Question Random thoughts about using pigtail power connectors for GPUs ?

[sycoreaper]

To get it out of the way, I know using the pigtail 6+2 on PCIe for the other power port on a GPU is a no-no.

How much power does the main 6+2 provide and how much additional does the tethered 6+2 PCIe cables provide?

Reason I ask, I'm wondering what the actual use-case for said tethered plug is these days and why they are still on the cables in today's world.

 

[Aeacus]

Reason I ask, I'm wondering what the actual use-case for said tethered plug is these days and why they are still on the cables in today's world.

Here's why:

Some video cards have 6 Pin PCI Express power connectors and others have 8 Pin PCI Express power connectors. Many power supplies come with a 6+2 PCI Express power cable which is compatible with both kinds of video cards. The 6+2 PCI Express power cable is made up of two pieces: a 6 pin piece, and a 2 pin piece. If you put the two pieces together then you have a full 8 pin PCI Express power cable. But if you split the connector into two parts then you can plug the 6 pin part into the older 6 pin PCI Express connector and leave the 2 pin part unplugged. That way, your power supply only needs to have one 6+2 cable to be compatible with both 6 pin and 8 pin PCI Express connectors.

Compatibility is the reason of the pigtail.

In similar sense, this is also the reason why CPU +12VEPS cable is 4/4-pin and not full 8-pin (for the most of the times). But there are PSUs out there who's +12VEPS power cable is also 8-pin (solid connector, without being able to split it in two).

How much power does the main 6+2 provide and how much additional does the tethered 6+2 PCIe cables provide?

6-pin PCI-E power cable is able to provide 75W.
8-pin, including 6/2-pin (if pigtail is connected) PCI-E power cable, is able to provide 150W.

Pigtail contains two additional grounding wires, which in turn essentially double the Amperage rating of PCI-E power cable (from 2.083A to 4.167A) and thus, double the wattage capacity as well.

To add to that, +12VHPWR connector (in use with RTX 40-series GPUs);
2x 8-pin -> +12VHPWR = 300W (for powering RTX 4070)
3x 8-pin -> +12VHPWR = 450W (for powering RTX 4080)
4x 8-pin -> +12VHPWR = 600W (for powering RTX 4090)

 

[sycoreaper]

Here's why:

Compatibility is the reason of the pigtail.

In similar sense, this is also the reason why CPU +12VEPS cable is 4/4-pin and not full 8-pin (for the most of the times). But there are PSUs out there who's +12VEPS power cable is also 8-pin (solid connector, without being able to split it in two).


6-pin PCI-E power cable is able to provide 75W.
8-pin, including 6/2-pin (if pigtail is connected) PCI-E power cable, is able to provide 150W.

Pigtail contains two additional grounding wires, which in turn essentially double the Amperage rating of PCI-E power cable (from 2.083A to 4.167A) and thus, double the wattage capacity as well.

To add to that, +12VHPWR connector (in use with RTX 40-series GPUs);
2x 8-pin -> +12VHPWR = 300W (for powering RTX 4070)
3x 8-pin -> +12VHPWR = 450W (for powering RTX 4080)
4x 8-pin -> +12VHPWR = 600W (for powering RTX 4090)

So then I'm not understanding the restriction.

You're saying the pigtail doubles both the amps and wattage, ergo also voltage, (V x A =W) of the 6/2 PCIe connector. So why does everything I've seen online say not to use the PT?

 

[Aeacus]

So why does everything I've seen online say not to use the PT?

Barring the usage of pigtail is obsolete advice, especially when you are using modern, good quality PSU with proper (thick) wire gauge.

Back in the day (15 years ago or so), there were instances where when using pigtail, resulted in PCI-E power cables melting. This was due to the reason that the PSUs (especially cheaper ones), used thinner wire gauge, which made the PCI-E power cables to heat up under load. Even more so when using pigtail. Hence why the notion was born not to use pigtail.

Oh, in my previous reply i wrote the official wattage rating of PCI-E power connectors. Though, power cables can handle far more. If interested, here's small article that shows the maximum wattage of each cable;
link: https://www.gpuminingresources.com/p/psu-cables.html

 

[Viking2121]

The pig tail is perfectly fine to use, there were issues back in the old days, thinner wire, not as good ends, But thats changed, been fine to use for years now.

The adapters on the other hand like duel molex to 6 pin, seen some molex to 8 pins, or even dual 6 pin to 8 pin, they are not advisable to use, better off just getting a better PSU than taking the risks epically on a higher power draw card..

 

[sycoreaper]

Interesting, thanks all. Odd that so many diagrams even today say not to use the PT and only use a dedicated cable.

 

[Zerk2012]

Interesting, thanks all. Odd that so many diagrams even today say not to use the PT and only use a dedicated cable.

It depends on the power supply cable gauge and how long the cable is.
It should say in the PSU manual.
If I remember right my Seasonic said 225 watts ( might of been 250 been a long time)for each cable. So if the card needed a 8 and 6 pin and the TDP was less than 225 watts using the pigtail was fine.

 

[Teknoman2]

The real question is why they have 2 x 6+2 on each cable when its highly inadvisable to use a single cable if the card has 2x8 or 8+6 power plugs?

 

[Aeacus]

The real question is why they have 2 x 6+2 on each cable when its highly inadvisable to use a single cable if the card has 2x8 or 8+6 power plugs?

Remnant from the old SLI/Crossfire days.

 

[kerberos_20]

Here's why:

Compatibility is the reason of the pigtail.

In similar sense, this is also the reason why CPU +12VEPS cable is 4/4-pin and not full 8-pin (for the most of the times). But there are PSUs out there who's +12VEPS power cable is also 8-pin (solid connector, without being able to split it in two).


6-pin PCI-E power cable is able to provide 75W.
8-pin, including 6/2-pin (if pigtail is connected) PCI-E power cable, is able to provide 150W.

Pigtail contains two additional grounding wires, which in turn essentially double the Amperage rating of PCI-E power cable (from 2.083A to 4.167A) and thus, double the wattage capacity as well.

actually on 6+2 cable, that extra two pins doesnt give any additional power, its just sense pins (grounds), it just tells GPU that 6pin can provide 150 watts
pigtailed can provide 300watts...but why its not advised to use single pigtailed cable is due to transient spikes, which can power spike above cable rating, so use them just for aditional power
say you have GPU with 3x8pin, then use two separate 6+2 cables and one pigtailed

but say if you have GPU with 150W limit paired with 2x8pin, pigtail can be used aswell, as that second 8pin is just there for transients (unless you overclock)