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Advice about psu!!!

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March 18, 2012 8:54:31 PM

guys need to no something quick when reading a psu specs and you see this on a 600w, 12v1 24a and 12v2 24a ....my question is ,is both the rails linked? what im trying to say is if my card needs 30a for example will it get that 30 ? 24 from 1 rail and 6 from the other ?? or does it work like this..... 12v1 is only for cpu use and not linked to 12v2 ????? please any advice is appreciated thanks

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March 18, 2012 9:23:03 PM

different rails are seperate.

but it can draw more current from one than the other.


as long as the total draw doesn't exceed the max then you should be fine.


make sure to go for higher than you need, it will make it work for longer because over many years you can get loss and it may not output as much power as when it was new, this is less likely to happen/become a problem in well made power supplies


if it is a combined rail then simply make sure the current draw doesnt exceed the max


if a card draws 200 watts. and is fed by a 12v supply from psu

then 200 divide 12 = 16.6/17amps

watts = current * voltage
current = watts / voltage




best thing to do is tell us what you are planning on using and we can recomend a psu to go for. when you see what we recomend then it will give you an idea on what to look out for because you may find a deal which is better than we could find
March 18, 2012 9:31:34 PM

shanky887614 said:
different rails are seperate.

but it can draw more current from one than the other.


as long as the total draw doesn't exceed the max then you should be fine.


make sure to go for higher than you need, it will make it work for longer because over many years you can get loss and it may not output as much power as when it was new, this is less likely to happen/become a problem in well made power supplies


if it is a combined rail then simply make sure the current draw doesnt exceed the max


if a card draws 200 watts. and is fed by a 12v supply from psu

then 200 divide 12 = 16.6/17amps

watts = current * voltage
current = watts / voltage




best thing to do is tell us what you are planning on using and we can recomend a psu to go for. when you see what we recomend then it will give you an idea on what to look out for because you may find a deal which is better than we could find


ok basically i was checking out my card(gtx 460 hawk) on guru of 3d and after their tests this card can pull 360w,30a when extremely OC'd so thats why im asking wether i would need a psu with 30a on each rail or is the rails combine?
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March 18, 2012 9:33:08 PM

and are you saying that different psu's come combine? is there psu's that are combine and others that arnt?
a c 87 ) Power supply
March 18, 2012 9:33:54 PM

If you are talking about a specific power supply and a specific graphics card you should name them.

I think it is fairly common if there is more than one rail and one of those rails is not fully loaded then another rail can draw more than what it is rated for if need be. So potentially if you needed 30A from one rail, as long as you are not drawing the full 24A from the other rail then that should be achievable.

The problem with this is that there are only two graphics cards in existence that I know of which will draw anywhere near 30A by themselves and that is when they are overclocked and those cards are the 6990 and GTX 590.
a c 248 ) Power supply
March 18, 2012 10:24:27 PM

Power supply recommendations are always for the system as a whole, the Guru3D results show the system using 347 W from the wall when it is at full load, this is not the power drawn by the GPU, that is only about 170 W. The system pulling 347 W from the wall means the PSU is delivering about 305 W(assuming 88% efficiency) to the system. The GPU itself is only using about 170 or about 14.2 A from the 12 V rail so you have plenty of power.

The main issue with the power recommendations is that people think the GPU is sucking down 30 A of power, it is not, the GPU, CPU, fans, drives, and motherboard all combined are able to suck down about 30 A from the 12 V rail which means your load is distributed across the rails.

Take a look at the link in my sig on single rail multi rail to clear up some more confusion.


@silvune - rails don't produce their own power and the ratings on them are the current limiter settings, you couldn't pull 24 A from both rails without the unit shutting down from over power protection because rail ratings are rarely additive, but you also cannot pull 6 A from one rail and try to pull 28 A from the other since the current limit is at 24 A it will still trip and shut down.
March 18, 2012 10:47:31 PM

nice article man ok so you could help me i have a psu with 12v1 been 16a and 12v2 been 20a ,the psu only has 1 6pin so im powering the card with the 1 6 pin and 2 molex to 1 6 pin .....my card stock at load should use about 170w guru says 205w ok so when i oc this card to its max what power am i to expect ? bare in mind i only have a 20a rail 260w ....on guru they state
Measured power consumption

Advertised GeForce GTX 460 TDP = 150W (768MB) 160W (1024MB)
System in IDLE = 167W
System Wattage with GPU in FULL Stress = 347W
Difference (GPU load) = 180 W
Add average IDLE wattage ~ 25W
Subjective obtained GPU power consumption = ~ 205 Watts

so if after they oc the card and the system pulls 503w subtract- the system power which is 167w that leaves you with 337w then take 80% of that which is 270w which the card is using am i right? and 270w been 22.5a so basically i would need a 550w 46a(23a + 23a) psu for the psu not to trip at max oc?
a c 248 ) Power supply
March 19, 2012 2:14:58 AM

That is a massive increase in the system power, putting subjective GPU power usage from the wall at 366 W, so its total power draw will probably be about 310 W from the PSU(assuming 85% efficiency for a bit of a worse case scenario) which means it needs about 26 A from the PSU, but its important to remember that it is getting about 75 W from the PCI-E slot, and not all of your connectors will share a rail.

Most multi rail PSUs also don't set their current limits so they add up to the max, take the EA500D for example, it has a max current capability of 37 A, but has 2 22A 12 V rails so that you can throw more than half the load on one rail without issue. It is also quite likely that the CPU and 24 pin main ATX connectors are on one rail with some molex's and SATAs, and the PCI-e connector and a few molex's and SATAs are on the other rail to help spread things around.

A single rail unit can help if you don't want to deal with balancing the loads, or an easier solution is to get a larger PSU(~650) so you have plenty of excess capacity and don't OC to the max. The sample they have had a 15% factory OC, their final results was a 41% OC for the core and shader and 22% for the memory clock which is pretty rediculous, stick to less than 20% and you will have rational levels of power consumption that is much easier to balance.
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