A typical household circuit can include several plug outlets, internal lighting fixtures or built-in appliances all on the same circuit. This means that whenever you plug your Christmas lights into the wall, they are sharing that circuit's capacity with anything else that may also be drawing power. If very few things are on the circuit, you have more wattage capacity. However, if your circuit already has several items drawing power from it, you may begin “tripping your breakers” in the fuse box. This happens when your wattage exceeds the amp capacity of the circuit.
Most household circuits are 15 or 20 amps.
It is recommended that all circuits never exceed 80% of their max wattage capacity.
15-amp circuits support 1,800 watts (80% capacity is 1,440 max watts)
20-amp circuits support 2,400 watts (80% capacity is 1,920 max watts)