Graphics PCIe Slot power?

I've been researching power requirements for a while now so I know how much headroom my PSU has on each rail but i'm a little confused as to the power a PCIe slot can provide.

I noticed on the Intel site that for X58 motherboards they rate each PCIe 2.0 slot as x16 and then in brackets x16 electrical.

Does this mean that if i'm running 2 PCIe slots at 8x (on a P55 board) that they will only supply 8x electrical?

I've also read here (near the bottom) that PCIe 1.1 was able to power up to 75w through the slot and that 2.0 can now supply 300w when properly implemented on the board. but I can't seem to find it anywhere else and am unsure if they got their info confused with the amount in total including PCIe power connectors.

Does anyone know for sure?

 

Ye the PCI-e slot itself can supply 75W by spec followed by 75W for a 6-pin PCI-e and 150W for an 8-pin giving you 300W which graphics cards are required to stay under (although some have been known to breach this which is fine if your PSU is up to scratch).

Someone else on the forums was saying recently though that 6-pin connectors are just as capable of 150W as 8-pin connectors are, the extra 2-pins are simply there to fool the GPU that it has all the connectors in place however it's not far off a 6-pin by circuit layout anyway.

 

Thanks. The extra 2 pins are there to make sure that the wires can handle the extra current without getting too hot, which is why it's much safer to use a proper 8 pin plug and not an adapted 6 pin.

Does anyone know whether the electrical power is affected if you run a PCIe slot at a fraction of it's maximum.

I would presume that if you taped up half a 16x card then only half the power would be able to reach it as each group of PCIe 1x pins provides enough power for each lane. But how would a P55 board handle this electronically?

The reason i'm interested is if it did only supply half the power I'd have a lot more headroom on my 1st 12v rail. By my calculations i'm already close to 20A out of 22A, but if the PCIe requirements were cut in half and passed on to the 3rd and 4th rails then i'd have an extra 6.25A to play with

 

Actually just read here that the 4x12v and 4x 3.3v are on a seperate part of the pin configuration to the lanes themselves.

Edit: Ok, just read that the 2 12v pins on the PSU connector, according to the ATX 2.2 (can't find one for 2.3) specification in part 4.5.1 of this document should use 16 AWG wire, which depending on where you read would vary from being able to handle 88.8w to 127.2w between them: (3.7*12 or up to 5.3*12). Both falling short of the 150w that 2x PCIe slots might require, not including any other 12v requirements the motherboard may have.

Edit again: I'd assumed that the power transmission figure was the one to go by but it seems that chassis wiring is for computers so according to the 2.3 ATX spec (found it at last) which suggests 18awg wires for everything except the 3.3v and pin 13. So it would cope with 2x 16A through the 24 pin connector without overheating.

 

Just found this thread, very interesting reading...

http://www.overclock.net/power-supplies/538652-shinji2ks-power-consumption-tests-updated-12-a.html

It seems that the GTS250 only draws 36w maximum through the PCIe slot the rest is through the plugs.

The 4870 on the other hand draws a lot more when not limited by the drivers.

Would still like to know if there is a reduction in power available through a PCIe 2.0 slot of it is set to run at 8x

I know that PCIE 1 running only 1 lane would provide 25w max, and running all 16 lanes provides up to 75w.

 

For anyone interested, I found this article by a french website that tested many graphics cards with a piece of kit that measured the power draw from both the slot and plugs, and has compiled a graph for furmark that shows how much is drawn just through the PCIe slot:

http://www.hardware.fr/articles/781-6/dossier-vraie-consommation-73-cartes-graphiques.html

It shows that some cards actually draw more than 75w through the slot and the translated text says it can vary depending on manufacturer.

"During our test, we also came across a faulty map a priori, a GeForce GTS 250 1 GB PNY home that had an overall consumption of 211.3 W at 130.9 FurMark with the single port PCI-Express (the second card, ASUS origin, showing a total consumption of 162.2W classic)! Dans le même genre, nous avons également eu une GT 220 DDR3 Club3D consommant 68.4w, contre 41.4w normalement … In the same vein, we also had a GT 220 DDR3 Club3D consuming 68.4w, 41.4w normally cons ..."

 

Only 3 reasons come to mind why they still have the external power connector...


1:They still have them in case some may attempt to overclock them,as it's not just high end video cards that get overclocked

2: lot of of these cards will end up in mid range and lower end Dell and HP systems,wich aren't exactly known for having good power supplies in them.

3:This one is kinda of a wild guess,but would be fun to see if the PCB on these new cards is the same,compared to any of the older generation cards of both GPU makers,as it isn't impossible that the Same PCB gets re-used with those new GPU's,in order to save development costs,and the external power connector just carries over as a consequence,even if it isn't needed.
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Which cards in particular are you talking about regarding points 1 and 2?

Judging from the French power consumption charts with the exceptions of the 9800gtx+ and GTS250, the 8800gt and 9800gt, 8800GS and 9600GSO (which were all basically rebadge jobs) GTS8800 320 and 640 and a few others (If they were using reference PCB designs when testing), the PCB's must be redesigned specifically for each model.

When a graphics card is designed to draw power from more than one source, the designers must assume that they will be coming from different rails and isolate each power circuit from each source (Slot, and any power connectors) otherwise if they were using a PSU which genuinely did have multiple rails (instead of what happens in most cases where one rail is divided and limited) then the varying fluctuations in the different currents would damage the hardware. So each power source has to be responsible for a certain part of the card.

