How To Select Correct Cable and Transformer For LED Lights

Question

I'm about to install some 12v LED lights and have been surprised by the amount of current required, especially if multiple strips are used, and the effect of voltage drop.

I will be installing RGBW 4-in-1 5050 chips. These are rated as 15W/m (each colour 3.75W/m). In total I am installing around 16m of strips (2x5m + 2(3x2m)).

1. Do I really need to size the cable from transformer to controller for 6.25A for every 5m run (assuming 15W/m)? Wouldn't using RGB not run each colour at full power at the same time, but rather a blend with reduced overall power? Or is it best to use cable that can take all the colours being on fully at the same time?

I was planning to run the entire 16m from a single 300W transformer, each strip in parallel. But this would mean that the cable run from transformer to an LED strip could be as long as 5-7m and this means that voltage drop comes into effect and I need a bigger cable diameter.

2. Would it be best to have a single 'big' transformer, or would it be better to have smaller transformers closer to each strip where the effect of voltage drop does not greatly increase the cable conductor diameter?

Any advice on the best way to install, cable and power these strips is greatly appreciated!

TIA

Answer 1

It's entirely up to you.

If you don't want to size for all colors being at full intensity at once, you will have several consequences during times when you do run full intensity at once.

• If you undersized the power supply, it will crowbar, causing the lights to flash and probably be uncontrollable.

• If you used "not legitimately certified" cheap Chinese power supply, it may burn your house down.

• If you undersized the feeder wires, you will get more dimming than you wanted at high brightness.

It is probably a Code violation to undersize the power supply. Code does not care about voltage drop and is only concerned with the wires being sufficiently sized to avoid overheating of the wire at any given millimeter of it. That is decided by amps alone vs wire size.

Most people who attempt LED strip lighting have some prior experience in hobby electronics. And most people coming from hobby electronics use hobby wire, where sizes like 10 AWG or 4 mm² are expensive/exotic. Simple trick: look at the "AC mains wiring" supply chain, where wires that size are cheap commodities.

Answer 2

The 'big transformer' I assume is a AC to DC power supply. Using 300W gives you a bit of a head room, always good. As far as wire sizing you can use whatever you want however note that the LEDs peak brightness is maximum current and drops off very rapidly. You could use a seperate wire for each color. I would use #10 or #12 wire to keep the voltage drop at about 1.25V. Your 'transformer' probably has a voltage output adjustment where you can compensate for this voltage drop. If you overcompensate you will drastically shorten the life of your LEDs. You can use speaker or alarm wire, it is not that expensive and will support what you want to do. Note I am talking about copper wire, if you use CCA you need to check as the resistance will be higher hence the voltage drop greater. When you have the sizing figured out etc calculate the heat rise in the wire and be sure it is within safe boundries. Always do this worse case or in your came max amperage.