affect other gauges.
Circuits need the correct size wire for the amperage of the current to prevent overheating. Amperage is the measurement of the amount of electricity going through the wire, and the unit of measurement is amperes, or amps. Electricians determine the amperage of a circuit by adding up the wattage of all the devices on the circuit and dividing that number by the voltage of the system.