PCI-E Split Cable for 2-cable RTX 3080 - 24/7 use?

There are lot of threads going around the topic, but I cannot seem to find the exact answer.

 

Question in short:

 

Will split PCI-E cables (included in the PSUs) suffice for 24/7 use at almost max power of 300W (we do scientific calculations for off-hours), or do we "really need" to wire two cables from PSU to the cards?

 

 

Question in detail:

 

We have two systems with following Corsair PSU:s

 

• RM750x (2018) - 750W
  
• HX1000i - 1000W
  

 

We are going to upgrade both systems to RTX 3080 cards (models with 2 power sockets), likely with headroom for OC as much as they allow, so possibly reaching the max 350-375W power use. So we're looking to get around 280-300W through the PCI-E cabling (split or separated - the actual question in here).

 

The system with HX1000i is also running GTX 1070 as a secondary card that will be powered by single cable of its own, as it only has one 8-pin socket and it's taking at max 200W total.

 

 

We have the required original cables for both the PSUs to run two separate cables for the GPUs, but the cable management would be so much cleaner with just one cable (and also easier for the airflow).

 

I understand its supported to run the split cable on PSUs that only provide one available, but is it unnecessary and recommended to be avoided, when there's option to use separate cables.

It's always better to run separate cables because of the voltage drop at high loads, and associated heat when a cable is overloaded.

one 8 pin PCIE cable is usually rated at 150W, so you can imagine what could happen if you try passing twice that.

 

I don't know if it will be spectacular, with smoke and flappy wires, but you'll certainly see voltage drop at the card, more so as the wires warm up. I imagine you'd get crashes pretty quickly.

 

I can only talk of my personal experience running only one cable on a 2080 : when i finally installed the second cable, the voltages red by HWinfo on the card went up by 200mV under load, and we're talking 180 - 200W through PCIE. that's already a significant drop

It's always better to run separate cables because of the voltage drop at high loads, and associated heat when a cable is overloaded.

one 8 pin PCIE cable is usually rated at 150W, so you can imagine what could happen if you try passing twice that.

 

Based on my understanding, the original, PSU provided 1-in-to-2-out PCIe cables are rated at the 150W each (8 or 6+2); (= 300W combined), so they will work up to full PCIe spec.

 

I guess what I'm asking here is (if my understanding is correct), how much "buffer" there is in the cabling to maintain the sustained 300W (= 25A both splits) for continuous load.

 

I can only talk of my personal experience running only one cable on a 2080 : when i finally installed the second cable, the voltages red by HWinfo on the card went up by 200mV under load, and we're talking 180 - 200W through PCIE. that's already a significant drop

 

If you did this with the HX1200i that you have in profile, it then matches much of my use-case here. Because then the other variables are out; so even powerful enough PSU would benefit from dual cables.

Thank you for the clarification!

 

This means we can try out the cards with pig-tail (including the overnight longer term calculations), then analyze the stability/performance and compare it to the 2 cable solution.

Based on my understanding, the original, PSU provided 1-in-to-2-out PCIe cables are rated at the 150W each (8 or 6+2); (= 300W combined), so they will work up to full PCIe spec.

 

I've seen others mention this about the pigtails and it confuses me -- isn't it still essentially 300W flowing through the mere 8-wires that plug into the PSU, vs. the 16-wires by using separate cables?

actually it's 3 wires per connector that carry the +12V. the 5 others are ground.

So if you plug just one pigtail you will see lower voltages at the card under load since the cables will warm up.

May work stock, but OCed, it's a recipe for BSODs.

If your PSU has enough cables, definitely run two, even if it's less aesthetically pleasant. The cables will stay cold and you will have minimal voltage drop.

I have a bunch of single PCIe cables, this way I can handle any power pig cards

 

Corsair sells them cheap so I suggest a handful

Edited October 25, 2020 by Vegan

I've seen others mention this about the pigtails and it confuses me -- isn't it still essentially 300W flowing through the mere 8-wires that plug into the PSU, vs. the 16-wires by using separate cables?

 

EDIT: Lots of apparently unverifiable "nice looking numbers" - edited away. In short, my understanding is, that its safe to use pigtail; its designed to deliver both ends at the standard (150W max at both totaling 300W). Johnnyguru has better and shorter answer below.

 

 

So eventually it boils down to your preferences. Electric-wise individual cables are better, for other reasons, they're not necessarily.

 

In my use case, I have little interest to "overwire just to be sure", because of cable clutter and extra flat cabling potentially intervening with the airflow of already tight-spaced dual GPU setup. I will try first to cope with one cable to 2x PCIe 3080 GPU and reconsider if I bump into any issues.

Edited October 25, 2020 by Kallex
Apparently unverifiable facts presented as facts

Thanks for correcting me. Actually I've continued Googling about the subject and apparently I have no fact basis on the ratings or the max currents.

 

I'll edit my post, I can't verify the data - its all fuzzy and all. But I think its still technically safe use the provided cables and they can withstand the max power - just not necessarily optimally.

