Question about multiple power supply rails, when powering a gpu.

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matt101

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May 19, 2010
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Hi all,
I am thinking of purchasing a gtx 770 relatively soon. Specifically this one : http://www.scan.co.uk/products/2gb-evga-gtx-770-sc-acx-28nm-pcie-30-7010mhz-gddr5-gpu-1111mhz-boost-1163mhz-cores-1536-dport-hdmi-d

However I heard from a certain youtuber, that you need 42a of power on your 12v rail for that card. But my power supply has 4 rails. The exact power provided is shown on the 5th page here : http://www.datasheets.pl/power_supplies/TP-650.pdf

I am unsure as to whether it will provide enough power to the rails, as I have no idea how to calculate the total when there are multiple rails involved. Not only that it says that the rails 3 and 4, are specifically for my PCI-E slots. So does that mean I just add together the amps of these two rails?

Thanks.

 
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markwp

markwp

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Jan 22, 2013
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You need a psu that can supply sufficient current (amps) to produce about 240 watts of power. The formula for watts is volts x amps. Since you know the watts (240) and the volts (12) you need to determine amps. Amps = watts/volts - 240/12 = 20 amps. Since a psu is ~80% efficient, you will need 20 amps/.8 or about 25 amps to properly serve the gpu.

Your psu says it will provide a max of 25a on either rail 3 or rail 4, but the total system amps cannot exceed 54 amps across all rails. If you use the connectors (one 6 pin and one 6+2 pin), you should pull ~12.5a from both rail 3 and rail 4, leaving ~ 29a for the rest of your system.

Your psu will handle the load.

There is a divided opinion on split rails - some like it, some don't. Personally, I like single rail systems - if the rail goes out, the entire pc shuts down. In a multi-rail, if a single rail goes, it may put excessive stress on the remaining rails and create excessive heat before they give up. To me, the chance of this heat creating fire is not worth the "value" of having only part of the psu go bad. If any part of the psu dies, I would replace it anyway.

Mark
 
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