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How many graphics cards (R9 280x) can motherboards handle without any powered risers? I think 2 is OK with any MB, but how about 3? Or is there any specific MB model that could handle even more of them?

I remember a thread talking about this and you seem to be spot on. If you're running 3 79xx/r280x just assume that you'll need a powered riser. I imagine that such risers are also getting a commensurate price squeeze.

In my experience..The answer is 3.

At 4 I seem to have a much greater chance of charring up the main MB power connector (Most cards will draw somewhere around 75 Watts through the MB).

I know, people will come in here and claim they run a dozen cards or whatever they want to claim without power risers.

I can only tell you what I have experienced in the few years of running my rigs. Without powered risers,at three cards I have only burned up one MB. At 4+ the number jumps.

As many as they fit i guess. Why should you require risers if there is space to put them on board?

I'd go for max 3 cards with unpowered risers, more at your own risk!

Cooling. You are not going to run three cards at 100% 24/7 next to each other on the MB.

I believe two would even heat crash often. I run my rigs open air with a two shelf system. The MB on the bottom shelf and the GPUs on the upper shelf using risers.

I had no issues with 3 unpowered risers on a matx board.

I would run powered risers on anything more then 4 but check if the motherboard also has an additional 12v input which some motherboards do like the Asrock which I would then run up to 5 unpowered risers.

For what I have understood, there's a risk that motherboard will burn if too much power is taken thru the PCIe- slots.

For the cooling purposes, unpowered risers would be just fine.

I've never used powered cables. I used to run three 5830s on each motherboard for a good year or two without problems. Upgraded all of my rigs a few months back to 7950s. I am running two 7950s on each board right now without issues. Only reason I didn't use three or more was because all of my PSUs are 750w. If my PSUs where 900+w, I wouldn't have even given powered risers a second thought.

Yeah well as I thought, there shouldn't be any problems with 2 or even 3 cards. But how about 4 to 6 in one MB? Is there ANY motherboard that could handle such a load?

Friend of mine ran five XFX 5850s earlier this Spring without powered cables.

Most boards have a power allotment on the PCIe Bus you can normally run 3 cards with out putting a drain on the bus but more then three you normally need extra power so you don't burn up the MB or the video cards. At least this is my experience