Cable sizing between psu and leds

Putting together a shopping list for my first project… Two 5m 300 led strings in parallel… According to the rule of thumb I’ve seen on this, and other sites, 0.06a per led… 18 amps at 5v per string… That’s a lot of current! I assume using a single 40amp psu connected at the centre between the two strings would be OK?

One thing… If I do things “right” that would require 12awg cables from psu to each string? Do people really size their cables like this, given the strings are unlikely to ever be left at full white for extended periods, and the cables will only be about 3m long. I question my logic as it seems 4mm2 cables are waay thicker than the copper tracks in the led string itself… And even soldering them to the pads would be tricky…

yeah 0.06a per LED is the “maximum” - but unless your application involves 100% white for extended times, you’re quite unlikely to ever reach that.

and you can always turn down the brightness which will reduce power requirements non-linearly.

there are rules of thumbs, but its probably best to take direct measurements (using a bench power supply for example) of the strip with the patterns and brightness you plan on using the you can come up with a better idea of what your typical power usage range will be and what wiring would be appropriate for that.

Hey @Robinlawrie! Sounds like these are 5V pixels in a 60 LED / m strip.

I would recommend you run a long 12 AWG bonded pair behind the entire length, and connect it to the strips at 3 points: middle, as you suggested, as well as at both ends. I think you can power it from the middle (best) or probably either end. If you need to keep the big wire aligned with the strip, you can zip tie every foot or so, or use silicone RTV adhesive to hold it in place.

I agree with timster: turn down the max power and you can use smaller supplementary power wires. 50% power looks 75% bright.

Here’s a recent real world datapoint.

I did a project last year that involved running 60/m 5V LEDs up to 270 in a line before power could be re-injected. I chose to run 10AWG behind each strip. This was actually because some of these fixtures were chained, so these rails could be used to delivering power to the next fixture in the chain, up to 500 total LEDs across three fixtures. It worked really well, all the way up to 2 MHz clock, and 100% power. We ultimately chose to run them at 70% max. That’s 30% less heat, 30% less load, some unknown percentage longer lifespan, and yet subjectively it looked only 10% less bright.

This is absolutely true! Let’s assume full white, the worst case scenario. At your injection point, 5 V may have dropped to 3 V from the thin copper traces in the strip, but the same current through your 12 AWG will have only dropped to 4.5V or so. Then when you connect it to the strip, that 3 V is buoyed back up to 4.5 V. Current actually flows backwards into the positive copper trace in the strip from the far injection point. The center between injection points in the strip may still be down at 4 V, but now that’s the lowest instead of having some trying to operate at 3 V. In this example, for simplicity in the explanation, I’m ignoring the mirrored voltage rise happening on the GND conductors.

Right! Instead, you want to make short pigtail leads that reduce your big wire down to something manageable like 18AWG within a few inches of the strip. In open air, 18AWG can actually safely handle 16A (and in my experience, that chart includes some pretty nice safety margins, especially for short connections). For the connection topology I recommended, note that it is only conducting half the max current, so 9A of your 18 per strip.

The quality of this 12 AWG → 18 AWG connection is kind of inversely proportional to how easy it is to make it. You can look up these techniques on YouTube, listed here from highest quality to lowest.

  1. Inline twist-and-solder, or window strip and solder, with heat shrink or electrical tape covering.
  2. Band splice (this is a specialized press that many contract manufacturers have)
  3. Dissimilar gauge butt-splice connectors using a proper crimper
  4. Terminal blocks with crimped ring terminals
  5. Wago 221-412 lever nuts can handle 12-24 AWG wire
  6. Low-temp “Heat shrink solder seal” connectors

I don’t recommend the vampire press-to-piece taps.

It depends on a lot of factors, but to take the simple/safe method that looks about right.

12 AWG for 40 amps, but if you are splitting it, you might drop that to 16 gauge. However, you want to figure out what your voltage drop will be, thinner wire will have more resistance and drop more voltage even if it’s rated for the current.

Sounds like you have about 20 ft of wire (gotta count both ways), with 16 gauge that’s 80 milliohms. At 18 A that’s going to drop about 1.44 volts not counting the voltage drop on the strip itself.

If you want to engineer it for 100% white, or rather, not fail it it happens to run close to full white, yeah. Many folks undersize things a bit, but you get into a gray area where you can start to have problems and/or safety issues.

Yes, the copper on the strip is fairly resistive, and will drop a lot of voltage too! The more power loss on the strip the hotter they get. This is why many folks inject power at multiple points.

Thanks for the pointers everyone… It does beg the question why 5v was chosen for these strings. Others are designed with 12v and that seems a much more sensible voltage to deal with on this kind of wiring.

One last question… Do the dotstar leds/pixelblaze tolerate any overvoltage at all?

I was thinking given these meanwell style supplies have a trim pot to tweak voltage, maybe starting at, say 5.5v would mean the voltage drop issue doesn’t take us so far under 5v by the further parts of the strings…

There are some advantages for 5V LEDs. Some 12V LEDs either use 3 LEDs in series per pixel (often limiting the addressable resolution you can get on a strip), or end up wasting the rest of the power as heat.

WS2815 and GS8208 are 12V LEDs that do some tricks and can run a R+G+B in series for a single pixel drawing the same power for any R+G+B combination. These are a bit more efficient when displaying certain colors, but are inefficient when displaying a single R/G/B color. 12V LEDs can handle a wider voltage range though, and so are convenient when dealing with voltage drop.

Many 5V addressable LEDs are specced for 4.5-5.5V. I wouldn’t take it to 6V.

In practice many work very well down to 3.5V or so without losing anything. Some maintain color balance, with some brightness loss, down to around 3V. At lower voltages, efficiency increases and there’s less waste heat.

The Standard Pixelblaze is tolerant of overvoltages without damage (up to 18v), but won’t operate at higher voltages, I’d keep it under 6V. The Pico has a hard limit of 6V, above which the regulator will be damaged.