Assuming you are referring to an AC circuit and not DC: The size of wire you need for 100 watts depends on the voltage of your circuit, the amperage rating of your fuse or breaker, and the length of your run. Most standard household circuits are 110-120 volts, so we'll use that as our starting point. Using the National Electrical Code (NEC) guidelines, for a 20-amp circuit you'd need 12-gauge wire, but for a 30-amp circuit you could get away with using 10-gauge wire. The NEC doesn't allow 14-gauge wire to be used in a circuit with more than 15 amps. So now we know that we need at least 10-gauge wire. The next step is to determine how long our run will be. For our example, let's say our 100 watt light is located 20 feet from our breaker box. We'll need to add 5% for every hundred feet past the first 50 feet, so in this case we'd add an additional 10 feet to our total length, giving us a total run of 30 feet. Now that we have all of our information plugged in, we can use one of several online calculators (like this one from Southwire) to determine the correct size and type of wire to use for our application. According to the calculator, for a 120 volt AC circuit with a 30 amp breaker/fuse and a total run length of 30 feet using THWN copper conductor Wireman awg 10/2 cable would work perfectly.