 

EDIT: As you state 16g to first connector explains a lot of it.

Edited October 25, 2020 by Kallex

Yeah apparently, I started to be "a bit too sure" on a matter, that I had no real expertise on.

 

Based on common information available, its seems to be safe to connect those 2xPCIe on pigtail, but just suboptimal. Some vendors do recommend (as in here too) go with separate cables and only pigtail if needed (in 3x connector cards).

 

I need to measure the wattage (once I finally get the two 3080s) of the cards and plan accordingly; mentally preparing to wire the additional cables if needed.

Yeah apparently, I started to be "a bit too sure" on a matter, that I had no real expertise on.

 

Based on common information available, its seems to be safe to connect those 2xPCIe on pigtail, but just suboptimal. Some vendors do recommend (as in here too) go with separate cables and only pigtail if needed (in 3x connector cards).

 

I need to measure the wattage (once I finally get the two 3080s) of the cards and plan accordingly; mentally preparing to wire the additional cables if needed.

 

RTX 3080 cannot do SLI, you need RTX 3090 now for that.

RTX 3080 cannot do SLI, you need RTX 3090 now for that.

 

I'm not planning to SLI 3080s. I have two different Corsair-powered systems where they are going:

 

- Corsair RM750x with single RTX 3080

- Corsair HX1000i with RTX 3080 + GTX 1070

 

I am doing Machine Learning research + gaming combined, want to get experience on multi-GPU setups for just that. The multi-GPU setup is my main workstation, the other one is for more regular use (and for different person).

Edited October 26, 2020 by Kallex

Actually, Corsair PCIe cables are 16g to the first connector.

 

And the "weak point" is the pin, not the wire.

 

And the thing that "ratings" never really tell you is that the wire or connector, etc. doesn't maintain lateral voltage across it's "rating". As the current goes up, the voltage goes down. Even within the wire's "rating". So that begs the question of "what does the 'rating' even mean?"

 

(It's a rhetorical question. I know the answer)

 

EDIT: Lots of apparently unverifiable "nice looking numbers" - edited away. In short, my understanding is, that its safe to use pigtail; its designed to deliver both ends at the standard (150W max at both totaling 300W). Johnnyguru has better and shorter answer below.

 

 

So eventually it boils down to your preferences. Electric-wise individual cables are better, for other reasons, they're not necessarily.

 

In my use case, I have little interest to "overwire just to be sure", because of cable clutter and extra flat cabling potentially intervening with the airflow of already tight-spaced dual GPU setup. I will try first to cope with one cable to 2x PCIe 3080 GPU and reconsider if I bump into any issues.

 

I'm still confused on this. Theoretically (and just out of curiosity), how can pigtail cables withstand the same amount of current when they have half as many wires connected at the source (PSU)? Wouldn't that still put too much current on the pins/wires at the source, whether or not there's a pigtail "diffusing" things downstream? Again, just curious.

I'm still confused on this. Theoretically (and just out of curiosity), how can pigtail cables withstand the same amount of current when they have half as many wires connected at the source (PSU)? Wouldn't that still put too much current on the pins/wires at the source, whether or not there's a pigtail "diffusing" things downstream? Again, just curious.

 

I think your question is proper, but you're mixing two things.

 

1. Wire thickness/pin thickness plays a role here.

 

It can be also thicker up to before pigtail and then thinner for example. As mentioned here, Corsair uses thicker cable up before pigtail is split, so it can handle both the "split" parts power/current.

 

2. Twice the amount of wires will of course double the capacity

 

That doesn't mean that the "undoubled capacity" can still be more than enough.

I think your question is proper, but you're mixing two things.

 

1. Wire thickness/pin thickness plays a role here.

 

It can be also thicker up to before pigtail and then thinner for example. As mentioned here, Corsair uses thicker cable up before pigtail is split, so it can handle both the "split" parts power/current.

 

2. Twice the amount of wires will of course double the capacity

 

That doesn't mean that the "undoubled capacity" can still be more than enough.

 

Ah, that makes sense about the thicker wires preceding the pigtail split. What about the pins at the PSU side, are those heavier duty too?

 

Again, I'm just asking this for understanding, not debating anything.

Ah, that makes sense about the thicker wires preceding the pigtail split. What about the pins at the PSU side, are those heavier duty too?

 

Again, I'm just asking this for understanding, not debating anything.

 

Oh I didn't think you were debating :-). This whole thing is confusing, I've spent a lot of time trying to figure out the bits and pieces.

 

We are running one cable (pigtailed) peaking to 320-340W power use for RTX 3080 during gaming, no stability or any issues so far.

 

We don't do 24/7 for 300W right now, but not due to doubts for cabling, just lowering the power use down to 240W as the "last 30% of power results just 10% performance". We have run overnight 320W total load occasionally, with no issues either.

 

Our systems have good airflow, in case it matters. Both systems are powered by Corsair; RM750x (Ryzen 7 2700X) and HX1000i (Ryzen 9 3950X).

 

Not a single crash or game crash on either systems since 1-2 weeks of normal Folding@Home and gaming.

