+12 Volt Rail Amp Rating

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Persondude2

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Sep 24, 2007
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I'm looking into buying a new video card, and the one that I want, the EVGA GeForce 8800GTS 640MB, says that it needs a "Minimum recommended power supply with +12 Volt current rating of 26 Amps."

What exactly does this mean? The highest amp rating I have seen for a power supply on Newegg is +12V@20 amps. Can I use two +12V 20 amp wires to power the card??

Thanks in advance for your help.
 
Check out the PSUs with a single +12V rail. For example, my PC Power & Cooling Silencer 750 Quad has a single, powerful +12V rail capable of providing 60A. It is SLI approved, and has two 6-pin PCIe and two 6/8 pin PCIe connectors for the latest video cards.

Regards,

Altazi
 
As Altazi said get a PSU that has a large single rail. The silencer 750 is kind of expensive and larger than you need. Get the Silencer® 610 EPS12V Power Supply instead. $119.00 factory direct with a 49A single rail. Check Neweggs price. The Corsairs are also single rail, even though they don't say so. I don't really like the cheesy modular cables on the Corsair though. Here's a link to Power Supply Myths Exposed! with some good information.

Edit: It's also a tier 1 PSU Official XS Tiered PSU Manufacturer Brand Listing Phase III
 
It actually should say something like "power supply with 'combined' +12 Volt current rating of 26 Amps".
And yes using ONE PCI-E 6-pin power connector will get the 8800GTS the extra power it needs and can't get from the PCI-E slot.
Corsair VX450W 33amps on the single +12v rail $70 after rebate




 
Im using a £20 (~$35 $40) PSU, no named brand from ebuyer. It provides 600 watts with 30 amps on the +12v rail. Im using an 8800 gts 320mb, the amp requirements are the same as the 640mb version. I have 1 hard drive, E6750 at 2.67ghz, and a dvd drive. Its working fine so far, under full load on games like oblivion :).

I dont know much about PSUs or if my PSU is actaully giving my gfx card enough power, is there a way to check?
 
If everything is working fine, that's a good start. Are you running any kind of system monitoring software, like ASUS PC-Probe? It will notify you if a voltage drops below a preset threshold.

You can always use one of the PSU calculators to see if yours is "in the ballpark". I like this one.

Regards,

Altazi
 
Thanks for the replies everyone. 😀 I think I understand now:



So if the power supply that I'm buying says this for output: +3.3@30A,+5V@50A,+12V1@18A,+12V2@18A,-12V@1A,+5V SB@2.5A

It will work for the video card because the two +12 rails combined are more than 26 Amps?
 

This is incorrect. WRONG!
"djcoolmasterx wrote :

No, you do not need it on one single rail. The 26Amps stated are for the entire system. So dual 18Amp rails would be enough."

You do not add the rails together. My old 305w psu has 2 18a rails = 264w = 22amps
 
What kpo6969 means is you need to check the total watts available to the +12v rail(s)
17-190-011-04.jpg

In the example of the 500w PSU above you can just make out that +12V1=18A and +12V2=20A for 38A.
But if you read the fine print you see that only 360W are available to +12V1+12V2. So 360W / 12W = 30Amps.
So instead of the expected 38Amps that power supply will only support 30Amps of +12 volts.
 
My suggestion is to stick with the single +12V rail. This avoids the hassle of trying to balance the loading on the multiple rails. Much easier this way.

Altazi
 
There is no hassle with balencing the rails. If the PSU is truely a dual rail PSU then its already seperated so that the CPU is on one rail with everything else taking the other rail.
Click to expand...
That is true, but if it is truly in compliance with the ATX12V specification then any unused current on the CPU rail will be unusable. Therefore, you are limited by the maximum amperage available on the second rail. With a single rail any unused current is available.
 


No never said that you add the amperage, instead you divide the wattage supplied to the 12v rails by 12v to get the overall amperage. In the example I gave you could typically expect about 30Amps for a 550W PSU(which I would expect is the minimum someone buying a 8800gts 640 would get).

Sorry if you misunderstood I just wanted to make clear that the 26 amps does not have to be on a single rail.
 
I need to buy Gainward 7800 GS, and Chieftec GPS-450AA-101A 450W psu, which has +12V1 14A and +12V2 15A = 29A total!

Should this psu be enough for this card? Thanks in advance
 
I can't find anything about Chieftec PSUs. My guess is that it came with the case. Either way I don't think they are reputable, I could be wrong. I would replace it with a name brand single rail. Corsair HX series is single rail even though they say they are multi rail.
 


You can find it on Chieftec site: www.chieftec.com

And Corsair is better I know that, but also a much more expensive... I don't have money for Corsair; it cost 4 times more the Chieftec! For example Chieftec 450W cost 50 Euros at my local shop, and Corsair 520W costs 100Euros, that far more expensive!

And why would you replace it with single rail??? What's the difference?

Thanks
 


Yes, I know that! But, Chieftec has 29A on both +12V! Gainward 7800 GS Golden Edition needs 6-pin PCI-E power connector!

 
What I meant is that I can't find it on any PSU lists or for purchase separately. I'm guessing that it isn't a very good PSU, but that's only a guess. For the single rail question see my post above or read this Power Supply Myths Exposed! #8.
 


thanks for that link, I now understand why You recommend a powerfull single rail PSU, rather than dual rail PSU; but on other side that powerfull single rail PSU costs much more then some normal dual railed 😀 :hello:
 
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