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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?

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).

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.

  • The connector and wire don't actually determine how much power is delivered to a device. It's the current capacity of the power supply (PSU) at the rated DC voltage that is the determining factor. When the connector or wire overheats and fails, then they affect the power delivery. Multiple wires & connectors are used to safely deliver the power within the rated capability of the connectors and wire. – sawdust Aug 15 at 6:35
  • @sawdust so in this example, the risk factor would be the wiring harness between the 8-pin connector and the two 6+2 pin connectors overheating? In theory it would be possible to power the whole card with a single 6+2 pin connector, provided the wires could handle the current? That's even more surprising, because that means the PSU maker provided me with a cable that is a potential fire/melting hazard? – WannabeCoder Aug 15 at 15:06
  • (1) Not really, assuming decent quality materials. (2) In theory yes, but that could violate the PCIe spec for power connections. (3) Not if the PSU has regulatory approvals, e.g. UL, CSA. The PCIe power connections have been derated (i.e. used to carry much less current than the connector manufacturer's and AWG ratings). – sawdust Aug 17 at 1:27
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My research so far has informed me that an 8 pin provides about 150W of power.

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).

In the same way the 6 pin connector is designed to safely carry the current for up to 75Watt.

Thus you can safely split on 8 pins connector as follows.

PSU with 300+ Watt -------- 8 pin ----> 6 pin.
                                  \---> 6 pin.

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).

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:

                   /------- 8 pin ---->            (max 150)
PSU with 300+ Watt -------- 8 pin ---->            (max 150)

To

                   /------- 8 pin ---->            (max 150)
PSU with 300+ Watt -------- 8 pin ----> 6 pin.     (max 75W)
                                  \---> 6 pin.     (max 75W)

and finally to

                   /------- 8 pin ---->            (max 150)
PSU with 300+ Watt -------- 8 pin ----> 6 pin --> 8 pin.   
                                  \---> 6 pin --> 8 pin. 

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.

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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  • I think the PSU will deliver as much wattage as is needed over even a single cable - the 12V rail that the 8-pins connect to can provide as much as 70A/850W. The risk isn't "insufficient power" the risk is your cables melting (fire hazard). The GPU is what determines the draw, so I suppose the crux of the matter is whether or not the GPU will try and draw up to its TDP (350W) even when only connected with 2x 8-pin (3x 6+2 pins). Based on my math, that's only about 6 amps per wire (18 gauge), which is plenty safe over that distance/voltage. – WannabeCoder Aug 15 at 21:12
  • Not sure about how much a PSU can deliver. If you have a 1000W unit with ahared rails, probably yes. If you choose economically and have a 450W fanless unit then probably not. (450 was plenty for I7-920, 6xDIMM, 4xHDD in RAID, HW RAID card, etc etc). But that was with a 250w Graphics card) – Hennes Aug 15 at 23:01
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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.

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.

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.

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.

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.

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).

But there are not 9, there are 6.

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.

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.

Notes:

{1} Some PSUs subdivide voltages into 1 or more rails that split the available total power (wattage) between them.

{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.

{3} Good power supplies have safety features to detect unsafe power variations. Not all do, however.

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