Driving led lights with 100ft wire


newbie and 1st time asking a question on the board.

I had a led pool light fail. These light are not made for salt pools and they are very expensive ($1K). So I reused the shell and installed led strips on a 4 x 4 plate and poored clear epoxy. On my bench, I can use the remote and make it work using a simple 12VDC power supply (laptop style). When I connect it to the pool 100ft wire (12AWG), the light does not work and I get 2.7VDC at the light. Without the light I get about 12VDC at the end of the cable. Is it possible to drive these leds without putting the power supply close-by? I would like to avoid running 120VAC on the 100ft cable. There are concepts that I'm not understanding in this scenario ?

by setamp
June 25, 2019

12AWG: 0.16ohm/100ft. two x 100ft: 0.32ohm. current: (12v-2.7v)/0.32ohm = 29A. If you don't want to use high voltage, you have to use thicker wires to transfer large current to 100 ft distance load. For example, if you want to have 10V at the light, you need the resistance of wires to be 2V/29A = 0.069ohm. 100ft wire should be 0.069/2 = 0.0345ohm, which is 5 AWG or 6 AWG wire (or 5 pieces of 12 AWG wires in parallel).

by wangwg88
June 25, 2019

Do you have a spec sheet for the LED strips you're using?

by minsookim1
June 28, 2019

Thanks for the info wangwg88. The 12AWG wire is already buried. Adding 4 more will cost mucho $$ and not easy to do.

Hi Minsookim1, I don't have the specs, but the set is similar to https://ledmontreal.com/en/rgb-led-strip-kits/300-led-rgb-strip-with-24-key-remote-led-montreal.html Maybe I will have no choice but to install a weatherproof box 4 or 5 feet from the pool and install the power adapter in it. I was trying to avoid having to put 120VAC on the 100ft cable, but it's probably not worst that bringing power to a lamp post.

I'm still evaluating what will be the best angle/option. Thanks.

by setamp
June 28, 2019

I'm trying to figure out why there is such a huge voltage drop when the LED strip is connected. There's no way the strip is drawing that much current, so the only way for there to be that much voltage drop is if the strip does not have any series resistors to account for a 12V input.

2.7VDC is within the range of a standard LED forward voltage, and the purpose of a series resistor is to handle the difference between input voltage and LED forward voltage. The strip may have series resistors, but it may not be designed for 12V input.

Try connecting the strip very close to the the power supply without the 100 ft cable and see if you still get a substantial voltage drop.

by minsookim1
June 28, 2019

1 Answer

Answer by setamp

It works on my bench with a 12V 8A power supply.

+1 vote
by setamp
June 28, 2019

DC current cannot transfer long distance. Edison used to try more than 100 year ago, but failed. Best solution for your case is to use AC 120V to transfer your 78.3W (=2.7Vx29A) power to the place near your lights, then use 12VDC power supply to transfer 120VAC to 12VDC. Of course, the 120V cable should be placed in safe place (e.g. hang on wall).

by wangwg88
June 28, 2019

Thanks for the answer. I will go that route. That explains why low voltage pool lights are 12V AC and the concrétion is made inside the light sealed enclosure. Does anyone know what is the physical phenomenon that is taking place in this context? Would it work if the 12vDC power supply was able to push 29A? I understand I would get other problems like heat dissipation issues and size. I’m just trying to understand the forces at play and learn the key concepts ;) Thank you again for your help.

by setamp
June 29, 2019

29A is calculated according to your info, but it's not accurate because your lights not working when 2.7V. You should check what's the current or wattage when the light is working well and connect powerful enough power supply, so no overload or overheat.

by wangwg88
June 29, 2019

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