The 8 pin PCI Express and the 8 pin 12 volt connectors are polarized differently so you won’t be able to plug one kind of cable into the other kind of connector. That is, you won’t be able to plug the wrong kind of cable in unless you try really hard. Unfortunately, the Molex Mini-fit Jr. connectors used by both kinds of power cables can sometimes be forced into a differently-polarized connector if they only have a few pins and you push hard enough. If the cable won’t slide in easily then you’re probably trying to insert the wrong kind of cable. The 8 pin PCI Express connector does have a small plastic bridge which prevents it from being plugged into an EPS 8 pin 12 volt motherboard connector. You can see the bridge in the image above between the rightmost two pins in the top row of the connector. But there’s no such protection to prevent EPS 8 pin 12 volt cables from being plugged into an 8 pin PCI Express connector on a video card. That combination may fit if you shove hard enough. And if you plug in the wrong kind of cable then expect fireworks. Some of the grounds and 12 volts wires for an EPS 8 pin 12 volt are reversed compared to an 8 pin PCI Express. Fortunately, most 8 pin PCI Express connectors are labeled “PCI-E” so people won’t confuse them with EPS 8 pin 12 volt cables. If the connectors aren’t labeled then you can tell an 8 pin PCI Express power cable from an EPS 8 pin 12 volt cable by checking the color of the wires which plug into the clip side of the connector. On the EPS 8 pin cable, the yellow wires (the 12 volt wires) go into the clip side of the connector. On the 8 pin PCI Express cable, the wires on the clip side are all black (grounds). That’s the same as it is with the 6 Pin PCI Express power cable. Of course, none of this helps you if your cable uses the trendy all-the-same-color-wires design which is popular with high-fashion power supplies. In that case you’ll just have to be very careful or hope the connectors are labeled.