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How to calculate GPU power needs


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#1 Alchemist

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Posted 06 August 2012 - 10:15 PM

Since I have been given a "hand-me-down" motherboard that was a high-end gaming rig when it was new, I am thinking of putting some newer cards in it and running some of the more demanding games myself. But I have ever bothered with high-end graphics cards before. The more I try to figure out how to setup the power, the more confused I get. It's not just the total capacity of the power supply you need to worry about, it's the amps available in the 12 volt feed. What is meant by the 12v "rail"? Some PSUs have all their power in a single "rail" and others have 2 or even more in some really high-end models. Since any PSU has several 12v connectors of different types, this is cannot be a synonym for the ATX motherboard connectors.

If a card requires 30 amps (which the manufacturers of several GTX 670 cards claim to require) is that total available from all of the "rails" or does it have to be available through one "rail"? Does the MB itself have to supply that, or does a PCI-E video connector supply that? Or is it good enough if the combination of them has that in total? Or is that really what the card itself requires? Because the tests I have seen claim that the GTX 670 only draws about 170 watts max, which would be about 14 amps at 12 volts. So are they claiming you "need" 30 amps, is that just their estimate of what the whole system would be using?

Anyway, if I can just figure out the meaning of "rail" and whether a single 12v feed needs to be 30 amps or more to run a GTX 670 or whether the PSU total from several 12v feeds being more than 30 amps is good enough, that would be helpful.

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#2 DJBPace07

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Posted 07 August 2012 - 05:14 PM

Unless you have an "Off Brand" power supply, any PSU rated for SLI or Crossfire from a good manufacturer will have what you need. When GPU makers put PSU suggestions on the box, they are usually pretty conservative and take into account the power needs of the entire system. Some power supplies have the 12V amperage split across multiple rails, others have just a single rail. That 12V rail is what the GPU needs. For example, take the ENERMAX NAXN 82+ ENP650AWT B 650W which has the output of +3.3V@25A, +5V@25A, +12V1@25A, +12V2@25A, -12V@0.8A, +5VSB@3A, which is more than enough on the 12V rails. Power supplies aren't exactly my strong point when it comes to PC's, however.

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#3 coxchris

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Posted 07 August 2012 - 08:19 PM

Power supply aren't my strong point too but I will give it a go

Each output level +3.3V, +5V, 12V1 ,+12V2 -12V, -/+5V has different voltages because motherboards have different components that operate on a different value of DC voltages

For example you have two rails on that power supply V1 and V2 those two "rails" handle25A on each rail. The PCI-express connectors have direct power to the V1 and V2 +12V rails.

Here is the website that I found for you on power http://www.playtool.com/pages/psumultirail/multirails.html and all about power supplies

What I do if I have a single gpu I calculated the recommended wattage of my card and add +/100w-150w to it so 400W+150watt = 550w psu. You can buy 650 and have it stable. A power supply only draws power as it needs.

With any single gpu in the system 650 is more than enough

With two (SLI) gpu in the system 750-800w

With 3 way or Quad 950-1000w (but that overkill if you have quad gpus)

Hope that help

Edited by coxchris, 07 August 2012 - 08:20 PM.

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#4 Alchemist

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Posted 08 August 2012 - 04:06 PM

Well, the question holding up my decision is what do "rails" mean and how do they limit power usage. But looking at the page you found, I see there is no clear answer because makers of PC power supplies don't follow guidelines very well. :( Since I only want to use a single GPU card I guess I'll go with the one I was looking at. At 700 watts and 25 amps on each 12v "rail" it should be powerful enough to handle the CPU on one "rail" and the GPU on the other. Even the GTX 670 at 14 amps.

Thanks for the help. Until standards improve there just doesn't seem to be a definite answer and experimentation is the only way forward...

Edited by Alchemist, 08 August 2012 - 04:07 PM.


#5 coxchris

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Posted 08 August 2012 - 05:50 PM

A power supply has 5 voltages each voltages related to a "rail" A "rail" is just a term to relate to a specific voltage Each rail is rated in amps So if I have 12 volt DC I guarantee I will have at least 25A on that rail.
http://answers.yahoo.com/question/index?qid=20100502003129AAl8jD2

They put limiting circuits in power supply to limited the power on each of the rails. I believe they put 5 limiting circuit on a power supply on each rail.
http://www.radio-electronics.com/info/circuits/transistor_current_limiter/transistor_current_limiter.php

The P1 connector (20/24 pin cable) is for the motherboard
the EPS-CPU connector is a (4Pin/8pin) that provides CPU with its own power.
PCI-Express connectors provide direct DC power from PSU to GPU.

I hope this explains your questions little bit more

Edited by coxchris, 08 August 2012 - 05:53 PM.

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