what's the importance of 12v rails?
 

if I'm looking at a psu, it has 4 12v rails. When thinking about power distribution to parts. Are there considerations involving this? Do I need to identify what plugs on the modular supply are are on what rails and put components on different rails? or is that not how that works? does it even matter?

Without 12v your system wouldn't run. The 4 12v rails are usually gpu dedicated.

The 12v is the main power your computer uses. Cpu, motherboard and gpu, along others component, use it to run. The more A the 12v (and 5v and 3,3v) rails have, the more wattage it can output. I personnally prefer single rail design over multiple ones, as you don't have to pay attention to spreading the load evenly between them.

While you check psu price's, you can almost tell the quality of the unit just by it's watts and A. Not all psu are created equal: i've come across 500w psu sporting 28A on the 12v rail, and others 500w with 16A... Take your time to check a quality silver or even gold unit: they are not THAT more expensive, and generally provide better voltage while being pretty efficient. Win-Win for me...

Just my 2 cents.

With PSU's that use multiple 12V rails you just have to make sure that you don't overload one of the rails. You could do that if you use a splitter on a cable to power 2 plugs for example. Otherwise, they work the same as a PSU with a large single 12V rail. I don't think that there is any evidence saying that one design is better than the other.

Which PSU in particular with the divided rails?

For the most part you will never have to worry as they only have so many connectors for each rail, and its hard to overload multiple rails.

-ST

As others have said, 12v is a modern day computers life line. I wouldn't worry about the amount of rails, so much as the quality of the power it produces. What power supply is it you are looking at?

No. All the PSU plugs are well identified. The molex 5V/12v normally are used for optical drives / HDD, the SATA power cables for SATA drives, CPU has its own plugs, the GPU gets power from the PCI Express power connectors (6+2), etc.

When buying a power supply you will get all the specifications on the PSU itself and on the web site which states the maximum load across different rails....when getting a PSU it's always good to get a higher capacity than you will need, 1) future proofing 2) not risking overloading or running close to load.
When getting a PSU always take into consideration your components, especially the GPU and its requirements, these will draw a lot of amps from the 12v rails - Usually you will get this information on the respective websites.

Otherwise as to your initial questions, no, the PSU takes care of all that, you just plug the appropriate connector to the appropriate device, you can't go wrong there as different connectors are keyed differently........Molex is done one way, SATA power, CPU, PCIe are all different so you can't go wrong there.

Some more expensive PSUs have dedicated rails for PCIe/etc.

Don't really need that, just get a good PSU that mmets the requirements of your components + higher capacity to be safe.

Just do a little research and look at reviews because some powersupplies may say they put out more then they really do.Just get a good name brand power supply. If it has four rails then it just means you can't go over a certain amount of amps on each 12v raill.

If you have a multiple rail power supply, just try not to use splitters and avoid daisy chaining devices, etc, and you should be fine.