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PSU Rails

Blindman231
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I have built a few computers since i went to college, but one things i consistently get embarrassed about it when me and the other person im helping build a PC are looking and PSU's, and they ask me "Whats the difference between have a single rail or 2 rails?", and i just sit there with a blank expression on my face.

I mean comon THIS PSU has 5 12V rails, but is still 950W. Someone educate the poor american!

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Single rail vs multi-rails PSUs refers to how the +12v is configured.

 

A single rail is a bit more "practical" as you can make full use of the total output of the +12v rail without needing to worry about having to accidentally trip off the Over Current Protection (OCP) in any sort of configuration. In theory you can output the entirety of the +12v capability on a single connector. On the other hand, a multi-rail say "no, you can't used the entirety of the 12v on single connector" and limits the amount of current passing through a set of cables via multiple of OCP points. Think of multirail units like the circuit box inside your house.

 

Some people preferred single rail, as they don't have to worry about accidentally setting the OCP off with a demanding system with a heavy overclock. While on a multi-rail unit, even if you have 1000w (~83A) on the +12v total, a poorly configured MR will not allow you to make full used of that 1000w. Let's say one the 12v is rated for 25A (300w) and it is monitor two PEGs connectors for your GPUs. The MSI GTX 780 Ti in this review will be able to power on that rail (the PCIe slot is also supplying to the card which is being supply from the +12v wire on the 24-pin that is from another +12v rail); however, some manufacturers didn't do this properly and have the CPU draw power from it as well, cause it to go over the OCP limits, and the PSU will shut off despite having capability to outputting more.

 

Of course, a quality modern day multi-rail PSU will be configured in such a way that no matter how you plugged in the cables, you should not set off the OCP aside from those are pushing their system to the limits with heavy overclocks under LN2 or such. If you so happen to set it off even under normal system, you can simply move the cables around to get around it.

 

The purpose of MR units are to add an extra layer of safety. If by some chance you actually pull the entirety of the 12v on a single connector on a high wattage single rail, something is going to be burn/melt/damage/etc, while a MR will limit that to a more safer range and hopefully prevent any major damage to occur. An example of this issues will be here (note the ending of that post, as it states if you happen to have a SR unit, you don't need to flip out and get a different unit)

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