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I want to buy and convert a 350W PC power supply into a 12v power supply for 20m RGBW led strips. I want to do this because I did not find any reasonable priced 12v power supply and simply because I want to build this myself.

I have found several articles and video tutorials on this topic and this seems pretty straightforward. From pure curiosity, however, I would appreciate any explanation why doing this requires cutting all yellow wires and joining them? If the power supply is 350W, means 350W / 12V = 29.2Amps (assuming 100% efficiency), a single yellow wire would not allow to pass this current due to the cross-section of the wire? (Because it will melt or something)

So, is the process of joining all +12V yellow wires mandatory and if yes why?

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You have a 350W PC power supply. That 350W is split across multiple 12V, 5V, 3v3 and all the other votlage rails you have on an ATX power supply. If you read the power supply's technical documentation, it will probably tell you in there what the current limit for each rail is.

So when you say you have 29.2A at 12V available, you're greatly over estimating the power.

The reason to join all the 12V rails together is a best practice idea. If there are multiple 12V paths in the PSU (which is probable) then you are keeping the load even between them all.

On top of that, you're reducing the voltage drop caused by the current through the wires. Often people talk about the current limit of a cable as if they expect the cable to melt. That is very unlikely to happen. Often the cable length will mean you'll have a 1 or 2 ohm resistance, at 1A, that means you're dropping 1 or 2V along the cable, so you're looking at more than 10% voltage drop due to cable resistance. Having two cables in parallel, you're reducing the resistance (and so volt drop) to half the value.

TLDR: - You have a lot less than 350W to play with - Best practice means joining the cables to spread the load - More cables mean less voltage drop

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  • \$\begingroup\$ You are right about the first part, that the power is split across all voltage rails. I wrote in general, but in the case of the PSU I'm thinking of, it has +12V single rail and the technical documentation states that it can provide up to 25Amps through that rail. Anyway, I think the second part of your explanation still holds, about joining the wires, am I right? Though I still don't understand, if its single rail from documentation, shouldn't one wire be enough, in theory? \$\endgroup\$
    – XMight
    Aug 22, 2018 at 11:36
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    \$\begingroup\$ @XMight - Be aware that if you had a single wire that was fat enough it would be enough. As this answer has stated it all comes down to voltage drop in the cable when you pull current through the cable. If you used lesser wires or a thinner wire the voltage drop will increase and it thus delivers a net lower voltage to your LED string load. Too thin of wire and it could melt the insulation and create a short or simply act like a fuse and open circuit. \$\endgroup\$ Aug 22, 2018 at 12:26
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    \$\begingroup\$ A single 18 awg carrying 30 amps may not melt the copper but the insulation, and then shorts out and something else may burn. Like your house. \$\endgroup\$
    – Passerby
    Aug 22, 2018 at 18:03
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    \$\begingroup\$ @xmight no. Just because it's one power rail does not mean just one wire is enough. At the typical gauge of an atx supply wire, 18 awg, it can only carry 10 amps, in free air, less when bundled like they are. There's a reason that the manufacturers and standard requires multiple wires for 12V, it's for safety reasons (also cheapness. Copper is expensive, and thicker wire is expensive) \$\endgroup\$
    – Passerby
    Aug 22, 2018 at 18:10

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