For anyone in the Clark Griswold school of holiday decorating, overloaded circuits are a real concern around Christmastime. Unlike Clark, you won't risk shutting down the grid service to your entire town, but you can certainly pop some breakers and possibly create a fire hazard. There are a few ways you can create an electrical overload, and all are easily avoidable. As for the decision to go with the animatronic Santa on the roof again this year, maybe you should let your circuits decide.
Overloading a Circuit
Standard household electrical outlets are on circuits that are rated for at least 15 amps of electrical current. Newer homes also have a lot of 20-amp circuits, including the outlets in the garage and outdoors. Lighting circuits (which often include outlets) typically are 15-amp. So what does the amp rating mean? It tells you how many lights and other devices the circuit can handle without tripping its circuit breaker. The rule of thumb is not to load a circuit over 80%, meaning a 20-amp circuit can safely handle 16 amps, and a 15-amp circuit can safely handle 12 amps. By adding up the amperage draw of your lights you'll know how many you can put on a single circuit.
Adding the Amperage
Check the packaging of your light sets. If that's not available, check the tag or plug on the light cord. If it gives you the amperage rating, you're all set. If it gives you only the wattage, divide the wattage number by 120 to find the amperage.
For example, if a light string uses 250 watts, the amperage draw will be 2.08 amps (250/120 = 2.08). You could get away with six of these strings on a single circuit, but five is a safer bet. Keep in mind that this is just the total for the Christmas lights; if anything else is using the same circuit, there will be less power available for the lights.
A single circuit may be serving several outlets and/or light fixtures. If you suspect you're getting close to an overload, divide the lights among two or more circuits. You can identify which outlets are on which circuits by switching off one breaker at a time and checking each outlet for power; any outlet that has no power is on that circuit.
Another potential overload comes from plugging Christmas lights into light fixtures, using one of those cheap screw-in adapters. You can do this safely only if the adapter is in good condition and makes good contact with the fixture base and you don't exceed the wattage limit on the adapter or the light fixture. This means you can forget about running all the outdoor holiday lights to one fixture, but it's better than causing a fire. Also, don't overload outlets and adapters by plugging in too many light strings. You can easily fry an adapter (since many of these are sketchy, to begin with), and the massive bulk of cord connectors protruding from the outlet itself becomes a shock and fire hazard.
When the weight of the plugs draws the plugs away from the outlet, it often exposes the blades of the plugs while they're still energized.
The LED Solution
LED lights are about 75% more efficient than conventional incandescent lights. Switching from old-style lights to LEDs makes your electrical load more than seven times smaller and has the same effect on your holiday electric bill. That alone might easily solve your overload problems. If you decide to make the switch, check the Energy Star website for a listing of qualified products. These meet the program's minimum standards for energy efficiency and warranty length. LEDs last a long time (or they should), so it makes sense pay a little more for quality.