Action Line: Christmas light limit can be calculated by homeowner
BY PHIL MULKINS World Action Line Editor
Sunday, December 02, 2012
12/02/12 at 2:41 AM
"The snap of a few sparks, a quick whiff of ozone and the lamp blazed forth in unparalleled glory," said screenwriter Jean Shepherd as "the adult Ralphie" narrating "A Christmas Story" in 1983. He described a practice frowned upon by fire chiefs everywhere: overloading electrical circuits with Christmas lights, which can cause house fires.
Knowing a few things about the power supply and Ohm's Law can help a homeowner determine how many lights can go on the house.
Every home has a main power line that enters from the electric company's pole and goes into the home's electrical panel, or breaker box. From there, individual 20-amp circuit wires go to other parts of the house for ceiling lights, wall receptacles, fans, dishwashers, etc. Any one circuit can have multiple 15-amp receptacles spaced along them, running through walls and the ceiling to the panel.
Even though most houses have 20-amp wall circuits, they still have receptacles rated for only 15 amps. Because Christmas lights use a continuous flow of electricity - unlike coffee makers or toasters - it is common to use only 80 percent of the circuit's capacity - 12 amps. Multiply that by the standard household voltage, 120, and you get 1,440 watts.
"The simplest way to install bulbs safely is to count the number of bulbs you're using on any one circuit, multiply this by their individual wattage and then divide that by the line voltage (120) which would give you the amperage (16 amps per 20-amp circuit)," said John Staires, electrical inspections supervisor for the city of Tulsa.
"There's such a wide variety these days of the types of Christmas lighting available and the lengths of light strings, I don't know if it would be within the capabilities of most homeowners to determine total wattage," he said. "Most strings of lights come with instructions that tell how many strings can be hooked together in a single run so as not to overheat the wiring. But if you have a box of old lights and no instructions, you can use the formula in Ohm's Law - bulb wattage times the number of bulbs in the string divided by 120 volts to get the amperage. And this should not exceed 16 amps per circuit."
Staires added that when buying new bulbs "make sure they have a 'listing mark' ("UL" being the most common but also those recognized by the Occupational Safety and Health Administration as "nationally recognized testing laboratories:" CSA, CCL, CSL, ITSNA, MET, etc.). We do get, particularly in home improvement stores, foreign-made sets of lights that aren't listed - and could well be hazardous. But any American product that's listed will have installation instructions."
Scott Bishop, service manager of Colburn Electric in Broken Arrow, said, "Most houses are on 15-amp receptacles even if the circuits are on 20-amp breakers. A 15-amp receptacle can handle 1,800 watts of power but the National Electrical Code, followed by most municipalities, says only 80 percent of receptacle capacity should be used (1,440 watts or 12 amps) to avoid breaker tripping. Even though they've got a 20-amp breaker on that circuit, other receptacles or even ceiling lights are fed by that circuit. So you can't go over 12 amps for any one receptacle - or a total of 1,920 watts (16 amps) spread out over two or more receptacles on that 20-amp circuit."
To figure how many watts can be on one wall receptacle, add up the wattage of each bulb and divide it into 1,440 watts.
Christmas Lights Etc. sells standard incandescent "twinkle lights" (2.5 watts each) and this divided into 1,440 watts is 576 bulbs per receptacle. The larger bulbs, C7 and C9, run at 5 watts, 7 watts and 10 watts (the higher the wattage the brighter the light) - 288 bulbs, 206 bulbs and 144 bulbs per single receptacle.
LED light wattage is much less than standard mini lights. A 70-bulb strand of LED mini lights compares to a 50-bulb strand of standard mini lights as follows: A 70-bulb strand of LED mini lights requires 4.8 watts, while a 50-bulb strand of standard mini lights requires 20.4 watts.
Staires added a word of caution about aging equipment: "Always examine the light strands for cracked or otherwise damaged insulation, and replace any lighting that shows visible damage."
Original Print Headline: Christmas light limit easy to figure
Phil Mulkins 918-699-8888
Allen Tyler untangles his Christmas lights outside his Tulsa home. JAMES GIBBARD / Tulsa World file