And if you all will look at the picture gary so kindly provided, you will notice that the "+2" portion has two black ground wires. The 8 pin connector, like the six pin connector has three yellow hot 12 volt wires.
I know what the manufacturers say, but can someone please explain how adding 2 ground wires can double the current capacity of a cable with the same number of hot wires?
It's not two ground wires. Anyone that thinks that it's two ground wires is mistaken.
Only one of the pins is a ground and the other one is the Sense1 pin.
When the Sense1 pin is jumpered/shorted to ground, on the graphics card, that tells the graphics card that an 8-pin PCI-E connector is connected and that it can draw up to 150 Watts through that PCI-E connector.
If the Sense1 pin is left open (i.e. unconnected/not shorted to ground) on the graphics card then it indicates to the graphics card that a 6-pin PCI-E connector is connected and that the graphics card is only allowed to draw up to 75 Watts from that PCI-E connector.