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[Solved] Would powering a Vega 56 (2x 8 pin required) through a single cable cause a power issue?


#1

Solution: The issue I was facing wasn’t a power issue. My Vega 56 not POSTing was due to BIOS auto settings (on both B450 and Z77) that only adversely affected Vega specifically. Manually disabling integrated graphics while in external GPU mode allowed to POST on B450. Had this running off the same lead with 150% power limit without issue (besides fan noise).

Original Post:

Basically, I got this new GPU (MSI Vega 56 Air Boost) and can’t get it to post on my computer. I want to confirm it’s not a power supply issue prior to RMA-ing it, but I don’t have a spare PSU nor do have the modular cables to confirm. I have a Rosewill Tachyon 650. The PCI-E lead I do have terminates in a dual 6+2 pin layout, which fits the 2x 8 pin required by the Vega GPU; the question is can it deliver through a single cable? The output, per Legit Reviews: [email protected], [email protected], [email protected], [email protected], [email protected]

I did check on two platforms: My original 2500k with ASRock Z77 Pro4-M - but would not POST: no POST beeps, no display signal. Bought a whole new platform (Ryzen 5 2400G, MSI B450M Gaming mobo) and still won’t POST, no beep, and no display signal, but the Debug LED indicates GPU issue. The system boots fine with integrated graphics and old R9 380X (single 8 pin).


#2

Just to make sure, the cable looks something like this?
image


#3

Yes, but times 2. My single pci-e cable terminates in 2x 6+2 pin.


#4

Don’t do this.

If your PSU doesn’t have enough (native) plugs don’t daisy-chain them, even if they’re on the PSU cable.

An 8-pin connector is specced at 375W going through it, that means 2 are specced to push 750W at peak (and usually they allow way above that), which means your PSU isn’t even specced to handle those 2 connectors.
Not to mention that your cables will get hot.

TL;DR: Get a proper PSU with 2 actual 8-Pin connectors.


#5

Actually this is an interesting question. I also have an MSI Air Boost Vega 56 with two eight pin connectors. But…the default power limit is 165w. Why does the card need two eight pin connectors? 165w less 75w from the PCIe bus means that the card should only need 90w directly from the PSU. IIRC, a six pin PCIe connector carries 75w and an eight pin carries 150w. So really, the MSI card should need at max an single eight pin (or dual six pin) config. Right?

Now I’ve flashed mine with a Vega 64 BIOS which drastically increases the power limit, so I really do need two eight pins. But with the default BIOS all that power seems unnecessary.


#6

The dual 6+2 is a native, non-modular cable directly from the PSU. That would be concerning if it were true, but the last I checked, 8-pin PCI Express auxiliary power is spec’d at 150W, so dual 8-pin is 300w, plus the bus itself supplies another 75W for a total draw of 375W peak. And given the general trend of more efficient GPUs, power consumption has actually gone down, especially in the high end. Vega 56 only targets 210W.

No adapters are being used. It is a single rail design capable of 54A output (or ~630W continuous power).
My last PSU (750w Rosewill Xtreme) was a quad-rail design, so there was some load balancing involved; I had to reference the cable with the rail as to not overload a rail. Since this is a single rail, I wanted to see if there was something a missed.


#7

PSU should be fine if it was working with your last system. Those cables with 2 6+2 on the end are fairly common and not a problem unless they are on a multi rail PSU and you end up overloading a rail. From what you have said it should be safe to say the graphics card is the problem.


#8

My PSU has very similar specs to yours: Single rail [email protected] delivered through a single cable. I’ve piped up to 275w (or maybe 200w depending on PCIe power draw) through that single cable without any issue, so I think it’s probably a GPU issue. If you want to be sure, you could try swapping out the PSU if you (or a friend) have a spare.


#9

Unfortunately, I don’t have a spare PSU. The good news is that I found that pack of cables for my PSU, and tried it with separate 6+2 cables with some interesting / mixed results: it booted! But only once, and once Win10 configured, it had me reboot, and it failed to boot ever since. Again, no POST, no beeps, no display signal. Just the debug LED stating “VGA”. Thankfully, since it was through Amazon, they’re shipping me another one.

Even more interesting news, I probably just upgraded my 2500k platform for nothing. Live and learn I s’pose.


#10

Not necessarily. I don’t have hard numbers to confirm, but I suspect a Sandy Bridge i5 would bottleneck a Vega 56, especially if you start overclocking it. The 8 threads in your Ryzen 5 will probably feed it better.


#11

2x 6+2 pin pci-e power connectors would be fine for the card.
It doesnt really matter in most cases.


#12

18 AWG cables can handle somewhere between 9 and 5A (depends on strands that make up the wire).
2 of the 8 pins are presence pins, so it is 6 pins carrying 3 circuits.
I would stay under the 225W per connector and 375W per cable load.

According to Toms Hardware, we are well within limits, even when overclocked.
image


#13

Vega 56 on a 650 watt PSU is fine, so long as the PSU can supply enough power over that cable from a single outlet on the PSU.

Vega 56 is only 220-250 watt TDP. There’s no way you’ll manage to draw over even 300 watts out of a Vega 56, not without it going nuclear, (or at least pissing you off with fan noise for basically zero gains) first.

Powering it via 2x 6+2 pins off a single cable should be FINE as long as the PSU issue i mention above is OK.

Put it this way. Real world i have a pair of Vega 64s powered off 2x cables. Each cable has 2x 6+2s on it and each Vega is fed from both cables, one to both rear 8 pins and one to both front 8 pins (tower case). Off an 850 watt PSU.

It is fine. I’m not overclocking because overclocking on vega is largely pointless.

Max system power draw i typically see from Corsair link is ~550-600 watts when both GPUs are heavily loaded on the balanced power profile.

And in any case… failure to post won’t be caused by this. Your Vega probably won’t even be drawing ANYTHING (most certainly nothing near even 200 watts) over the dual 8 pins whilst posting. So… i’d say your post problem is unrelated and you should RMA it.