PCIe and Risers understanding Questions

[nookiegirl]

Hi there,

it seems that allmost everybody here always knows how many powered and how many unpowered extenders are required. Is there some kind of formula you guts use?

Or do the short slots always need powered risers?

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[ISAWHIM]

The PCIe bus can only "safely" supply about 75watts total.

Each card can pull up to 25watts... Through the PCIe rail. (Poorly designed ones will pull more)

They "pull" whatever you can not supply through the 6-8pin connections, through the PCIe-rail.

EG, if your 6-8pin connections can supply the full 250watts(20a@12v) needed for each running card, almost no power will be drawn from PCIe rails. (almost)

However, some 12v rails supply up to 25a, some only PEAK at 20a, thus, in constant operation, they will TRY to pull power missing, from the PCIe rail.

This is where it is an "issue"...

The pins have a better contact rating which can handle the 25w feeding to the card. The "riser cables" are cheap thin wire, and not sufficient for 25w to be delivered through the PCIe connection. Thus, the wires melt.

If you only have 3 cards, and a good 20a(250w) 6-8pin connection on them, you do NOT need to worry about PCIe bus supplemental power.

If you have only 18a(216watts) on your 12v rails, and the remaining 34watts of power has to come from the PCIe... (in a 250watt draw card)... Then you NEED powered risers.

If you are going to use more than 3 cards, you should have powered risers for more than two of them. (eg, 4 cards, 2 should have powered risers, or a powered-jumper in one slot.) {Again, if you are NOT using cheap china risers with thin wires, and you ARE able to supply the majority of power through the 6-8pin connections.}

Remember, we are running in "constant operation", which these cards and MoBo's were NOT designed to do... (Not the consumer stuff).

[ISAWHIM]

The best way to determine if you need "powered or not", is to simply do the math.

If your 12v rail supplies 80a (960w), and your PCIe can supply (75w), and you have four cards that pull a maximum of 250w (1000w total draw)...{always calculate for max}...

Then you can put 4 cards (1000w) without powered connections, because...
(960w + 75w = 1035w) Thus, you should have a surplus of 35 watts... or rather, only 40watts will be "pulled" from the PCIe, at max-load. (40w / 4 = 10w per PCIe slot, which the thin wires can handle.)

NOTE: I run 6x 7970's which pull only 230w max/constant. (Total cards: 1380watts + 45watts for the other stuff. {Mobo, SSD, Ram, CPU}) = 1425watts max, at the wall. (Thus, it is actually about 20% less than that, at the mobo and PSU, sue to inefficiency at 90% load on 120v power.)

My 12v rails supply 115a (1380w max/constant) + (75watts from PCIe potential) = 1455watts. Thus, I have a surplus of 30w, or I draw only 45watts from PCIe at max/constant, which is 7.5watts per slot. (Less, because 45 watts is going to the other stuff, but that ALL still goes through the 14-pin connector on the Mobo.)

[nookiegirl]

thanks a lot for the explanation!

[chungenhung]

And if you are overclocking... you are more likely to need powered risers.

[spinx]

im running 4x 7950 clocked core@1100 and memory@1600 without powered risers, working great so far... perhaps the motherboard will die eventually, time will tell