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I'm planning on buying a new R9 280X, to crossfire with my current one. However, I have a 600W PSU which may not be able to supply sufficient power for both, as each ~300W require under load, and the rest of the computer hasn't been counted in yet. Can I use another PSU and connect it's PCI-E power cables to the new GPU, and leave the other GPU and the rest of the computer connected to my current PSU? I have a MSI 990FXA-GD80 motherboard, capable of supporting both GPUs.

EDIT: Tested it with a GTX 580 using a 500W PSU for the motherboard, and connected a 350W PSU into the PCI-E, and it worked perfectly, but requires good timing.

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    picking up a new PSU with actually enough power's the smart thing to do here. – Journeyman Geek Dec 15 '15 at 22:06
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    There are two reasons I prefer the 2 psu option, first of all, I don't have to sell my old PSU, which may not sell for as much as I bought it, secondly, I already have a ton of spare old PSUs between 280-400W, so it's essentially free! A new PSU is usually more expensive then 2 combined anyway – Matthew Dec 15 '15 at 22:09
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    There are actually cases that will do 2x PSU for the more extreme builds out there. 900D by Corsair has bays for two PSUs for when you absolutely need dual 1KW+ PSUs to power your dual 18-core Xeons, four Titan Xs, and custom water loop along with an army of low-RPM high-pressure fans. All he's doing "wrong" is increasing his chance of a PSU failure just by virtue of having two of them. – Arthur Kay Dec 15 '15 at 22:43
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    If one of them fails, only 1 gpu stops working anyway. Besides which, PSUs rarely fail unless they are badly built such as the cheap chinese high-wattage ones. – Matthew Dec 17 '15 at 20:35
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    Even so, a PSU failing is already very, very rare, and even if it does fail it's no disaster as little data is lost (if you remember to save!). Dual PSU setups aren't just for people who can't get enough from 1 single high wattage one, and a special case isn't needed either - just leave the side panel open and put the secondary PSU outside the box. – Matthew Feb 19 '16 at 20:51
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Yes, the add2psu adapter can be used for that as a drop-in component. However, you could also paperclip short two of the 20-pin leads (which I really don't recommend, and it's a particular pair you'd need to look up) which will cause your second PSU to always be active instead of waiting on power from the motherboard to return on the one pin. This is more or less what add2psu does AFAIK, but more elegant.

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    The focus of the question was on whether it was ok to connect 2 different PSUs to one component, but thanks. Personally, i think that add2psu and similar products are ridiculously overpriced; the paperclip option is very easy to use. Just connect the green slot to any black slot, and the PSU boots up. Most PSUs have a switch at the back anyway, so the paperclip doesn't need to keep being connected and disconnected. – Matthew Dec 17 '15 at 20:32
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    As I read the question, it seems to be asking about multiple PSUs for one machine, not so much one component. I.E. second card solely off of the second PSU. PSUs supply slightly different voltage over the same lines, so I personally would not mix two supplies on one component, as it might adversely affect stability / overclocking in weird ways. – Arthur Kay Dec 18 '15 at 2:13
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    The graphics card is plugged into the motherboard, and the PCI-E power is from the second PSU, meaning both are used on the same component, as the motherboard supplies some power aswell. – Matthew Dec 19 '15 at 18:16
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    Ah, true. But they take separate paths through the power circuitry and phases before they hit the GPU die, so it shouldn't matter. – Arthur Kay Dec 19 '15 at 21:08

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