I have a 2080Ti and a Corsair HX1000i. I've always had a single cable with two plugs. Everything was fine - just turning off the MK11 in one particular situation (I did not suspect that the peak of power). I was sure it was a game bug. Other demanding games work fine. Today I found other forum threads that suggest the RTX2080Ti requires two cables. So I added a second. And you know what? MK11 works fine now. The 3080 sucks the same power as the 2080Ti - so two cables are better (in fact it's the only reasonable solution). It probably depends on the power supply, but the HX1000i likes a double cable.

I have a 2080Ti and a Corsair HX1000i. I've always had a single cable with two plugs. Everything was fine - just turning off the MK11 in one particular situation (I did not suspect that the peak of power). I was sure it was a game bug. Other demanding games work fine. Today I found other forum threads that suggest the RTX2080Ti requires two cables. So I added a second. And you know what? MK11 works fine now. The 3080 sucks the same power as the 2080Ti - so two cables are better (in fact it's the only reasonable solution). It probably depends on the power supply, but the HX1000i likes a double cable.

 

Thanks for the heads up! We actually have crashing issues with Doom Eternal on one of the systems - the 750W powered one (other one hasn't played enough).

 

I might try out the double cabling to see if it resolves (if the game gets played still).

if games are crashing it may be time to reset windows

Added this friendly note - I try not to have toned response, but I'm confused Corsair vs. Corsair contradicting responses. This latest response so far stands out as seemingly false. I love the products, love the quality of support, now help me sort out which response is right and which one is wrong:

 

 

These cards can easily pull over 300 watts from the PSU with some cards shipping with a default power draw of 380watts. The PCIE slot is spec'd to supply 75watts and a single cable from the PSU (even with pigtail connections) is only rated at 150watts. You should be using a single cable for each connector on the GPU. So if your GPU has 2 PCIE power connectors you should be using 2 separate cables from the PSU. If it has 3, you should use 3.

 

Now that's a bit bold counter-statement late to the party, when there has been statements elsewhere (including this thread, by Corsair personnel) that pigtail is specced for 300W. So I find the pigtail capacity listed in the quote contradicting on pre-research I did and even the answers within this thread alone. I also personally am not aware of any RTX 3080 cards above 350W TDP coming with only 2x connectors, but I understand that is out-of-spec situation if those go to 380W. Our cards are 340W TDP cards (Asus TUF Gaming RTX 3080 OC - with default manufacturer limits).

 

Other thread(s) mentioning the 300W limit (again by Corsair employee):

https://forum.corsair.com/Forums/showthread.php?t=157109

 

Our use case isn't anymore 24/7, but nonetheless - I really would like some official clarification of the "misuse of product" part below.

 

Using a single cable with the pigtails you risk overdrawing from the single connector and then melting and burning up the connector and eventually the PSU. BTW this sort of damage isn't covered by the warranty since it is technically misuse of the product as you installed the GPU incorrectly. All manufacturers of current GPUs are recommending one cable per connector due to the risk of these exact issues and to ensure the GPU has the power required to operate.

 

I just did as thorough search as I could from Asus TUF Gaming RTX 3080 OC edition. Nowhere I found the recommendations (even less requirements) for cabling. Only recommendation is 850W PSU.

 

So, if you're effectively saying, that our (I consider high end) Corsair HX1000i or our RM750x are not covered with warranty, could you please pinpoint the exact documentation for that. We only use the cabling provided with the product, verified to the spec in this thread and the other one linked.

 

I get the part, that for voltage stabilization and OC capacity. But if you're right and suddenly your pigtail must not be used with Corsair PSUs, there must be official statement of it. Or some actual, specific links to sources, where I can verify the claims.

 

TLDR; I'm not arguing with you out of principle. So far I just have contradicting sources and none as frightening that "Frying up the PSU, not covered by warranty because of misuse".

Bottom line: The rating of pins, wires, etc. is a max rating before it gets so hot that things catch fire. If it's rated at 10A, that means it's only a matter of time that at 10.1A it gets so hot that connectors melt. It's not like you can go from 0 to 10A without consequences. :cool:

 

 

Long ago I did some testing of various wires with a adjustable power supply I built. What i found was generally as the current rose the insulating coating tends to release noticeable fragrances before the wire becomes incandescent. Plastic insulation tends to melt too adding to the stench.

 

Any point of resistance will likely show up on the IR camera fast. I checked my last build close to be sure I did not screw it up with cabling and even checked USB wires to be sure they were not hot with BCC 1.2 active. BCC 1.2 calls for 5A of current at the USB port which is an issue for PSUs which tend to lack adequate 5V power for use as a charger.

 

USB cards with a molex or SATA power connector are stupid. The old LM7805 can do the job from the 12V line way cheaper.

 

https://hardcoregames.video.blog/1972/10/17/lm78xx-regulators/

 

I need to do more work on that post from my old electrical days. Back then TTL was all that was available to make PONG etc.

Edited January 19, 2021 by Vegan