# Voltage Drop on long runs of LED Christmas Lights?

I'm looking to invest in some quality LED Christmas lights for my house and try to hang them in a way that would wrap the entire house and all the roof lines. I think that the total length covered is just under 500' with the peaks and valleys.

I'm looking at buying a spool of Christmas Light stringers and LED bulbs similar to the products below. Although it's not mentioned in the specification, the wire is 18awg.

Based on my rough math and Ohms law, I've calculated that this entire spool, when lit will draw less than 3 amps (I understand this to be well under the limits for 18awg wire and a single circuit), but I have concerns about the voltage drop. Using online calculators, I've calculated the Vdrop for 18awg copper with a 3A load to be ~20V at 500', and ~10V at 250'. I'm not sure about the tolerances of LED bulbs, but I can't imagine running them at ~100V AC is healthy when they're designed for 120V.

What is the maximum length I can run without causing a fire hazard, impacting the life of the bulb, or causing any other problems? A product rep for the store said I could run all 500' in one run with no problems. Does that sound legit?

For reference, the products I'm considering are pasted below:

• Many, but not all, of the strings of lights I've seen have the maximum string length printed on the package and/or on the label attached to the string. You might check that before you buy. Commented Nov 19, 2019 at 19:56
• actually, running them at 100v would make them last longer and run cooler. The downside would be a slightly dimmer appearance. How much dimmer? that depends on how they are wired, but it should be roughly linear to the voltage, so 100v is 20% dimmer than 120v. I would go for it, and if you can tell, chop them into 2 or 3 branches. Commented Nov 19, 2019 at 20:33
• Keep in mind that the actual LED doesn't operate at 120v. Each bulb has electronics that step down the voltage to a few volts to drive a plain ol' LED. Depending on how this is done, the input voltage may be allowed to vary quite a bit while maintaining full brightness of the actual LED. Commented Nov 19, 2019 at 20:39

Causing a fire hazard

That is decided by the presence of a non-faked UL Listing (with file number). Listings from other NRTLs such as CSA or ETL are also acceptable.

Note that most mail order, everything from Alibaba and AliExpress, and almost anything from eBay or Amazon Marketplace (even if shipped with Prime) is abusing a loophole in Customs/safety regs to sneak in entirely dangerous and defective Cheese junk products. The dead giveaway is the "CE" label they stamp on almost everything, almost always faked; as CE is a voluntary mark only meaningful inside the EU. CCC is another popular fake mark.

impacting the life of the bulb

It's LED. What will kill the bulb is the driver electronics being cheap; or weather. Voltage drop won't affect it one way or the other.

Causing any other problems

Personally, I think running a 500' cord as a single run is a mistake. It will be extremely unwieldy to install. It has a huge surface area for potential damage, i.e. one hit knocks out 500' of cord instead of a 50' section. The spools I'm seeing actually don't come with plugs; you are meant to cut the cord to shorter lengths to suit your installation, and put on as many plugs as you need.

So you should be splitting your power feed a fair bit, not least for handling purposes.

• Thanks for the reply, I've contacted the company (American, based in the US) and they assured me it is UL listed, as its a commercial supply place. Admittedly, it can be faked, but I will verify. Commented Nov 19, 2019 at 20:25
• w/r/t the single run - I don't think I could do a 500' run either, but I could see a few circumstances where longer runs could come in to play (i.e. the entire perimeter) and was wondering if voltage drop or any other electrical principle imposed a "safe" maximum. Again - thanks for the reply. Commented Nov 19, 2019 at 20:28