The relevant rail for this question is the 12V rail, where by far most of the power is drawn from in a modern desktop. PSUs differ in their specifications, but in my example 850W PSU the 12V rail can output the full 850W**, further limited by a max amperage of ~70A. Basic electrical formulas indicate this makes sense: Watts = Volts x Amps. This tells us that the 12V rail - where the 8 pin connectors on the PSU connect to - can supply plenty of power, far more than the GPU in question needs.

On the surface, it certainly appears as though it would only provide 300W of power, but I can't understand why PSU makers would give you an 8-pin to 2x 6+2-pin connector if it meant that each of the 6+2 pins could only provide half of the power as a normal 8-pin (because, again, they're supposed to be functionally equivalent). I've been trying to understand PSU cable specifications but have been unable to find a definitive source.

This question can be broadly broken down into 3 parts: the power source (PSU), the power transfer (the wires connecting the PSU and the GPU), and the GPU itself.

NDFilterCanon

Sort of yes. The 8 pin connector can carry up to 150W of power from the source to the sink (read: source is PSU, and the sink is the graphics card).

First, the PSU. The ATX standard determines a lot of the numbers thrown around here: 6 pins being restricted to 75W, and 8 pins being restricted to 150W. These standards are reasonable, safe, and followed by virtually all consumer desktop hardware. However, they are only standards, and electricity listens to physics, not standards. All power in the computer comes filtered through the PSU, subdivided into different voltages for various pieces of hardware. Broadly speaking, these voltages are divided into "rails" - so everything of a single voltage (12V, 5V, 3.3V, etc.) comes out of the same power source*. Each of these rails is limited to a certain share of the total power.

A common cable included in modern modular PSU's is the 8-pin to 2x 6+2 pin. However, based on reading (such as PCI-e Power - Does 6 + 2 Equal 8?) an 8-pin is effectively the same as a 6+2 pin. If that's the case, then how can one 8-pin connector effectively supply two 6+2 pin connectors?

NDFilterhenry's

Editing to add the following info due to a very unhelpful bot based comment: It should not come as a surprise that due to law, PSU providers generally assert that the products they produce are safe to use. With regards to adding citations, unfortunately much of the information here is not citable. PSU manufacturers do not typically publish data such as the cable specifications, or even the pinouts (although these can be reverse engineered/measured easily enough).

I like diagrams better as I'm a visual learner. If this diagram is correct then there is nothing to be concerned about. It would basically mean you can run a RTX 3080Ti using one 8 pin to to dual 8 pin that is normally supplied with the PSU or use a splitter and it will still be more than enough to carry the wattage. I'm running four RTX 3070's GPU's exactly like this & drawing between 110 to 120 watts each card out of a Corsair HX 1200 watt PSU. I also have two RTX 3080's with exactly the same wiring configuration pulling up to 212 watts each out of a Corsair HX 1200 watt PSU, this is still below the limit, neither even get warm let alone hot to the point of melting. I've seen this question bandied around the internet in so many forums & personally I've come to the conclusion that it's been made way more confusing than it actually is. The diagram explains what the cables can carry.

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I did a significant amount of research on my own, as well as learning a lot about basic electrical concepts to figure out the answer to my question. First off, I am not an electrician and my answer here certainly doesn't meet any professional or legal standards.

NDFilterfor Mobile

{2} Realistically a PSU would shut itself down long before this, as the other rails are also drawing power and the maximum output of the entire power supply is also 850W.

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NDFilterSony

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Now those last two cable are pinned in such a way that the graphics card is told it is allowed to draw up to 150W. But the PSU mostly likely cannot deliver that so it will not work.

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Secondly, the extra 2 earths in the 6+2 pin exist to inform the client device that it's plugged into an 8-pin cable, and not a 6-pin cable. This lets the client device know it can draw 150W of power through the cable safely. Why is it safe? Well, the extra grounds have nothing to do with it. It's all about what the cable is rated to do: typically, a 6+2 pin cable will use thicker wires (or any other method to decrease the resistance) so that the cable can carry the extra current without overheating.

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Sure, 1 8 pin socket on a card can take 150w max from a cable, meanwhile that cable can can carry 280+ depending on cable guage and insulating rating and have multiple connectors on it to supply power to multiple connectors on the card.

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Isnt the pci slot drawing 75 watts though... so your mostly drawing 300 watts over 6 wires which puts you back at 4.2 amp per wire

Using three 8 pins connectors would signal the graphics card that it is allowed to draw up to 150W per connected cable. Now if the PSU handles this then you are golden, but most likely it only expects to deliver 300W on that 8 pins connector. so you end up starting with this:

And this leads to the misinfo about "8-pin to 2x6+2 pin". It's all about what the PSU can output from one of it's sockets and what the cable can carry safely. If the cable that came with the PSU terminates in 2x6+2 pin connectors, it seems likely that the PSU can output 300W from a socket, and the cable as a whole is rated to carry 300W. Using it shouldn't cause an issue. (caveat: unless you run into problems with multi-rail PSUs where there can be a benefit to distributing power load across multiple rails, although these days single-rail PSUs are quite common. And even if you do run into a multi-rail PSU problem, it should just trip the OCP circuit and turn the system off rather than be unsafe.)

