the PCI-e slot is said to be good for 75 Watts. Azurewave aw-nb037h driver for mac.
one six-pin cable is said to be good for 75 Watts.
an eight-pin cable driven by an 8-pin outlet on the mainboard is said to be good for 150 Watts.
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Simply splitting a six pin power outlet into an eight-pin adapter to the card may not provide enough power. This manifests itself with uncontrolled power-off when you start an intensive graphics process. (the power supply overloads and shut off to avoid damage).
Some cards requires so much power that they require their own power supply.