Splitting 12V rail for one GPU.

[Unicorn_FTW]

Hi,

I'm building a new gaming PC and I'm a little puzzled by the dual rails on the PSU I've selected. This is the PSU I've chosen:

Be Quiet L8 700W (http://www.bequiet.com/en/powersupply/373)

And the GPU I've chosen is this:

Asus HD 7970 (http://www.asus.com/Graphics_Cards/HD7970DC2T3GD5/)

The GPU requires two 8-pin PEGs, the PSU has 4 available. But, the PSU has dual 12V rails, providing 36 and 30A of current. My question is would this PSU be sufficient for this GPU (allowing for overclocking headroom) and the rest of my system (i5 4670K, which I plan on overclocking too).

I realise there is a sticky on this topic, but it would be great to get some feedback which is specific to my config. Thanks in advance!

 

[alkemyst]

What price point you buying at?

700W should be fine for a 7970.

I would buy a Seasonic, Antec HDP. Usually your video card will be the loudest.

 

[Unicorn_FTW]

Price point is around £50-60 (I'm in the UK).

Yeh, I thought 700W would be more than enough. I'm wondering is the 600W version of the above PSU okay aswell then? Bear in mind, I do hope to be overclocking both GPU and CPU.

Also, there should be no worries with these PSUs using two 12V rails then?

 

[aigomorla]

is it enough? yes..

700W is most definitely enough.

Also, there should be no worries with these PSUs using two 12V rails then?

just make sure u use the proper 8pins.

The pins on the board and gpu are different, make sure u dont force the wrong headers and yes, you wont have any worries.

 

[Unicorn_FTW]

Thanks for the response aigomorla.

For sure 700W is more than enough, I guess the same is true for 600W too. The thing with the rails is the problem. The PSU comes with 4 PCI-E connectors (all 6+2 pin) but how do I know which is on what 12V rail? The documentation for the PSU doesn't state this at all.

This is why I'm a little worried that 30A on either 12V rail might not be enough, if for example, it is shared between a set of PEG connectors and say the 24 pin ATX connector.

Maybe I'm being a little paranoid?

 

[aigomorla]

only way to find out would be to open the PSU voiding your warrenty and see how many places the yellow wire comes from, as that is the 12V rail.

Are you being a bit paranoid?
Possibly.. if its a bronze/silver/gold 80+ rated PSU, they are all soild, and u dont have to stress... however YMMV... u may run into the fluke bad luck of a bad PSU.

Also i dont think u'll run into a situation where your cpu and gpu will be under heavy stress at the same time for long periods of time unless ur a folder.

600W would be cutting it in the edge...
Approx 120-150W for the cpu when overclocked... add about 50-75 for all the additional.. (usb + fans + hdd) + 300W for the GPU.
~ 525W (MAX Theoretical Guessimation) <-- 100% load approx value...
a number only 1% of the real users ever see, unless ur infected with a virus or a folder or some crazy render using both gpu and cpu.

if everything was pulling power, your max...

Honestly op, id stick with a 700W.
Headroom is indeed very nice...


Jonny has a great article on rail splitting.
You should check it out and not worry too much about it.

Im sure the PSU has splitted them in a way so wouldnt overshoot on it.
Most PSU makes dedicate a rail itself to the board and another to the 8pin to offload amperage on the single rail.

 

[Blastman]

The GPU requires two 8-pin PEGs, the PSU has 4 available. But, the PSU has dual 12V rails, providing 36 and 30A of current. My question is would this PSU be sufficient for this GPU (allowing for overclocking headroom) and the rest of my system (i5 4670K, which I plan on overclocking too).

Easily enough.

From manual …

12V1 30A … FDD, HDD, PCIe1, PCIe2, SATA, 24pin
12V2 35A … PCIe3, PCIe4, CPU (P4 + P4)

… you have 35x12 = 420 watts for just graphics and +12V EPS, which is lots.

Also, the rail ratings are usually pretty conservative -- meaning that the individual rails will usually pull considerably higher than what they are rated for. For example, the Enermax Triathlor Bronze 550W psu has 2 rails rated …

12V1 25A
12V2 25A

… but the manual states the OCP (over current protection) trip point is 35-45A … 10-20A above what the rails are rated at. So, one could likely easily run up to 30A or slightly more on one of the Enermax rails no problem. If bequiet is anything like Enermax, those rails will easily pull their rated wattage and then some.

 

[Deders]

I don't think you'll have problems with either the 600 or 700W models but I would suggest using PCIe 3 and 4 for the graphics card as up to 75W for each graphics card is supplied through the 24pin cable, via the PCIe slots themselves.