Can I use a PCI-e cable in the 8-pin motherboard plug?

Your graphics card power connector has 8-pins and may look the same as the plug for your motherboard, but you should never try to connect one to the other.

This is both because the connector is physically different, but also because the cables are wired differently and might damage your components.

It is luckily very easy to see what cable goes where since they are all marked on CORSAIR power supplies. As you can see in the above picture, the connectors are either labelled as PCIe or CPU. The PCIe cables go to your graphics card, and the cable marked CPU will mount in your motherboards 8-pin header. You are also able to split the CPU cable in two should your motherboard only have room for a 4-pin connector.