# what size cable for powering an LED sculpture?

I've got four 60 amp 5 volt power supplies, and I'm sending power to a sculpture with 3000 RGB leds. So here's the total power picture we're looking at...

3000 * 5 volts * 3 leds per pixel * .02 amps = 900 watts. At 5v that's 180 amps.

My power supplies are rated for 240 amps, so I'm comfortably within my margins there. Also, my piece will only have around 50% of the lights on at any time, also the lights won't be white, so they will be using less than .02*3 leds of power...

So. What's our safety margin here... something like 500 watts seems like a good normal load, with occasional spikes higher.

(if any of this seems bogus, feel free to point that out)

My main question is this... What size cabling should I use to send power up to the art piece from the power supply box? I was looking at NEC standards and it was saying something like 4 gauge... but it was only asking me about amps used... And I think that's for 120 volts. Does it matter that I'm only sending power at 5 volts?

The cables from the power supply box to the sculpture will be around 20 feet long.

(Oh - and if you want to check out the art piece in progress, I'm posting a lot of it here... https://www.facebook.com/skylightpdx/ )

• 180 Amps is solidly in welding cable territory. That makes 20 feet a very long cable, you'd be better siting the PSUs much closer to the LEDs.
– user16324
May 5, 2016 at 23:31
• Yes, it does matter. You'll want to make the wires oversize from the NEC recommendataions, because you can't tolerate the same voltage drop as a 120V circuit. A 2.5V drop in a 120V circuit is only 2%, but in a 5V circuit, it's 50%! Ideally, you'll use the "remote sense" feature of the power supply to allow it to compensate for the cable losses. May 5, 2016 at 23:32
• so it sounds like maybe the answer might be to move the power supplies over to the art piece and run power to individual runs of LEDs in smaller blocks. I could power 100 or 300 leds at a time relatively easily... then we'd be talking more like 90 watts / 18 amps per cord...? May 5, 2016 at 23:42
• Due to the high resistance of Flexible Printed Circuit, the thin copper that the led strips are made of, you want to distribute multiple runs from the power supplies to the leds anyway. Since each run will carry less power, they can be smaller. Multiple 18 AWG runs carrying 5 Amps instead of a single 10 AWG run carrying 50. May 5, 2016 at 23:45

Since you have 4 supplies, at a max of 180 Amps, that's only 45 Amps each, if equally divided. Since your intended application is much less than that (50% on at any given time, no White), you could do this with just 3 supplies, but since you have 4 might as well use them.

Due to voltage droop, from the resistance of a cable at high currents, you want to reduce the amount of current a single cable carries. So you will want to have multiple runs of power from a single supply to the LED strips.

Based on a quick calculation using 5VDC, 18 AWG, 5 Feet (10 Feet both ways), and 5 Amps, it shows a voltage droop of 0.32 Volts. So the LEDs, at the power input, will only see 4.68 Volts. If we cut the current in half, that's 0.16 Volt drop. More reasonable. If we go to 16 AWG, a larger size, at 5 Amps, it's still a drop of 0.2 Volts.

And that's before you get into the voltage droop from the FPC of the led strip itself.

So what you need is a short run of a large AWG cable, from the power supply to a distribution bus, and then multiple parallel runs of smaller cables to multiple spots in the LED strips.

Adafruit has a nice writeup of a similar project, using 80 Amps. Key points, see Power and Construction, but the whole thing is a good read.

Note the single AC Supply, to 12 AWG Fused cables, to a power distribution block, to multiple smaller cables to the led strips.

Keep the cables as short as possible to reduce voltage drop. If your Power supplies have an adjustment pot, increase the voltage to compensate. And the power supplies should be next to the sculpture. Having Mains AC power boxes installed near the sculpture isn't expensive and will be better.

• I think I can hide the power supplies behind the center panel of the piece, which will mean all my cables will only have to be 6 feet at longest from there. So I think that will be good. :) May 6, 2016 at 0:43