As we can see in the tests, the PCIe slot draw is very different for similar cards, for instance the GTX 280 draws 39.5 from the slot whereas the 275 which is basically a cut down version draws 54.2 doing the same task. If the tests were all done thoroughly (and no-one assumed that just because of a rebadge that it would be exactly the same) then the identical results for the cards mentioned above point towards the same basic PCB. There is of course room for the manufacturers to design their own PCB's which would account for the PNY and Asus power draw discrepancies. (I'd be worried if I had a GTS250 that drew as much as the PNY)

Another thing to note is that more often than not, when 2 cards with the same GPU were measured with different memory amounts, the card with the larger memory draws less power than the smaller one, at least from the 12v rails (the first page in the article says that they could only measure the 12v or the 3.3v from the slots, not both). So either the 3.3v compensates for the extra memory or the boards with the larger memory have more efficient components, or both.

I'm actually quite happy that my cards draw significantly less than 75w each from the PCIe slots as it balances out the 12v loads quite nicely. I was worried for a while that all the components that relied on the 1st rail were getting close to the 22A limit. It seemed silly that the 3rd and 4th rails (both 25A) were only supplying 8A each.

 

For PCI-E all the power provided through the slot is from the first little bit at the front (near the display outputs) of the card. The 16x slot with x8 or x16 electrical is referring to how many PCI-E lanes are actually connected, these are what provide the actual bandwidth to the card. They don't provide the power to the card, hence why a any PCI-E card can run in any slot.

I have my PCI-E x16 graphics card in a x4 open ended slot so I can put a RAID card in the full x16 slot (PCI-E x8 card) I don't need the full bandwidth for the GPU since it is only a server.

 

I was a little confused as to why Intel would specify 16x data and 16x electrical but it does seem as long as it isn't a dedicated 1x slot then it will power up to 75w. thanks

 

So let me understand this, your worried that your first rail on your PSU will become overloaded? What do you have plugged into it? My Revolution 85+ supplied 30A per rail, so I could power a whole low power rig from each rail... (it supplies 840W from just the 12V rails!!!)

 

Motherboard :: 30w? :: 2.5A? (guestimate, overclocked MSI P55 with power saving features and turbo on, overclocked to 180 bclk)
Memory :: 6w :: 0.5A
Fans :: 1.03w :: 0.09A (These are all figures from the Manufacturers websites)
Disk1 :: 7.2w :: 0.60A
Disk 2 :: 8.3w :: 0.69A
Optical :: 15.6w :: 1.30A
X-FI Gamer:: 11w :: 0.92A
Floppy :: 5w :: 0.42A (yes I still need a floppy)

Total so far: 84w :: 7.01A

If I was to add 2x 75w (2x 6.5A) into the mix for my SLI setup, then the 19.5A total wouldn't leave much headroom for upgrading. What if I wanted to use the firewire slot? they can power up to 50w.

Also because a lot of devices use more power in the initial surge of starting up (disks use 2A to spin up) I might find I couldn't even power on any more.

(edit: EPS 2.91 requires that each 12v rail can handle an extra 2A of peak power for a minimum of 12 seconds, but the requirement for all 12v rails together is 6A)

Now I know I have a little headroom to overclock the motherboard a little more and hopefully reach 4GHz one day.

 

Erm... This is assuming the the 24 pin and 8 pin connectors run off the same rail... How many rails does your PSU have? 3?

 

4 rails, I'm not including the 2nd rail which is dedicated to the CPU.

The PCIe slots will be powered via the motherboard from the 12v pins on the 24pin connector. This is one of the main reasons why they added an extra 4 pins (with an extra 12v and 3.3v), with 20 pins they couldn't get enough power to the PCIe slots.

 

I'm sure some of the 8pin connectors power must go towards the GPU's though, as 12 * 22 is 264W, which is insane for any CPU...

 

The 8 pin CPU is normally one rail, the 24 pin is negligible really and the PCI-E connectors usually have their own rails. Check your PSU documentation and it should tell you which connectors are on which rails. With 4 rails you should be fine with any combination as the manufacturers usually spread them out. If it has multiple PCI-E connectors then it will be fine as the power should be spread in a way to support SLI/Crossfire systems.

EDIT: and you can't really damage your system too easily by overstressing a rail, since it just wont supply the power and you will probably get crashes when you fully load the system. If this happens just try switching connectors around and use different ones (so as to get a different rail.)

 

I think he's more worried that too much is getting pulled on his first rail as his motherboard and all other devices apart from CPU draw power from it, apart from the GPU PCI-E power cables of course.

Edit: If it's a good PSU it'll just shut down. I know mines good to 940W for peak usage, but it'd auto shutdown around there anyway to stop damage happening...

 

I asked MSI and they said that only the CPU draws from the 8 pin connector (2nd rail), maybe having 22a would allow for dual chip setups?

The 3rd and 4th rail (25A each) supply the graphics connectors but in my case they only need to supply around 126w (10.5A) each.

 

I'd rather it didn't get to the point that after investing in some hardware, I plug it in and find my PC shuts down or crashes.

Unfortunately the connectors on the PSU itself are different sizes, the first rail is 6 pins, the 4th is 8.

 

On dual socket systems you tend to see 2 of them. It is curious I'll give you that. Using the 8pin connector is the only way I can see for it to work really, otherwise how would you be able to run 4 cards in SLI/Crossfire...? Otherwise just those cards would run at 25A alone and even on the bigger brother to my PSU will only(!) supply 30A per rail...