why are there multiple rails on x1950xtx

[xtera]

Does a single x1950xtx need multiple rails or do i use a dongle and connect
one 12v rail the 6pin pci express i see shows 2 12volts does 1950xtx use
both 12rails for crossfire or does it need that for a single card too>>>

 

[First of One]

Just so there are no misunderstandings, there are two 12 V *pins* in the
6-pin PCIe connector, not two *rails*. In the link I sent you previously, on
12 V, the X1950XTX is expected to draw about 6 A through the PCIe connector.
6 A is apparently excessive for a single pin to handle.

So, if your PSU doesn't have a 6-pin PCIe lead, then the 6-to-4-pin adapter
dongle should see both 4-pin leads plugged in, because each 4-pin lead
contains only one 12 V pin.

In Crossfire, each of the two cards has its own power connector, so you
would need to connect four 4-pin leads. Though at that point if your PSU
doesn't have "native" 6-pin connectors you should consider upgrading the
PSU.