Feeding a GTX 970 properly

[Steven85]

Hi, I'm building an ITX machine which is quite small. In it I can only fit a GTX 970 card (that's the best I can get). Problem is that the PSU only has one PCI-E 8-pin cable. Now I could use that, and one two-molex -> 1x6-pin adapter to feed my GTX 970, but that would give me only 210w at best (PCI-E port + 1x6-pin + 2xmolex = 75w + 75w + 2x30w = 210w), since I wanna overclock it I would prefer to have the full 225w at my disposal.

Now I've measured that my build only needs 120w to operate at full throttle when using the onboard graphics card. That's 10A at most (in fact it's a bit less since I measure the output at the wall plug which always measures it more than what it really is, anyhow). The PSU is brand new and is rated for 34A @ 12v, continuous use. That is to say that it has 24Amps to spare, the GTX 970 (as it stands, I have unlocked it) can't use more than 19 Amps, so I am well within my PSU's limits when I add my GTX 970.

So my remaining problem (as I initially said) is the 1x8pin that I have instead of 2x6pins. Since -however- 1x8pin=150w, same as 2x6pins can't I use some sort of adapter (a Y cable?) to turn my 1x8pin into 2x6pins? I'm asking 'cause I read in some other threads that that's a bad tactic, but it is the only one I can currently use to get my GFX card to operate at 225w. Funny thing is that it can take the overclock temperature-wise, if only I could feed her with enough current I would be golden...

So what should I do, would such a cable/adapter help me? Or it would -somehow- still not feed my GPU with enough juice?

edit: What I have in mind is such a cable: http://www.ebay.com/itm/151388366997

 

[Quix]

I've never had any luck with those adapter cables. No matter what I do, they always seem to make the card fail to boot. I think the molex leads on most power supplies are not designed to put out > 50watts on molex leads. The 8-Pin to 6-Pin I don't have any experience with.

You can't just add up wattages like that, that's not how it works. There are a large number of different variables including how the rails are arranged if it's a multi-rail PSU and tolerances on particular runs. I'd try it and see, you're not likely to actually blow anything. If it doesn't work out you can source a more powerful mini-ITX power supply, unless it's a proprietary design and then it's either mod the crap out of something or get another case.

 

[Steven85]

I've never had any luck with those adapter cables. No matter what I do, they always seem to make the card fail to boot. I think the molex leads on most power supplies are not designed to put out > 50watts on molex leads. The 8-Pin to 6-Pin I don't have any experience with.

You can't just add up wattages like that, that's not how it works. There are a large number of different variables including how the rails are arranged if it's a multi-rail PSU and tolerances on particular runs. I'd try it and see, you're not likely to actually blow anything. If it doesn't work out you can source a more powerful mini-ITX power supply, unless it's a proprietary design and then it's either mod the crap out of something or get another case.

But if I connect my Asus Strix which only needs one single 8-pin adapter on my PSU it has no problem. Clearly the power supply is far from being the issue. I even had my Strix unlocked and overlocked way over, the PSU clearly is made to last such amperage.

My problem is clearly with the molex adapter, it outputs so little current that it is basically useless... that is why I wanna make use of the 8 pin adapter to its fullest potential. Does anyone had any experience with that?

PS: My PSU is not multi-rail. It's a single fat rail. I always buy single rails, easier to handle.

 

[LordEC911]

The 8pin to 2x6pin should work just fine. I wouldn't trust them to go over spec but I don't think you will have a problem with using them the way you described.

If you don't have your GTX970 yet, I think there is a compact one (m-ITX?) coming out that takes a single 8pin.
Edit- Gigabyte Mini-ITX GTX970

 

[Steven85]

The 8pin to 2x6pin should work just fine. I wouldn't trust them to go over spec but I don't think you will have a problem with using them the way you described.

If you don't have your GTX970 yet, I think there is a compact one (m-ITX?) coming out that takes a single 8pin.
Edit- Gigabyte Mini-ITX GTX970

I already have this: http://cdn.overclock.net/1/17/900x900px-LL-17c73841_gtx_970_pcb_34642.jpeg

Sadly it can only take 2x6 pins, when I bought it that gigabyte was not anounced, then again I think my cards cooling is a little better.

I have unlocked the card's power limit and it routinely hits 1450Mhz on the core and 7800Mhz on the ram department. It's not a Strix, or a MSI TwinFozr level of performance, but it comes close which is quite surprising given how small it is.

I routinely outperformed the GTX 980 setups with this little beast... that is of course when I have proper power.

So you say that I should go ahead and buy that adapter, right? Am I going to see a large difference from what the molex adapters gave me (pain and suffering )?

 

[EdZ]

1x8pin to 2x6pin will have zero issues. It is within power spec. The 6pin connectors themselves can take a lot more load than their rated 75W, so the only weak point for overclocking would be if the 8pin to 2x6pin adapter used very low gauge wiring. And eve then, the wiring run will be very short so the issue will be one of inefficiency rather than safety.

 

[Steven85]

1x8pin to 2x6pin will have zero issues. It is within power spec. The 6pin connectors themselves can take a lot more load than their rated 75W, so the only weak point for overclocking would be if the 8pin to 2x6pin adapter used very low gauge wiring. And eve then, the wiring run will be very short so the issue will be one of inefficiency rather than safety.

My card most often than not stays below the 200w mark, but it has some spikes to ~210 watts, it does not happen too often, but if the wiring is of low quality as you say would it lead to lower performance at those moment, or -worse- to a lockup as the card won't be fed with enough current for the clocks that I would be running?

 

[EdZ]

My card most often than not stays below the 200w mark, but it has some spikes to ~210 watts, it does not happen too often, but if the wiring is of low quality as you say would it lead to lower performance at those moment, or -worse- to a lockup as the card won't be fed with enough current for the clocks that I would be running?

If your card is 2x6pin, then the max specified draw would be 225w (2x75w from the 6pin PCI-e power connectors, and 75w from the PCI-E card edge connector).

The wiring would not affect performance or cause crashes. It would simply become warmer than it is rated for. Only in the very cheapest of PSUs (the kind with the instantaneous peak rated load displayed on the box rather than the sustained load) would you find cheap enough wiring for this to be dangerous. Do not use cheap PSUs with expensive GPUs and you'll be fine.
Your PSU will be the weakest link in power delivery, not the wiring or connectors.