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Question about power requirements with more than 1 rail.

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enacku
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Wednesday, October 22, 2014 3:07 PM (permalink)
Let's say your 1000watt psu has 2 rails, each of the rails support 60amps
 
The gtx 980 requires 30amps. If you have two cards on one of the rails, are the requirements 60amps (30amp + 30amp)? Or do the amps not get added together like the required wattage? If each card pulls 200watts, you need 400watt minimum, but would required amps be something around 45-50amps @ 400watt? instead of 60amps @ 400watt?
 
Lastly if you have 2 12v rails, are certain psu cables on 1 12v rail while other psu cables are on a different one? Hope I explained my questions clearly, thanks.
 
 
 
 

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    bob16314
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    Re: Question about power requirements with more than 1 rail. Wednesday, October 22, 2014 5:10 PM (permalink)
    First off, a GTX 980 isn't a 30A card, it's a 13.75A (165W) card..See the Details here..Of the 13.75W and 165W, the card can get 6.25A and 75W through the PCIe slot itself..Amps are calculated by dividing the wattage by the voltage (165W / 12V = 13.75A)..Yes, you add the amps together and the watts together when considering load..Here is a handy calculator for various conversions..If someplace says a GTX 980 needs a minimum of a 500W PSU with 30A on the +12V rail for example, they're talking about wattage and amperage to run the whole system, not just the graphics card.
     
    If there's two or more +12V rails, some cables are fed from different rails, of course..You would need to go to the PSU manufacturer's website (or some other reliable source) and look at the specs/diagram to see which rails feed what..That information is also sometimes found in the manual for the PSU and sometimes on the PSU itself..There's some dated articles/info here and here and here about multiple rails and other stuff, but the same holds true today.

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    dave851
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    Re: Question about power requirements with more than 1 rail. Wednesday, October 22, 2014 10:06 PM (permalink)
    bob16314
    First off, a GTX 980 isn't a 30A card, it's a 13.75A (165W) card..See the Details here..Of the 13.75W and 165W, the card can get 6.25A and 75W through the PCIe slot itself..Amps are calculated by dividing the wattage by the voltage (165W / 12V = 13.75A)..Yes, you add the amps together and the watts together when considering load..Here is a handy calculator for various conversions..If someplace says a GTX 980 needs a minimum of a 500W PSU with 30A on the +12V rail for example, they're talking about wattage and amperage to run the whole system, not just the graphics card.
     
    If there's two or more +12V rails, some cables are fed from different rails, of course..You would need to go to the PSU manufacturer's website (or some other reliable source) and look at the specs/diagram to see which rails feed what..That information is also sometimes found in the manual for the PSU and sometimes on the PSU itself..There's some dated articles/info here and here and here about multiple rails and other stuff, but the same holds true today.




    Just want to add onto this, some power supplies will say things like 60 amps max per rail, but only a total of 50 amps can be drawn from both rails at any given time. Not all power supplies are like this but it is still considered good practice to try and load the rails evenly. 

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