Why do I need more than one 12V rail?

What's wrong with a PSU with one really powerful 12V rail?

I'm new to PSUs and doing some shopping. Planning to upgrade from my current 430w to the 600w minimum that my ASUS Crosshair mobo says I should have.

The 12V rail that I need, according to the mobo, has certain specifications. It's to plug into the 8-pin slot on the mobo, to power the dual-core cpu. So if that's what it's for, why would I need more than one rail?

Multiple 12V rail systems are for...what hardware?
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More about rail
  1. Another GC...
  2. aziraphale said:
    Another GC...

    Do you mean two total? Not sure what you mean. Maybe since my current card is old, I don't get it - GeForce 6600. It has nothing you plug into. I've read that new ones require a 6-pin plug? Are you saying that aside from the mobo 8-pin connector which is for the CPU, graphics cards also take 12v rails?

    It's the last thing I'm going to upgrade on this system.
  3. There's nothing wrong with a PSU with a single 12V rail, in fact that couldn't be further from the truth. A single 12V rail is the best configuration.
  4. That's what I've always wondered about. Almost all the medium to high end PSU's have multiple rails, but I've heard that they're really just one rail from an engineering standpoint, that they are split into extra rails for the purpose of ensuring that no single rail gets over 20 amps.

    In a review of a single rail Silverstone, Anandtech writes:


    The second special characteristic is the single 12V rail. In a time when other manufacturers are building up to six 12V rails in their PSUs, Silverstone has come up with a single rail able to pull a load up to 54 amps. If we look to the actual Intel ATX12V specs, it states that there should be no rail with more than 20 amps for safety reasons. That's a fine step but not possible when you listen to the graphics card manufacturers. They ask for up to 30 amps from a single 12V rail which would make every OCP kick in if reached since they lie at around 24 amps. The result would be a shut-down of the PC while running the actual application. With a single 12V rail Silverstone went in another direction. If you have enough power to supply every component in the PC from a single rail, there would never be any problem since the PSU is regulating all the power drawn from just one source.

    This might be a valid way to design a power supply, but, even though we don't know what it is, Intel probably had a reason for designing the specifications around a 20 amp per 12V rail limit. Silverstone backs up their design choice by stating that there is no application which could force this kind of power supply to fail. We have not heard of any problems in the field with a single high current 12V rail, and in our tests we weren't able to provoke this power supply into failing. We will make sure to pay close attention to this during the tests.


    So, an Intel specification led to more rails to regulate amperage. Both of the Antecs in our two main PC's have 3 12 volt rails with maximum of 18 amps on each. Can't wait to get a 3850 or 3870 for each! (Come on second W2 my upgrade jones needs that refund!)

    Johnny Guru's power supply articles are reliable:

  5. Yep Jonny Guru is great. I find it funny that it's "safe" to run up to 20A, it's not like 20A is really a safe amperage.
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