PCIe 6-pin/8-pin Connector Question

[box o rocks]

I see that a 6-pin connector is rated for an additional 75W of power. And that a (6+2) 8-pin connector is rated for 150W of additional power. But the only extra wires on the 8-pin connector are 2 extra ground wires. Without adding yellow (+12v) wires, how does that increase the wattage and current carrying capacity of the 8-pin over the 6-pin?

Somewhere I think I read that the middle yellow wire was unused on the 6-pin connector, but is used when connected as a 6+2 pin connector. Is that the reason no extra yellow wires are included when used as an 8-pin?
Trying to make sense of this...

 

[nukemaster]

It just allows the video card to detect that it has an 8 pin connector(I have never tested to see if it even does).

With 3 wires holding the 150 watts or 12.5 amps is not an issue at all.

I do think it is strange they never added more 12 volts to it, but I guess the figured that the 6 pin was already good for it so just "detect" and cable/connector good for 150 watts

 

[Eximo]

A standard six pin connector actually only has 2 12V+ lines and 5 wires, the added yellow wire there doesn't have to be populated unless doing a 6+2 config.

One of those added wires is a ground, the other is actually a signal wire that carries no load. This informs the GPU that there is an 8-pin connector plugged in and it is safe to start.

Wire gauge is also a factor, but your standard 18 gauge PSU wires can carry about 60W per 12V wire without a significant increase in temperature. So a 6-pin is closer to 120W capacity and an 8-pin around 180W. They are underrated to keep temperature down and so that there is a little wiggle room for sudden spikes.

Corsair likes to use 16 gauge (bigger) wire and put two 6+2 connectors on a single Power Supply PCIe plug. Perfectly safe, but it is a very common question on here.

 

[box o rocks]

So, my assumption was correct? The average 6 pin connector (with 3 yellow wires) is basically an 8 pin in-waiting? If so...
...and if the wire size was at least 18 gauge, that would mean using a 6-pin to 8-pin adapter was perfectly safe. No?
(assuming the +12V rail it is connected to has the capacity, of course)

 

[nukemaster]

Yes it should.

I would never test it on the other power supplies I have with out 6+2's. 300 and 430 watt units.

 

[Math Geek]

would be able to in theory but in reality the psu itself is probably not capable of giving the extra power. that's where the whole theory breaks down.

the adapter does not generate any power so if the psu can't give it, then fooling it into thinking it has an 8 pin connection is not going to produce more power. the fact that the psu does not have an 8 pin and only a 6 pin tells me the maker of it knows that it can't give that extra 75w or it would have put the 6+2 on there from the start.

man i hate the dang adapters so much, gives people hope that their bottom of the barrel junk psu can somehow magically double it's power by simply adapting a connection. i'd personally would never use one nor advise that anyone else does either.

 

[nukemaster]

Truth be told it is only 12.5 amps.

Across 3 18 gauge wires this is nothing(even more so due to the short runs and the wires are not held together by another outer casing like an extension cord or anything.).

I know my poor old 300 watt only has 22 amps. I would NEVER try to run an 8 pin card unless my cpu was very low powered(and even then).

It is all about how much 12 volt current one has. You are 100% right that these adapters do not make power. If you happen to have an early pci-e generation unit with sufficient power, it should not be an issue.

 

[box o rocks]

@ Math Geek
Well, the thing is... I have this 525W Dell workstation PSU with 3 x 18 amp +12V rails rated for a max of 500W. It only has one 6-pin. The 6-pin PCIe connector has 3 purple/3 black wires. My GTX 960 needs an 8 pin connector to work. So I used the 6-pin to 8-pin adapter that came with the GTX 960.

To test it, I ran HWinfo64 while running Furmark. HWinfo showed the GTX 960 peaked at 154W @ 99% core load. I assume the PCIe connector is on one of the 18 amp +12V rails alone*. So, since 18A @ 12V = 216W, I am assuming that using the supplied adapter is safe. The wires never got above room temp to the touch.

*
+12Va = CPU?
+12Vb = PCIe?
+12Vc = drives & everything else?

 

[Math Geek]

the math works on that one. those are decent psu's made by delta. it'll probably be ok but not something i'd use as a long term solution.

 

[nukemaster]

The distribution of the 12 volt rails is generally marked with striped wires. So yellow, yellow + black , Yellow + green and so on.

It is also important to know that multirail power supplies almost always have a larger 12 volt rail feeding them. The rails are generally just current limiters for safety(just in case you want to stack adapters to run everything off one LP4(molex) connector ). At one point it was a part of the ATX spec if I remember right.

Since you said you have 500 watts, you have already seen this combined rating. You have about 41 amps for all 12 volt devices and as long as no rail goes over 18 and the combined does not go over 41.666 you are good(from a power standpoint). In general a quality power supply will simply shut down on overload(and a cheap one will make fireworks in your computer and take out other parts).

 

[box o rocks]

The only other heavy load on the +12V rail(s) is my W3670 Xeon CPU.
Thanks guys. I think I'm good now. Learned something in the process.

 

[nukemaster]

Good old 1366 cpus(it may be as heavy of a load as the video card under very heavy stress).

Glad you learned something. I did too

I did not know that pci-e power connectors only required 2 x 12 volt wires for 6 pin connectors(all my power supplies had 3 x 12 and 3 x ground).