Photocell timers
A photocell timer senses daylight. When it gets dark, it switches the lights on; when it gets light, or after a set number of hours depending on the model, it switches them off. Because it reacts to actual light levels, it follows the changing sunset through the season without being reset. Place the sensor where it sees the evening sky but is not lit by the display itself or a porch light, which can make it switch off early.
The photocell timer is listed at $11.00 each, and the 4 timer case at $43.00, which is $10.75 per timer. A house with lights fed from two or three outlets needs a timer at each outlet, which is where the case makes sense.
Estimating the load
Every display should be planned against what a circuit can carry. A common household outdoor circuit is 15 amps at 120 volts. A widely used rule for loads left on for hours is to stay at or below 80 percent of the breaker rating, which for a 15 amp circuit is 12 amps, or about 1,440 watts. Other devices on the same circuit count too, so check what else shares it.
To estimate the load, multiply the number of bulbs by the watts per bulb printed on the packaging or listing. The difference between LED and incandescent is where the planning changes:
| Bulb type | Typical watts per bulb | Bulbs per 1,440 W |
|---|---|---|
| Incandescent C9 | About 7 W | 205 |
| Incandescent C7 | About 5 W | 288 |
| LED C9 | Use the listed wattage | Many times more than incandescent |
With incandescent C9 bulbs, a single circuit supports only about 200 bulbs, which is roughly 200 feet of 12 inch roofline. LED C9 bulbs draw a small fraction of that, so the practical limit on an LED display is usually the rating of the wire and connectors rather than the circuit.
Wire and cord ratings
Each cord has its own rating, and a run must stay under the lowest rating in the chain. The SPT-1 18/2 lamp cord is listed at 8 amps or 960 watts. Check the rating printed on your socket wire and plugs as well, and treat the lowest number as the limit for that run. With LED bulbs this limit is rarely reached on a residential roofline, but it matters on long commercial runs that chain many sections together.
Planning outlets and cord paths
- Mark each outdoor outlet and note which breaker feeds it.
- Assign each roofline section to the nearest outlet, keeping lead cords short.
- Run lead cords up downspouts or along trim, where they are less visible and out of foot traffic.
- Use jumpers of lamp cord to bridge gaps between separate roof sections instead of long runs of socket wire with empty sockets.
- Put a photocell timer at each outlet that feeds lights.
Outdoor safety basics
- Plug outdoor lights into GFCI-protected outlets.
- Keep plug connections off the ground and out of standing water, pointing downward so water drains away.
- Use cords and plugs intended for outdoor use, and do not run cords through closed windows or doors where they can be pinched.
- Inspect cords at setup each year and replace any with cracked insulation.
This page is general guidance for planning. If you are unsure what a circuit can carry, or you want dedicated outdoor circuits added, have a licensed electrician check it.
Timers and cord. Photocell timers and lamp cord are available from Christmas Light Supplier. Christmas Light Supplier ships every order free, the same day it is placed, with no order minimum and no added fees.