Can someone explain how SLI works with PSU?

[zeock9]

Do graphics cards in SLI mode still require their full power connectors to be plugged in?

I have, for example, 'SLI-certified' CM 700W PS with two 6+2 PCI-E connectors, both of which are being used by one single GTX 275; does this mean that my PSU isn't really capable of running higher end GPUs that require two power connectors in SLI?

Or do cards in SLI mode only requre one of their power connectors each to operate?

And if not, do people running quad SLI setups must have more than one PSUs in their system? because even top of the line 1200W+ PSUs only have six 6+2 connectors, which means 3-way SLI would be their max.

I'm a little confused!

 

[iiiankiii]

Most cards you buy come with molex to pcie adapter. You can use those adapters to add a second card.
molex to pcie adapter

You have to plug in the pcie power to all cards. As long as your PSU is strong enough to support 2 cards, it'll be fine.

 

[ViRGE]

With only a handful of exceptions, video cards require that all of their PCIe power sockets are filled. So for a 2 card setup you would need to be able to supply 4 PCIe power connectors.

Most likely you have an older PSU from when cards with multiple PCIe sockets were not as common. A 700W PSU should still be able to drive SLI under some circumstances, but realize that 2 video cards add up quickly.

As iiiankiii notes you can always get Molex adapters to work with. Another option is getting an adapter that can convert 8pin to dual 6pin (fun fact: 6pin = 75W, 8pin = 150W = 2x6pin). In either case you'll want to do the math to be sure your PSU has enough wattage and has enough amperage on the right rains to support the cards you wish to use. A pair of Radeon 290s and a pair of GeForce 970s would be quite different, for example.

And if not, do people running quad SLI setups must have more than one PSUs in their system? because even top of the line 1200W+ PSUs only have six 6+2 connectors, which means 3-way SLI would be their max.

Actually many high-end PSUs come with 8 PCIe connectors these days. The Corsair AX1200i for example is 1200W unit with 8.

 

[mistersprinkles]

I would not SLI/CFX any 2 powerful cards with a 700W PSU. 2 low end cards yes. 2 970s no way. That's a 1 GPU PSU with some overhead, as far as I'm concerned.

 

[Jumpem]

I would not SLI/CFX any 2 powerful cards with a 700W PSU. 2 low end cards yes. 2 970s no way. That's a 1 GPU PSU with some overhead, as far as I'm concerned.

Look at the total watts drawn for a 970 in the reviews that have been done. It looks to be around 300W. A 500W PSU is plenty for a single 970, and a 700W is plenty for two 970s.

 

[Borealis7]

it all depends how much combined total Watts the PSU can supply on the 12V rail(s).
i think 50A (=600W) would be enough, taking into account the added 75W PCIe slot power.

 

[weevilone]

I've not been able to get me non-overclocked rig with 4790k, SLI reference 980s above 451W at the wall so far, and that's including the other gear on the UPS.

 

[Jumpem]

I've not been able to get me non-overclocked rig with 4790k, SLI reference 980s above 451W at the wall so far, and that's including the other gear on the UPS.

Hmm. I had been wondering if I could get away a 4790k and SLI 970s on a Corsair HX520.

 

[hojnikb]

Hmm. I had been wondering if I could get away a 4790k and SLI 970s on a Corsair HX520.

If you dont overclock anything, then its doable. That PSU has ~480W on 12V, so its enough. Single 970 is 145W (so 290W for two), 4790K is 88W...