I'm powering a 980ti from both of the mini 6 pin power supplies, one of them has an 8 pin split. It's a flashed card for Mac Video Cards and I'm using the wires that came with it.
Everything has been working fine for the last month I've had it. And I've been doing crazy renders that rev my 12 cores up just about the entire last 4 weeks. Not a single issue or crash.
I'm not sure what a "2x mini 6 -> 8 pin + 2x SATA -> 6 pin" is. Is that what I've got?
No, what you are doing now is.
Cable A: mini 6pin -> 6pin
Cable B: mini 6pin -> 8pin
So, what actually happening is.... Your card draw
1) up to 75W from the slot, Nvidia usually make good use of the slot on high end card. So, let's say it draw 70W.
2) up to 75W from the 6pin. So, again, we assume another 70W from one of the mini 6pin.
3) all the rest from the 8pin. So, it may be up to 110W from the other mini 6pin.
Since no manufacture will intentionally make a card that draw more than 75W from the PCIe slot (well, apart from the RX480). So no matter the card is the base model or OC model, they are only designed to draw more (when required) from the 6pin / 8pin, but not from the slot.
The cMP's mini 6pin official limit still 75W, same as normal 6pin. However, it can actually deliver up to 120W before the cMP shut itself down. So, in your case, even thought the 8pin may constantly draw more then 100W, the cMP still working as usual.
MVC know this limit very well. May be he did fine tune the card and make sure it's power draw always stay within the real world limit. Or simply your 980Ti always pull less then 120W from the 8 pin even under 100% load.
However, the power draw vary between cards. If another brand's 980Ti (or any other 250W TDP card) draw power as follow.
1) 60W from the slot
2) 65W from the 6pin
3) 125W from the 8pin
It still stay within the 250W TDP limit, however, the cMP will shut itself down when the 8pin draw more then 120W.
This is why some 980Ti users reported that their cMP suddenly shutdown when stressing the GPU.
Since the 980 Ti is not designed for cMP, no manufacture will try to make the 8pin power draw stay within 120W. In fact, it make more sense to utilise the 8pin but not stressing the 6pin / slot to 100%.
Anyway, it's a little bit hit and miss. It's depends on if you get a good card, from which manufacture, and how's the firmware programmed to draw the power.
If you want to make sure everything stay within the official limit, and you don't want any extra PSU, or mod. You can...
Cable A: 2x SATA -> 6pin
Cable B: 2x mini 6pin -> 8pin
In this config, 2x SATA rated up to ~110W, and the 6pin should only draw up to 75W. 2x mini 6pin rated up to 150W, and exactly fit the 8pin power draw limit.
So, even though any 6+8pin graphic card really draw 75+75+150 = 300W. The cMP will still able to perform, and in fact, everything stay within official limit.
For anyone already fill up all HDD / optical bay. There is another method which to make sure won't over stress any mini 6pin to reach the 120W limit.
Cable A: 2x mini 6 pin -> 8pin
Cable B: 8pin (female) -> 2x (6+2)pin
So, we will plug Cable A into the cMP's logic board, and connect Cable A and B together, then plug one of the 6+2 into the 6pin, the other 6+2 into the 8pin.
In this case, the power draw will be something like
1) 70W from the slot
2) 110+70=180W from Cable A
Since Cable A is connected to both mini 6pin, therefore, each mini 6pin only need to deliver around 90W. A little bit above the official limit, but still well below the shut down protection limit.
Anyway, since your card works fine. So, obviously no need to do anything.
And OP has 2 HDD bay empty, he can choose to get a 2xSATA -> 6pin cable to power his 980 Ti.
