Psu dual rail - please explain

teulk · 30 Jun 2007 at 01:03

Ok ive just bought myself an antec eartwatts 430w PSU, now it has two +12 17a rails so that would be combined as 34a ??? Now ive also just bought a x1950gt which ive been told (if i can remember correctly)needs a total of 31a........the psu only has 4 molex so im guessing they are the two 12v rails with two molex connectors on each.....Now the gfx card requires two power leads so that would be the two rails used but i would then have to use the other two molex connectors to power the rest of my stuff i.e hdd, drives,fans ect. Now this its the bit im worried about, if the gfx card needs 31a and the rails connected to it are shared by the rest of the stuff in my PC will this affect the running of the card - i.e how much Amps it actually gets........any of this make sense ??. If id bought a PSU with 6 molex connectors then would i be right in saying two would rails would be seperate from the other 4 so then i could give the gfx card two dedicated power supplies ??? Forgive my ignorance on this matter.

semi-pro waster · 30 Jun 2007 at 02:24

I'm afraid it isn't as straightforward as a simple additive calculation for the rails, 2x17A is probably about 30A total (without going into the mathematics of it as I can't be bothered to work them out properly right now). An X1950GT doesn't need 31A, or at least it will happily function on a lot less.

I'm not 100% on how Antec splits the rails on that PSU but in most cases if you overload one rail then the rails combine to become in effect a single rail PSU which will easily cope. Each molex does not equal a separate rail but you will probably have two 'looms' coming from the PSU, each one signifies a separate rail usually. With the graphics card you have I wouldn't be too worried about splitting them up, if possible I'd use one loom for the graphics card and the other for the hard drives/optical drives but I doubt it will matter in the least.

teulk · 30 Jun 2007 at 03:21

Thanks very much, so i can rest easy then that it should all run fine - i doubt that my card will ever get "maxed out" so i dont have to worry about it running at max load

Nullvoid · 30 Jun 2007 at 11:07

Hey teulk.

The 12v rails on the earthwatts are only artificially separated to comply with the atx standard of no more than 18A per rail. In reality they will act like one big single rail (with a combined 30A, not the 34A you get just by adding them together), and you should have no problems whatsoever in running your system (assuming you don't have too many peripherals, 30A is enough to run any single card).

Oh and what you're saying about cables has me a bit confused. A single x1950gt will only need 1 pci-express connection, which the 430w earthwatts indeed has. You will be using the molexes for older hard-drives/optical drives.

teulk · 30 Jun 2007 at 18:37

Nullvoid said:
Hey teulk.

Oh and what you're saying about cables has me a bit confused. A single x1950gt will only need 1 pci-express connection, which the 430w earthwatts indeed has. You will be using the molexes for older hard-drives/optical drives.

Its the Sapphire X1950GT 256MB GDDR3 Dual DVI TVO AGP version so i was under the impression it required two power connections i.e two molex - am i wrong in thinking this ???

Nullvoid · 30 Jun 2007 at 20:41

No you're perfectly right, I'd just forgotten that it was an agp card, sorry!

There's still no need for concern however. Whether you carefully split up all your peripherals to make for a balanced load on the cables available, or pile them all onto one cable, the supply will still just happily go about its work.

teulk · 30 Jun 2007 at 20:58

Nullvoid said:
No you're perfectly right, I'd just forgotten that it was an agp card, sorry!

There's still no need for concern however. Whether you carefully split up all your peripherals to make for a balanced load on the cables available, or pile them all onto one cable, the supply will still just happily go about its work.

Thanks