Here are some great tips for determining the proper wire gauge, ampacity, and the maximum wattage allowed. Determinining the proper size wire to use can be easy if you know what amprage and wattage a wire can carry per wire gauge. The trick is to have the right sized wired fiited to the power demand it will have on the circuit.
Wire gauge is the physical size of the wire, rated in gauge size. For instance, common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire. The gauge of the wire dictates the amount of current that can safely pass through the electrical wire. Electrical current is is measured as ampacity.
Ampacity is defined as the measurement of how much electrical current can flow through an electrical wire safely. This amapacity should match the circuit size, meaning the circuit breaker or fuse that protects it.
Appliances are marked with a tag that informs you of the maximum wattage (load) that it takes to run. The maximum wattage of the appliance should never exceed the maximum wattage rating of the circuit it is connected to. If it does, a dedicated circuit should be run that is capable of handling the load.
One more thing to keep in mind is to select wire that best fits your needs. Some wire is stranded, while other wire is solid. The solid wire doesn't always pull as easy in conduit with a large number of bends, but is much easier to place under wire terminals like those on switches and outlets.