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PCI-E 3.0 Slot Power

post #1 of 3
Thread Starter 
This is in the place that makes the most sense for it to be in, feel free to move it though.

I have Google'd around a bit, and I can not seem to find the maximum amount of power a PCI-E 3.0 slot can provide, a lot of sites say 300W.. Which is just 75W (PCI-E 2.0 power, maybe 3.0?), 75 (a 6x pin) and 150 (an 8 pin)

So for reference:
  • PCI-E 1.0/2.0/2.1 can do up to 75W through the PCI-E slot itself.
  • 6x Pins can do a theoretical max of 75W.
  • 8x Pins can do a theoretical max of 150W. - I also do not quite get how an extra two pins can attain double the power, well I assume two of the cables on a 6x pin are ground. Which makes each of the four "pins" left to do ~20W each. (18.75W).

Also interested in how my 7950's can run at above the maximum (Assuming 8x PCI-E 3.0 is 75W).
Two Six-Pins, and the PCI-E power total 225W, whereas a 220W 7950 BIOS throttle my cards at stock so I use a custom one that does 286W..

TL;DR What is the maximum amount of power a PCI-E 3.0 slot can deliver?, How does a 6x Pin deliver only 75W, while a 8x Pin can do 150W (Theoretically).
 
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Seven Seven Five
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post #2 of 3
I could be wrong, but I don't think any of these devices are doing current or power sensing to enforce these "limits" at all. Some graphics cards can pull over listed TDP and past these kinds of 75+75+75 calculations and so on—e.g. GTX 470 before power limiting kicks in can draw over 225W easy even without an overclock, and it gets dual 6-pin and the slot only. After overclocking, a bunch of people are drawing over those figures.

In fact, as I recall, the numbers are just in the specifications; Nvidia and AMD and non-graphics vendors specify and make add-in cards that usually are compliant with these specs / guidelines.

So the limits are a matter of policy and not physical (electrical) capability. Use thicker wires or PCB traces, and they can support more current before melting stuff and/or too much voltage drop, as a result of the greater cross-sectional area and thus lower resistance. High-end motherboards for 3+ graphics cards use often have those supplementary power connectors to the mobo for physical reasons. As for the policy, it sort of helps ensure that you're using power supplies of a certain standard that can hopefully handle the power draw... if the manufacturer is honest, which they often aren't. That's most of the deal behind the 6-pin vs. the 8-pin connector. Though IIRC there might have been something about sensing planned with the 8-pin, though nobody actually does it?

As far as I know, PCIe power slot spec is unchanged with 3.0. Hence the policy should still be 75W. The number of data lanes being used is irrelevant: the power provided via the slot on the +12V and +3.3V lines are on those pins to the left of the notch. To the right of the notch are the data lines. Those aren't for power delivery.

In reality, for cards with external power, you usually see that much more power is pulled from the PCIe power connectors to the PSU than through the slot (for example: click).

I didn't check the spec. You can check it if you want if you're interested in whatever rules or policy they have. As for practical usage, don't dwell on it, as it doesn't really mean anything.
post #3 of 3
Theoreticaly 300 watts.

From Bit-tech:
Quote:
However, PCI-E Express 3.0 provides no more power than its predecesors - but that's actually a good thing. Having >300W drawn from each 16xPCI-E slot would drive up the cost of motherboards, as the copper traces on the motherboard would need to be thicker. Manufacturers might even need to add extra layers to try and route these high power traces and their associated electromagnetic interference away from sensitive data traces.

PCI-SIG says 300 watts:
Quote:
Q: Does PCIe 3.0 enable greater power delivery to cards?
A: The PCIe Card Electromechanical (CEM) 3.0 specification consolidates all previous form factor power delivery specifications, including the 150W and the 300W specifications.

Looking at the way PCI-Sig worded it, the actual PCIE slot still only outputs 75 watts of power. Considering what Bit-tech said that you would need thicker copper traces it's not likely the slot itself will provide more than 75 watts of power.

Reading from the actual PCI-Sig documentation:
Quote:
A 300W add-in card can receive power by the following
methods:
􀀹 75W from x16 PCIe® connector plus 150W from a 2x4 connector
plus 75W from a 2x3 connector.
􀀹 75W from x16 PCIe connector plus 75W from a first 2x3
connector, plus 75W from a second 2x3 connector, plus 75W
from a third 2x3 connector.
– Note that this is not the preferred approach.

That is how you would achieve 300w, but wouldn't it just be easier to get two 2x4 connectors for 150w each if you wanted to reach the 300w?
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