How many C9 LED lights can run on one circuit?
The number of C9 lights an installer can put on one circuit comes down to three numbers: the circuit's breaker rating, the wattage printed on the bulb, and the rating of the cord and timer between them. LED C9 bulbs draw a small fraction of what incandescent C9s did, so a single outlet can run thousands of feet of LED roofline. The limit usually shows up in the cord or the timer before it shows up in the breaker.
Circuit capacity: 15 amp vs 20 amp
A standard 120-volt circuit's capacity is amps × volts. A 15-amp circuit is 1,800 watts. A 20-amp circuit is 2,400 watts.
Christmas lights run for hours at a time, which makes them a continuous load. The standard practice for continuous loads is to stay at or under 80% of the breaker rating: 12 amps / 1,440 watts on a 15-amp circuit and 16 amps / 1,920 watts on a 20-amp circuit. Plan every install against the 80% number, not the breaker number.
Find the bulb wattage
Use the wattage printed on the bulb packaging or spec sheet for the exact bulb on the job. Commercial LED C9 bulbs typically draw well under 1 watt each. A traditional incandescent C9 draws 7 watts.
That gap is why LED changed C9 installs. At 7 watts, a 15-amp circuit at 80% runs about 205 incandescent C9 bulbs. At 0.5 watts, the same circuit runs 2,880 LED C9 bulbs.
The formula
Bulbs per circuit = (circuit watts × 0.8) ÷ watts per bulb.
Amps for a run = (bulb count × watts per bulb) ÷ 120.
Example: 1,000 LED C9 bulbs at 0.5 watts each is 500 watts, or about 4.2 amps. That's a 1,000 ft. run at 12 in. spacing, one 1,000 ft. C9 socket spool filled with C9 LED bulbs.
The cord and timer are usually the real limit
The breaker is only the first limit. Every cord, plug and timer between the outlet and the lights has its own rating, and the lowest one sets the ceiling for everything it feeds.
The SPT-1 lamp cord (zip cord) Christmas Light Supplier stocks is rated 8 amps / 960 watts. A leader cord built from it can feed up to 960 watts of lights, less than the 1,440 watts a 15-amp circuit allows. Socket wire has its own rating printed on the spool, and prebuilt light strings carry a "maximum connected" figure on the tag.
A photocell timer has a rated load as well. Add up everything plugged into it and keep the total under its rating. See Photocell timers for Christmas light installs for placement and load.
Watch for other loads on the circuit
Exterior outlets often share a circuit with a garage, porch lights or other outlets. A circuit that's fine on install day can trip on a cold night when a space heater or freezer on the same breaker kicks on. On residential jobs, ask the homeowner what else is on the outlet's circuit, or run the display from two outlets on different breakers when the load is close to the limit.
Exterior outlets are usually GFCI-protected. Water in a connection can trip a GFCI even when the load is well under the limit. Keep plug connections off the ground and out of gutters, and point them downward so water drains off.
Worked example: 2,400 ft. of roofline
A 2,400 ft. roofline at 12 in. spacing is 2,400 LED C9 bulbs. At 0.5 watts each, that's 1,200 watts, or 10 amps.
Circuit: 10 amps fits under the 12-amp continuous limit of a 15-amp circuit, as long as nothing else significant is on it.
Cord: 1,200 watts is over the 960-watt rating of one SPT-1 leader cord. Split the display into two feeds of 1,200 bulbs (600 watts each), each on its own leader cord.
Timer: confirm the photocell timer is rated above 1,200 watts, or put each feed on its own timer.
Parts: two 1,000 ft. spools plus a 500 ft. spool (2,500 ft., with 100 ft. of slack for terminations), 2,400 bulbs in 25-packs, and male and female vampire plugs for the custom ends.
Frequently asked questions
How many LED C9 lights can I put on a 15-amp circuit?
At the 80% continuous-load limit (1,440 watts), a 15-amp circuit runs about 2,880 LED C9 bulbs rated 0.5 watts each. Use the wattage printed on your bulbs: bulbs per circuit = 1,440 ÷ watts per bulb. The cord and timer ratings may limit the run before the breaker does.
How many C9 lights can go on a 20-amp circuit?
A 20-amp circuit at 80% is 1,920 watts, which is about 3,840 LED C9 bulbs at 0.5 watts each or about 274 incandescent C9 bulbs at 7 watts each.
How many watts does an LED C9 bulb use?
Commercial LED C9 bulbs typically draw well under 1 watt each, compared with 7 watts for an incandescent C9. Use the rating on the bulb packaging for exact load math.
Why does my Christmas light circuit keep tripping?
Either the total load on the circuit is over its rating (often because other devices share the circuit), or a GFCI outlet is tripping from moisture in a connection. Check what else is on the breaker, and keep plug connections off the ground and pointed downward.