Common sense: Consult with a master electrician as you install your lights to ensure that you run them according to city code and electrical standards. The following article is a guideline.
The question: How many lights can I run on a household breaker?
For a 20-amp breaker don't exceed 1900 watts if it is dedicated to only Christmas lights. For a 15-amp breaker, don't exceed 1400 watts. Keep in mind that if the circuit is shared, these numbers must be modified. (Otherwise, a teen, a curling iron and a blow dryer could blow the breaker and turn off your Christmas lights.)
Since most Christmas lights and bulbs come with a wattage rating (ie. a C9 bulb is commonly 7 watts or a mini lights string 25-50 watt range, depending on its length), we've elected to address this question with watts instead of current.
We calculate the figures above by using a simple formula.
Power (watts) = Voltage (volts) * Current (amps)
So, for a 15 amp breaker we see the maximum rated power is:
Max Power = 120 V * 15 A = 1800 watts
But we don't want to come anywhere close to that max. Most electricians use 80% of max as a guideline.
1800 watts * .8 = 1440 watts max (15-amp breaker)
(We rounded down to 1400 watts in our short answer.)
For a 20 amp breaker, the calculations are similar.
120 V * 20 A = 2400 watts * .80 = 1920 watts max (20-amp breaker)
As stated above, keep in mind that any appliances, lights, additional outlets, on the same breaker will deduct from this total.
Calculate the total watts on the outlet by adding together the wattage of all the light strings that are directing and indirectly (through splitters) connected to the breaker. Keep in mind you might have 4 outlets on a single breaker so understand that this calculation is per breaker NOT per outlet unless an electrician or builder has installed a single dedicated outlet on ONE of the breakers in your service panel.
Have an electrician help you figure out what is what in your home's wiring before starting your Christmas light installation.
Hope these basic guidelines help. Questions? Be sure to call a professional.
If you are wondering how many lights can be run in series, take a look at this article.
Keep blowing breakers?
Consider going LED next season.
Most LED light strings are rated at under 3 watts per string and most LED bulbs that screw into traditional 18 AWG wire are under a watt per bulb.
Want an easy way to estimate how many watts in your design?
Head over to our Energy Cost Calculator to quickly estimate how many watts you'll be installing.