| We are putting light bulbs on a large outside tree in a city park. We | have six 100 foot strings of 100 C-9 lights. We have two 20 amp GFI | circuits available. I plan to put 300 lights on each circuit. My math | | shows 2300 watts available and C-9 bulbs use 7 watts. This comes to | 2100 watts. I will use 12 guage extension cords. The light strings | and bulbs are commercial grade. This all looks good on paper, but I am
Normally the design current for a 20 amp circuit should be 80%, which would be 16 amps here. Your wattage calculation appears to have assumed
115 volts. What matters is what the total current is for the 100 bulbs with the voltage supplied (should be close to 120 volts in the USA).
If the bulbs are truly 7 watts at 120 volts, this is 17.5 amps total for three strings on one circuit. You could get away with this working. If you can find a couple shorter strings and back the number of bulbs down to say 500, you can make it work more within the code requirements. Reducing one string in each of the 2 sets to 74 working bulbs, either by a shorter string or by leaving in some dead bulbs clustered in the back of the tree, would get you under 16 amps.
| worried because there is not much room for real world error. Does | anybody have experience with this many lights? Wasn't there a movie | about a guy causing a large scale blackout when he threw the switch on | his house decorations? I don't want to be that guy. Any help you can | give me would be appreciated.
That's just movie fiction to impress people. There's also the urban legend of the kid hitting a power pole with his bat just as the lights went out in a past big NYC blackout.
You are not going to do much more with a big error in this than burn out some plug or cord. At worst, it could burn down the tree of the hot spot created is close to it.
Surely you have an electrical inspector office around there. While this might not fall under inspection issues by not being a permanent install, maybe they can give you some advice on a short term setup, especially if you are an employee of the same jurisdiction (city?).