# Thread: Single vs. Multiple +12V rails: The splitting of the +12V rail

1. micro ATX User
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I have a question. As you said that 12V2 line is reserved for CPU. So if a power supply has two 12V rails, 12V1 and 12V2. Both, say, with rating of 18A each. Now we have installed CPU of say 7-8A. Will the rest of 10A remain unused? Or can that be used and given to other pc components when required?

2. Originally Posted by WJohn
I have a question. As you said that 12V2 line is reserved for CPU. So if a power supply has two 12V rails, 12V1 and 12V2. Both, say, with rating of 18A each. Now we have installed CPU of say 7-8A. Will the rest of 10A remain unused? Or can that be used and given to other pc components when required?
There are a lot of factors. First we need to know the combined 12V capacity. If it is only 25A combined, then you could have a load of 7A on one rail but nothing is "trapped" since the other rail can only draw up to 18A. But if the combined 12V capacity is like 30A and there is only a load of 7A on the first rail, you can only use 18A more leaving the remaining 5A inaccessible. This is one of the few failings of a multi rail design and IMO, a 18A max two rail design is not acceptable on a PSU that has at least one PCIe connector. If a PSU is powerful enough to run a video card that requires a PCIe connector and 18-20A or so is the max for each rail, then there should be at least three rails: one for the CPU, one for the mobo/molex/sata and one for the PCIe connector(s). But this on a case by case basis.

3. Flux Capacitor User
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Originally Posted by WJohn
I have a question. As you said that 12V2 line is reserved for CPU. So if a power supply has two 12V rails, 12V1 and 12V2. Both, say, with rating of 18A each. Now we have installed CPU of say 7-8A. Will the rest of 10A remain unused? Or can that be used and given to other pc components when required?
1. 12V2 is not always reserved for the CPU. That was just an example.

2. The unused capacity of the rail is technically "lost". However, most PSUs have their OCP set in a way that if one rail is unloaded or not loaded very much, the other rails still have enough total capacity to be able to take advantage of the power that is not being used on that rail. For example, let's say we have a PSU with four 12V rails having a combined maximum of 60A, but each rail has its OCP set at 20A. With such an arrangement, even if you leave some of the rails partially unloaded, the other ones have their OCP set high enough that they can make up the capacity you "lose" from not fully loading the rails. You just have to make sure that you don't exceed the combined maximum +12V rating, even if when you add up each rail's individual OCP limit it comes out to more.

4. Agreed. That thread is full of morons... both single and multiple +12V rail fan boys.

5. Flux Capacitor User
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I would say the NCIX forums are the black hole of the internet, except that dubious honour really belongs to the Tom's Hardware forums. NCIX is pretty close though.

6. I particularly liked the part where the DaleF person said that the "Splitting of the 12V rail" thread stickied here was written by someone "misinformed" about how PSUs work. Facepalm x 10.

7. Yeah I saw that they have Corsairs new PSU line at NCIX already and noticed that thread. I thought it was hilarious, should have made some popcorn.

8. Originally Posted by Smirnoff
I particularly liked the part where the DaleF person said that the "Splitting of the 12V rail" thread stickied here was written by someone "misinformed" about how PSUs work. Facepalm x 10.
My favorite post was the one where the guy said that the single +12V rail made him feel like the PSU was more robust and better quality.

Umm... using less parts makes a PSU more robust?

9. Originally Posted by jonnyGURU
My favorite post was the one where the guy said that the single +12V rail made him feel like the PSU was more robust and better quality.

Umm... using less parts makes a PSU more robust?
Yeah, less parts, higher reliability

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