Christmas Light Power Budget Calculator
Plan Christmas light strands by circuit amperage and bulb type.
Find the safe number of strings per outlet plus monthly running cost.
Stringing too many holiday lights on one outlet is the most common cause of tripped breakers in December and a non-trivial cause of house fires in January. The math to do it safely is short, but very few people actually run it.
A standard US household circuit is 15 amps at 120 volts, with a code-recommended continuous load limit of 80 percent. That gives you 12 amps of usable continuous current per circuit, or 1,440 watts.
watts_per_circuit = 120 V × 15 A × 0.8 = 1,440 W
Incandescent mini-lights pull about 0.4 W per bulb. A 100-bulb strand is 40 W, so a single circuit theoretically holds 36 strands. In practice, manufacturers limit you to three strands end-to-end before the first strand’s wire overheats from carrying the downstream current — that limit is on the strand, not the outlet, and ignoring it is what melts plug ends. Run multiple chains via a heavy-duty outdoor extension and a powered splitter rather than daisy-chaining beyond the manufacturer’s stated max.
LED mini-lights pull around 0.04 W per bulb, ten times less. A 100-bulb LED strand is 4 W. The same circuit holds the equivalent of 360 incandescent strands worth of LEDs. Modern LED strands are also rated for 40 to 50 daisy-chained connections rather than three, so the practical wiring becomes much simpler. If you are still using incandescents, the upgrade pays for itself in a couple of seasons of electricity savings on a moderately decorated house.
Cost-wise, the difference is striking. A 1,000-bulb incandescent display running 6 hours a day for 45 days at 16 cents per kWh costs about $17. The same 1,000 bulbs in LED costs about $1.70 — that is the entire month for less than a single fancy coffee.
Outdoor extension cords have their own limits. A 16-gauge 50-foot cord is rated for about 13 amps continuous and is fine for any LED display, but a long incandescent run can exceed it. Use 14-gauge or 12-gauge for incandescents, and check that any cord you use outdoors is marked for outdoor use (the W on the jacket).
Last item. GFCI outlets are required for outdoor decorating in most modern code, and a GFCI that trips during a downpour is doing its job. Before you reset it for the third time, check that the cord ends are off the ground and that no connections are sitting in a puddle.