Watts, amps and breakers
Christmas light circuit and amp calculator
Find out how many C9 bulbs one household circuit can carry and how many circuits your display needs. The default assumes 1 watt per LED C9 bulb; change it to match your bulbs.
The math
Usable watts = amps × volts × load limit − other load. Bulbs per circuit = usable watts ÷ watts per bulb, rounded down. Circuits needed = display bulbs ÷ bulbs per circuit, rounded up. A 15 amp breaker at 120 volts and 80% gives 1,440 usable watts; a 20 amp breaker gives 1,920.
LED vs incandescent at a glance
| Bulb | Watts | 15 A | 20 A |
|---|---|---|---|
| LED C9 (low) | 0.5 | 2,880 | 3,840 |
| LED C9 (planning figure) | 1 | 1,440 | 1,920 |
| Incandescent C7 | 5 | 288 | 384 |
| Incandescent C9 | 7 | 205 | 274 |
This is why LED C9 bulbs changed roofline lighting: a whole house can often run from one outdoor outlet, where incandescent strings needed several circuits.
Circuits are not outlets
Several outlets often share one breaker, and exterior outlets may share a circuit with a garage or porch. Before plugging in, find which breaker feeds each outlet and add anything else on it (a freezer, a garage door opener, other decorations) to the "other load" field.
Timers and control
A photocell timer switches a run on at dusk and off after a set number of hours. It does not change the load math, but it keeps the circuit from running all day. Check the timer's own amp rating against the load you plug into it.
Frequently asked questions
How many LED C9 bulbs can go on a 15 amp circuit?
A 15 amp, 120 volt circuit carries 1,800 watts; at the 80% continuous-load limit that is 1,440 watts. At 1 watt per bulb that is 1,440 bulbs; at 0.5 watt it is 2,880. Subtract anything else plugged into the same circuit first.
Why use 80% of the breaker rating?
Holiday lights run for hours at a time, which electrical codes treat as a continuous load. A breaker should carry no more than 80% of its rating continuously, so the calculator plans to 80% by default. You can change it, but lowering it adds margin; raising it does not.
What wattage should I enter?
Use the number printed on the bulb or its packaging. If it is not listed, 1 watt per LED C9 is a conservative planning figure; many LED C9s draw closer to 0.5 to 0.8 watt. Incandescent C9s are about 7 watts, C7s about 5.
Does the wire have its own limit?
Yes. Every cord, spool and plug has an amp rating printed on it, and a single run should not exceed it even if the breaker could carry more. The calculator shows the bulb limit for an 8 amp SPT-1 feed as an example; check the rating on your own wire.