In older versions of the ATX specs Intel had required the CPU to have its own rail separate from the
motherboard and other components. It had also specified that no rail have more than 240 VA available(20A @ 12V), this is what lead to multi rail power supplies in the beginning. Both of those requirements have now been lifted allowing for power supplies with a single 12V rail and 12V rails rated for more than 20A which is very useful.
All power supplies convert the AC(alternating current) power that comes out of the wall into DC(direct current), and most units only have a single 12V DC source, in a single rail unit all of the yellow 12V wires are tied directly to this source, in a multi rail unit they are split into bunches(rails) and tied to the source, there is a controller chip monitoring the current going into each rail and if it exceeds a set limit it will shut down the power supply. Some smaller cheaper units claim to have multiple 12V rails to comply with the old ATX specs, but actually just tie all the bunches together with no over current protection so while the label makes it appear as a multi rail unit its actually a single rail unit, this helps cut costs.
One of the big myths about multi rail units is that because of the overcurrent protection you can end up with “trapped power”, XFX uses a power supply with a 200W 12V DC source as an example:
In a multi rail unit they show the 12V source split into two 100W rails, and when loaded up with 75W on each rail from the graphics card adding 50W for the CPU onto either of the rails would cause it to shut down even though it had 50W remaining for the source and a single 200W 12V rail would have been able to handle it all fine.
The lie in this scenario is that multi rail units have their over current protection(OCP) limits set such that you wont end up with trapped power, in this scenario the unit would likely have its OCP limit set at about 130W on each rail so that there would not be an issue of “trapped power”, this is why for most units with multiple 12V rails you cannot simply add the rails together to figure out the total power available from the 12V source, you must look at the total power listed for the 12V rails listed below them. On the units where the sum of the listed max currents for the 12V rails does add up to the stated limit of the 12V source the true current limits are usually set 3-5A(36-60W) higher than the label shows.
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