Connecting 4 GPUs to 2 PCI-e on PSU

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wtfStorm

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Nov 14, 2014
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Been searching for few hours now and still haven't found a clear answer so decided to ask here.

The setup:

The question:
As can be seen from the PSU specs - they mention "PCIe Connector - 4". However, there are only two 8-pin slots. What's that about? Does it mean these two 8-pin slots can be used to power 4 GPUs that are to be connected with two 8-pin PCI-e splitted cables (that come with the PSU )

If the answer is "yes", then am I right in assumption (bear with my probably below average awareness level):
- 8-pin PCI-e cable provides 150W of power
- PCI-e slot (though it's used via riser, but the riser itself is 6-pin powered) provides additional 75W
Since 2 GPUs are connected to one PCI-e slot on PSU, we get:
150W / 2 + 75W = 150W power for each GPU
Is this correct? If so, what can happen to PSU/GPUs if both GPUs connected to same PCI-e slot on PSU consume more than 300W total?

Thanks.
 
Solution
Mark RM
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Thanks for the link. However I've already read few of such threads, including this one and none of them answer the questions I've asked in my original post.
By the way, my Z270-P mobo is mentioned as the first one in the thread you've linked :)
 
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I'm sorry, but you seem to completely miss the point. Have you even read my post? In the thread you've provided the guy uses 6 GPUs with 1000W PSU. I'm going to use 4 GPUs with 750W. I'd think I'd made a pretty detailed description. Thanks.
 
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First off, this arrangement is perfectly fine and you're making it confusing.

The PSU has two non removable non modular PCIe connectors and two modular ones you can attach... for four total. (that means 4x6+2)
Second, don't use SATA connectors to PCIe connections, they are low grade, low gauge wiring and prone to overheating and failure or fire.
If you need more PCIE connectors for direct connection to the cards risers, then feel free to split the six pin PCIe connectors from the PSU with Y cables
OR you can use PCIe six pin from molex adapters, but use no more than TWO of these per Molex wire that runs from the PSU

Four RX 580 class GPU's will run on that PSU just fine.

 
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@Mark RM, first off - thanks for the coherent and on point reply. However I have more questions.


Not exactly true: CX750M has 1 x 8-pin for CPU and 24-pin for mobo non-modular connectors. And, as already mentioned, 2 x 8-pin PCI-e modular connectors. So I still don't quite get why they mention "4 PCI-e" connectors.


I've tried to do this today, but the length of splitted 8-pin PCI-e Y-connector (the cables that come with the PSU) doesn't allow to connect both PCI-e riser and GPU itself. Moreover, if able to connect like this, we would get 150W per GPU total, not 150W (per 8-pin) + 6-pin<->sata/molex which is something around 40-60W if I understand the scheme right.


Ok, so will such 6-pin to molex adapters do the job? To be precise:
- I connect each PCI-e riser power with 6-pin to molex adapter
- because such adapter has 2 molex on the end, I connect both of them to molex cable from PSU (6 pin labeled "peripherals" on PSU)
- finally I'll connect no more than 2 x 6-pin->2-molex adapters per molex cable that runs from PSU: considering such PSU cable has exactly 4 molex.

Sorry if I'm being too rigorous, but I want to make sure I get this all just right and stable. Thanks.


 
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I have CX 750M. And yes, last pic was from cx650m (a mistake on my part), however it has exactly the same non-modular cables: 1 x 8-pin for CPU and 24-pin for mobo. So it doesn't really change the point, as in my questions :)
 
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Alright I think I'm clear, this version has two modular PCIe cables that are already daisy chained. Each cable has two PCIe connectors for a total count of four, the problem is they're too close close together to be useful in a four card arrangement.

The PSU comes with two molex chains with three adapters on each (instead of four unfortunately).

What I would do is use one 8 pin extension cable on each PCIe cable connected to the power supply enabling me to hook up all four of the top connectors on each card directly via the PCIe cables included. Each of those wires can feed two cards assuming (correctly I hope) you plan to under volt and power limit them via BIOS mod. So yes in theory you could potentially ask for a 300 watt draw with two eight pin cables coming off one line like this, but really it never works like that in mining.

Next I would hook up that six pin cable you showed me to the two molex adapters and further I'd split that six pin adapter you showed me with a purchased Y six pin cable and hook up two of the risers. each cable draw from the PSU would be 150w. Doing this to both included molex cables allows all risers to be powered.

In reality , I expect the system to actually draw 1) a full 150watts from each molex cable 2) around 8-130watts from each PCIe cable depending how you configure the cards. That's just how AMD cards work they don't seem to balance the draw out of the PCIe slot, they just suck back the 75 watts there and take whatever else they need from the eight pin cable on top (easy to measure with a riser in place).

This is why I don't like the SATA connectors, they are only rated for a small draw on 12 V. A single molex can deliver 132 w according the specs, SATA less than half that.

I hope that helps a little with those questions
 
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Solution
@Mark RM, thank you for such a detailed answer.


Actually the length of modular PCIe cable - 2nd branch of the daisy chained - is just enough to connect 2 GPUs which will be near each other, so I'm going to connect them in this manner.


So the connection will be like: PSU->6-pin molex cable->molex to 6-pin adapter->Y-splitter 6-pin adapter->2 powered risers.
However, as can be seen from molex to 6-pin adapter photo - there's only one 12V cable per each molex end (yellow). Are you sure these 2 molex are capable to provide 75W EACH? Moreover, how will it be possible to have 150W per 2 molex, since on the other end there's 6-pin connection to PSU? (I know I may be asking something silly, but I'm no electrician).

 
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yes that wire will carry 150W, it's the connectors we're most concerned with here, if you hook up a sata connector like this it will get HOT. If you hook up a molex it won't. IS this out of spec? Yes, but it's also standard practice for these small scale mining expeditions.

What makes this work well is quite simple... you have additional grounding via the 6 pin at the top of the card, the ground plane is common, it prevents your single 12v wire to the risers from getting over hot... this is the same method used to change a 6pin PCie 75 watt connector to a 150W eight connector, you add more grounding.

That's all I can add to this conversation.



 
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Thanks. What if I get 2 of 6-pin->molex cable with 4 molex connectors on it, buy 4 mentioned 6-pin->2 molex adapters, and connect each adapter to riser's 6-pin using single molex end (from 2 available), will it work? Thing is it's hard to acquire 6-pin Y-splitters in my area, and I'd like to avoid the hassle of ordering Y-splitters for couple of bucks via online store.
 
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No, the dual molex to PCie 6pin adapter is designed to fill the PCIe connections from both molex connections, using only one molex connection will leave the six pin PCIe connection non functional.

There is such a thing as single molex to PCIe 6pin adapter, but usually they use such thin gauge wiring it's not worth bothering with.
 
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I see. Does that mean I could connect 2 risers with 2 of such dual molex to 6pin using single 6-pin->4 molex PSU cable and be fine?
 
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