This brings us to our hypothetical GPU - it asks for three 8 pin connectors. Assuming it is responsible (I have not tested it), it will only function if it it senses three 8 pin connectors plugged into it. At that point, it assumes it can safely draw all the power that the ATX spec says it can: 450W (ignoring the PCI slot itself). Importantly, it will evenly draw this power across all 9 (3 for each 8-pin) 12V wires connected to it. This is pretty straightforward and safe - but alas, we have our goofy 8 pin to 2x 6+2 pin cable. If you've been following along, you will realize that the GPU will still pull all of the power it needs across the wires - because it assumes everything else is following ATX spec. It detects 3 8-pins connected to it (6+2 pins, but again they're the same) and will try to pull the 350W it needs across the 9 12V wires. (Here we are using the max estimated TDP of the theoretical graphics card).

My research so far has informed me that an 8 pin provides about 150W of power. One rumor has it that the upcoming 3000-series cards from Nvidia might utilize 3x 8-pin connectors. In that scenario, can you safely use two 8-pin to 2x 6+2 pin cables to connect the GPU (note: this would leave one of the 6+2 pins unplugged). Ignoring the power from the PCI slot itself, this would either provide 300W of power (for the 2 8-pin connectors facing the PSU) or 450W of power (for the 3 6+2-pin connectors plugged into the card).

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I recommend reading https://en.wikipedia.org/wiki/PCI_Express which has some nice background information on how the connectors are keyed.

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Firstly, it's worth noting that the question about "8 pin to 2x6+2 pin" is misleading, because we're talking about a power supply cable. The end that plugs into the PSU is typically not going to be a PCIE 8-pin. There's a chance it has the same pin out, but most PSU cables will have a 'this end into PSU' and 'this end into PCIE socket' indicator, which indicates that there is indeed some difference between the two ends. Note that this is different to 12VHPWR connectors, which are bi-directional.

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So in summary: if the cable came with your PSU, you're probably fine to use it in any configuration. If it's an aftermarket cable, exercise caution.

Regardless of however the 8-pin to dual 6+2-pin cables split themselves, at one point during the transfer they are limited to a single 8-pin cable. The PSU doesn't care - the power is coming off the same 12V rail no matter how many cables it's traveling through. The GPU doesn't care - it's getting its power one way or another. But remember the detail about the wires catching fire? It's a risk, and one we need to be wary of. The safe ATX spec says we can do 450W across 9 wires: that's 50W per wire. More usefully, it's ~4.2 amps per wire (50W / 12V = 4.17A). Our scenario? 350W over 6 wires: ~4.9A per wire. So if we connected the 3 8-pin requirement over only 2 8-pin cables (2 8-pin to dual 6+2-pins), we are breaking the ATX spec.

NDFilterglasses

In that scenario, can you safely use two 8-pin to 2x 6+2 pin cables to connect the GPU (note: this would leave one of the 6+2 pins unplugged). Ignoring the power from the PCI slot itself, this would either provide 300W of power (for the 2 8-pin connectors facing the PSU) or 450W of power (for the 3 6+2-pin connectors plugged into the card).

The power must still be carried from the PSU to the GPU, and it's here where the ATX standards are the most restrictive, and for good reason: the PSU supplies power and the GPU will take as much power as it needs, but neither does so with much regard to what's between them. If the PSU can't supply enough power, it will shut itself off*. If the GPU can't get enough power, it will shut itself off. If the wires get too much power, they will melt - or worse, catch fire. Accordingly, the ATX standards set the safety margins (6 pins for 75W, 8 pins for 150W) far below the point where this will occur. But to further confound the issue, both 6 pin and 8 pin connectors have the same number of current-carrying wires in them: 3. While there's some debate as to the purposes of the extra pins, the 3 hot wires within both cables indicate that both can carry the same amount of current. This is why a 6+2 pin is effectively equivalent to an 8 pin cable - there's no increased electrical risk. At the end of the day, the only real difference between the 6 and 8 pin cables in general is to inform whatever is using them of how much current is intended to be provided. It does nothing to stop anything from drawing more.

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Where it gets to be a bad idea is aftermarket cables. If, for example, you used a PCIE 6-pin to 2x PCIE 8-pin (something I've seen advertised online), it's quite likely to end in tears as neither the PSU nor the PSU 6-pin cable are rated to carry 300W. It might work with adequate cooling, of course, but it's certainly not recommended.

Through online calculators and speaking with people more familiar with electricity than myself, this does not appear to be a fire hazard: the 18 gauge wiring that is typically the standard for these cables isn't a fire hazard until around 15-20A per wire (over a 1 to 2-foot run). There's lots of variables in this calculation though, so it's best to stick to the ATX standard and never run splitters like the 8-pin to dual 6+2 pin.

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