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Hi there,

I have posted this question on Tom's hardware (http://www.tomshardware.co.uk/answers/id-2994362/connect-external-psu-380x.html) without much help. I am confident the Bitcoin community can help me better and maybe explain me better too :)

Basically I have this setup:

• a mini-ITX HTPC with an Antec FP-150-8 PSU, max 150W continuous power; +3.3V and +5V maximum combined output: 65W (thus the 12V line should be 150W-65W=85W at max 10A); 2 SATAs hard drives and an i3 4340 CPU are connected
• an Asus R9 380X GPU which I will connect to this HTPC's PCIe
• a be quiet L7 300W PSU (http://www.bequiet.com/en/powersupply/46) which powers on at the same time of the HTPC's PSU, which has two 12V rails rated max 18A (combined total of 252W max)

This GPU has an 8pin connector for extra power and provides a splitter which gives 2x6pin ends; I am planning to  connect both rails of the external 300W PSU to these 2x6pin ends by using the 8pin EPS header (2x4pin actually, that is intended for CPU) for one end and two molex for the other end; this is under the assumption that they are internally on the two 12V rails.
Adapters:

• http://www.amazon.com/Express-adapter-inches-sleeved-Angeles/dp/B005H3KH9E for the 2x4pin CPU wires
  
• http://www.pchelpforum.com/attachment.php?attachmentid=60013&d=1451483197 for the 2 molex 5.25"
  

These two would pick from the 300W's 2 rails and collect (via this adapter: http://en.community.dell.com/resized-image/__size/550x0/__key/communityserver-discussions-components-files/3746/adapter.jpg ) into the GPU 8pin power input.
My reasoning is that the GPU can only draw 75W from the small PSU (via PCIe) and the rest will come from the external PSU, thus splitting through the two rails will also make the intensity lower on each of the two ends and possibly lead to a "safer"/more reliable setup. I have also checked the adapters' wires, and they should be able to handle up to 10A.

From what I am reading this setup should be doable, however people is scaring me off doing this; it would be great if I could understand why it's not possible/advisable.

Please help :) as I am a newbie of these things I might have some huge mistake in thinking going on here...

Um, the problem is the pci-e lane you will plug the GPU typically try to draw up to 70 watts from it and that need to be powered by the motherboard. If your PSU is rated 70W and can actually do 70W, you're already using most of it with the CPU, fan and other minor components.

If you want to do this i think you would need to replace the motherboard's PSU.

The motherboard's PSU is rated 150W; the 3.3V and 5V can in combination draw up to 65W (second manual), thus the amount available to the 12V line should be 85W max (still at 10A max).

Edit: so you think the GPU will just not work correctly if it can't draw those 70W/75W from the tiny PSU available in there? I mean, it will not use the external PSU for extra power in such case?

I'll re-iterate. For most GPU, it will try to draw up to 70w from the motherboard. The PCI-e pick up the rest of the consumption which can be up to 300W for some high end amd card, can go up to 575W i believe.

If the motherboard's PSU can really do 150W, this could work, but i don't know if its designed for this, if it can handle the draw etc.

Thanks for your replies. It's not a problem of draw as the PSU allows max 10A and even if those all those 70/75W were available for the GPU, it would only draw at 6.25A. The real problem is probably that those 85W @ 12V are not fully available for the GPU...I think there might not be enough for 70W even.

For this card the extra load drawn from the 2x6pin is 225W according to this review: https://www.techpowerup.com/reviews/ASUS/R9_380X_Strix/21.html

The exact details for end watt consumption depend on what algo you are mining as well. If you want to mine Ethereum, its going to be a bit under 200W i think. But the GPU may still try to draw 70W from the mobo.

I think the GPU draw power first from the PCIE power connectors, if it is not enough, it will draw from the MB.

I think its crazy running a high power GPU with such low power PSUs.

Lately there are too many people who want to mine ETH but don't know exactly what they are doing.

Don't plug in power to the 2 PCIe connectors on the GPU from 2 different PSUs! Either use both 2 PCIe on the GPU from the external PSU or connect both 2 PCIe with the internal with some Molex adapters, however looking at the specs with what you have neither will work because the cables aren't there.

Go on Craigslist or Kijiji and buy a +600Watt PSU second hand for like $20 and do it right.

If you don't find a used one then buy a brand new one for like $35. Most 500-600Watt PSU have at least 2 PCIe connectors.

The secondary PSU should be just single rail. It is very difficult to distribute the power among several rails.