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I know that a 8 pin pci-e rail has 150 watt. How about a 8 pin CPU rail?

I have a 6gpu setup and for my second PSU I'm trying to utilize the 2 8-pin CPU rails:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817438006

Any help would be appreciated, thanks!

This is not a good idea because the PSU's are setup very precisely to supply power to different components. Using a CPU rail could go beyond the limits of your PSU, and I would not recommend it. Your independent research is probably a better source of information than my non-expert opinion xD

Better to use molex and convert it to 8-pin.

To add to that warning, I've modded a few power supplies and found the 4/8 connector on 2 of 5 units ( most were dead awaiting an autopsy ) were to a separate 12V area from the peripherals 12V section, the others tied in to the same as peripherals.  If you don't mind voiding the warranty, make sure to unplug it from the wall and connect a power supply tester until it won't light up before opening ( to discharge everything ) and take a peek inside.  Cheat and follow the non-black lines and see if they all go to one big glob of solder or if there are separate areas for CPU and Peripheral 12V.  If they go to the same spot, I would recommend using it as a second PSU and one of those dual power converters so that they both go on at the same time and the ' main ' one is for the CPU and that one is only for GPUs and fans.

No issue as long as your 12v and grounds go where they are supposed to.

http://www.mangione.com.br/upload/cablemod2.jpg

12 volt is 12 volt. So long as you have the correct 12v/grounds going into the correct pins, and you load balance your power supply if its multi rail, it'll work fine.

If you direct plug your cpu 8-pin into your pci-e 8 pin, you will fry your video card.

Thanks alot for the info guys!

How about max watt output? Is it same as 8 pin PCI-E at 150watt?

An 8 pin PCI-e is rated for 150w but is capable of more. Depends if your PSUs wires are 20AWG, 18AWG, or 16AWG. CPU 8 pin has one more 12V wire compared to PCI-e 8 pin and should be capable of putting out more power.

Thanks, interesting reading.

I hope this is a joke. A molex typically has only one 12V wire in the harness.

Same. Molex converters are bad. You are asking for melted wires. Literally.

Using the 8-pin CPU connector can be an advantage, especially on multi-rail supplies.  They tend to have a rail dedicated to the CPU connections that you couldn't use otherwise.

Don't listen to the muppet who says not to use it, he/she/it is clueless.

Yeah, especially the quad rail supply the OP linked.  That CPU connector is probably the only connector on one of the four 20 amp rails.

The rule is 1 proper Pci-e power connector and one molex converter per card, never had any problems doing it that way.

You will one day. The PCI-E connector has 6/8 wires for a reason. You'll get away with it on graphics cards but ASIC hardware often goes well beyond the 75w/150w PC standard.

Yeah don't use molex!

Some time ago I had some HEX16A miners from technobit, they use about 80 watt each and are powered by a molex connector. My PSU only had one cable with molex connectors (like most modern PSU's do these days) and when I had one miner on the cable it already got quite hot. When I had 2 on the same cable it became too hot to hold for over 30 seconds. When I added a 3rd one the cable became too hot to even touch.

To OP:
I understand why you're asking this, but you might want to consider buying an extra PSU. Usually PSU's are most efficient if they're being used for about 50-75% of their capacity. And besides that, it's probably quite some money you've spent on GPU's, why would you risk losing them (or the PSU)?
Maybe it's just my philosophy, but I would never do "tricks" with PSU's to save a few dollars, I would go for the safer solution to be sure it will run like it should.

Molex have a 150 watt capacity, just make sure you don't cross that.

Well it is 11A so that is 132W on the 12V rail but most molex connectors will die under a load like that.  They are routinely used for much lower wattage especially in the last decade when MB and GPU got their own dedicated high current connectors so the Molex connectors were left to power fans and CD Roms.  They don't get much love in the over engineering department.  Plus is it a pretty old connector design without any retention clip so it is pretty easy for them to work themselves slightly loose.  Even a barely loose connector can increase resistance by a magnitude.  High current + increased resistance + cheapest possible part they could source + bad luck = fire.

If the OP setup isn't using the 8 pin CPU connector and OP can find a suitable adapter I would use that.   The 8 pin CPU connector is designed to provide up to 250W to the motherboard. 

From my experience getting people setup with all manner of equipment, 8 pin ATX = roughly 8 pin PCI-E from any reputable brand. With unbranded stuff its a lottery what you get anyway, but something like Corsair would work fine.

and don't buy an off brand PSU.  A sold rig (or server or workstation) starts with solid reliable power.  Power issues (beyond the PSU just dies or explodes) can cause all kinds of weird instability issues.  Files getting corrupted on disk?  It could be a bad drive or it could be bad power.  Keep getting driver crashes?  It could be bad power.  Due a memory check and it reports bad memory? It could also be bad power.  Weird performance issues, hard lockups under high load or CPU/GPU throttling back?  It could be a defective component or bad power.

Since bad power can show up as almost any computer problem starting with cheap power supply is just setting yourself up for failure.  Plus quality name brand PSU (corsair, enermax, xfx, and my personal favorite SeaSonic) generally are built better, have "beefier" caps, more overhead, and carry long warranties.  SeaSonic has 5 year warranties, pretty much no questions asked.  The same SeaSonic PSU has outlived three or four rebuilt workstations.

Spend the coins and start with solid reliable power.  The PSU will still be useful long after you stop mining.

Ofc, but now I said "per card" hinting at graphics card usage where anything but a 7990 should be fine with a 6/8 pin + molex coverter. Besides 1x6 pin connector can handle 150w+ easily, just look at the overclocked antminers.