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May 13, 2013, 04:09:14 PM |
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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).
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