3 PCI power from 2 ?

Hi, need some advice. I've got an
1.OCZ ModXStream Pro 700w Silent SLI Certified Modular Power Supply
2.XFX ATI Radeon HD 4890 1024MB GDDR5 PCI-Express Graphics Card
3.OcUK Value ATI Radeon HD 4890 1024MB GDDR5 PCI-Express Graphics Card - Retail

I'd like to crossfire the two 4890s however the power supply has two "red 6pin plugs" for the pci-e cards. All would be well except the XFX card uses both of them.

In my bag of cables I've got a two molex into a 6 pin but it only uses 5 of the pins - one of the middle ones isn't used. Like item scan code: LN9039 (can't post urls yet!) Is this the right cable? Any particular way to use it?

Is there Y cable to split one into two? Can i get a cable to use some of the molex cables?

Bit confused as to what to do so advice welcome!

Thanks

Dave

A "y-cable" would be bad, the whole point of 6 pins/8pins 2 connectors is to supply higher currents (easier than thicker wires) , if you used a Y-cable would be doubling the current draw over the joined cable this could over load the rail on the PSU, and even worse over the wire causing it possibly to melt.

Quote:

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Originally Posted by oolon

A "y-cable" would be bad, the whole point of 6 pins/8pins 2 connectors is to supply higher currents (easier than thicker wires) , if you used a Y-cable would be doubling the current draw over the joined cable this could over load the rail on the PSU, and even worse over the wire causing it possibly to melt.

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So are you saying with this PSU and these two graphics cards I can't have them both in?

The point of extra connectors is simply to get around standardised limits on what the power supply should be providing on different rails. Those standards went out of the window a long time ago, but to get their cards certified as ATX the manufacturers still follow them.

Your power supply has two 12v rails rated at 25A each. That should be enough for a 4890 each, so all you need to make sure you do is balance the load so that either you have a card on each rail, or you have both cards on both rails. Your cables should be labelled as to which rail they are on, but you might need to do some digging to find out which one the molex connectors are coming off.

EDIT: technical drawing tells you all you need to know (MUCH kudos OCZ for making it available):
http://www.ocztechnology.com/res_old...XSP500-700.pdf

All the modular connectors are 12V2. That means no matter the cable arrangement you're going to be powering two 4890s largely from one 25A rail, which might still be okay, but I wouldn't.

What was saying is using a Y-Cable, without considering what rail it is pulling power from is a bad idea. Some PSUs (like mine) have a single rail, so that is not a problem however the current draw down the cable should not be over looked, as there is a limit on how much a wire of any given thickness can carry, and wires (for cheapness) are normally rated for a maximum expected, and a cable with a single plug on it with a maximum allow current, is quite a "known" ammount. However using a two molex to 12V2 is ok, as the molexes were on a different wire back to your PSU, so long as you don't put lots of other things on it with a splitter. I has two connectors for a reason... current draw, a single would over rate the connector.

Quote:

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Originally Posted by oolon

I has two connectors for a reason... current draw, a single would over rate the connector.

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I had never considered wire gauge before :O Good point.

So out of interest, as i mentioned above the cable I have only has 5 of the six pins in, one of the middle ones isn't there. Is this still the right sort of cable or is that missing pin required?