Safe Christmas illumination

December 17, 2004 1:50 PM

How many strands of Christmas tree lights (100 lights/strand) can I safely plug into a single outlet and/or connect to each other?
posted by Dr. Zira to home & garden (12 comments total)

Suppose your outlet is rated for 15 A (check the fuse or circuit breaker rating) at 110V (USA). A typical strand of lights consumes 25-50 Watts, check your box for the value. Let's assume 25 Watts. Then,

Number of strands * 25 Watts per strand = 15 A * 110 V = 1.65kW

So the number of strands must be less than 1.65kW/25W = 66. To be safe, divide that number by 2. So 33. Your mileage may vary as I may have dropped a sqrt(2). Adjust 110 V to 240 V if you live elsewhere.
posted by fatllama at 1:59 PM on December 17, 2004

In other words, go nuts. Just unplug them when you leave the house.
posted by modofo at 2:00 PM on December 17, 2004

fatllama, maybe the little tiny bulbs use that little power (although I recall them being rated at about 2 or 3 watts).

A standard sized light bulb (such as you might use outdoors) uses about 5 - 7 watts (C-9 & C-7 size bulbs).

USA electrical plugs are rated to 1500 watts. That's 214 bulbs per outlet, MAXIMUM, if you want to be safe. If you want to push it, you could do 300 if they're definately 5 watt energy saver bulbs.

You have one further complication: To draw 1500 watts safely at 120 volts you need 14 AWG wire. Your christmas lights probably use 16 AWG wire, which is more for 800 watts and under applications. That means it will get hot and might start a fire if you connect the lights in series. If you connect them all directly to the plug, this complication won't happen.

Again, like fatllama says, if you live elsewhere, recalculate for your area. I believe in the UK, for example, plugs are rated to 13 Amps @ 220 volts, or 2860 watts maximum.

One more thing: If you're overloading the outlet, don't expect the breaker to trip immediately. If you're only, say, 15% over the maximum of the breaker, it may take a long time (say 10 minutes) before the breaker heats up and trips.

HTH.
posted by shepd at 2:30 PM on December 17, 2004

If you have the box the lights came in, it usually recommends a maximum number of strings of that type to plug in to one outlet, which probably has a big safety margin...
posted by TheOnlyCoolTim at 2:37 PM on December 17, 2004

We've had 7 strands (700 lights) on our tree for a couple of years now, with no problems.
posted by CMcKinnon at 2:44 PM on December 17, 2004

Most light strings say never plug more than 3 in a row. I ignore that, but I won't suggest that you do so. Extension cords also say not to plug one into another, but I have three in a row in a couple of places and then six or seven sets of net lights going into the power stake at the end.

I have about 20,000 lights on seven outlets, fwiw. Appears to be spread over two breakers as I was turning random ones off the other night experimenting.

I measured several of my newest strings of 50 bulbs and they came out at 18 watts. I measured a string of 500 lights at 200 watts.

The large chistmas lights that used to be really popular take much more electricity. I put one string of them up with some other lights, threw a breaker. I took them down and put up about ten more 100-light strings of the tiny lights and everything is fine. I forget the wattage of that string (of 25 bulbs) but it was really high compared to any other decorations that I have up. I might just throw those away (I bought a big box of them for a quarter at a garage sale...heh).

To save energy and for additional safety, I have mine on timers so they come on at dusk and go off before I go to sleep.

You can pick up timers at Wal-Mart or any of the big chain stores around.

Next year I intend to do a lot of work around the house and yard so I could have up to a couple hundred thousand lights if I decide to go that crazy with it. Never too many places to plug shit in anway, what with my desire for fountains and birdbaths and landscape lighting and all of that jazz.
posted by weretable and the undead chairs at 3:05 PM on December 17, 2004

sheph speaks the truth. I forgot about any current capacity limitation of the outlet which should never ever be exceeded, but I know that wire ratings are actually quite conservative. Again, err on the side of safety, but my initial calculation gives the right order of magnitude if you substitute the fuse/breaker values at your house. By the way, here is a page listing some Christmas light (the miniature kind) estimates.
posted by fatllama at 3:17 PM on December 17, 2004

er, shepd, sorry.
posted by fatllama at 3:18 PM on December 17, 2004

Thanks! Electricity confuses, yet amazes me. ..

/simple caveman
posted by Dr. Zira at 10:00 PM on December 17, 2004

Shouldn't the worst case be tripping a breaker?
posted by knave at 11:00 PM on December 17, 2004

knave, that's the hope.

But, if your christmas lights use inferior cable you could be starting a fire hazard. It's unlikely, but possible. And when it's made in China, the barely possible is happening, right now. :-D

Do what the box tells you, IMHO.
posted by shepd at 7:13 AM on December 18, 2004

Get an extra smoke alarm for the room the Christmas tree is in. Test all the smoke alarms in the house.
posted by theora55 at 4:46 PM on December 18, 2004