ATX 24-Pin 12V Power Connector Pinout (ATX v2.2)
Below is the complete pinout table for the standard ATX 24-pin 12V power supply connector as of Version 2.2 of the ATX Specification (PDF).
More Information on the ATX 24-Pin 12V PSU Connector
The power supply connector can only be plugged in while pointing a specific direction on the motherboard. If you look carefully at the following illustration, you can see the pins take a unique shape, one the motherboard matches in only one direction. The original ATX standard supported a 20-pin connector with a very similar pinout as the 24-pin connector but with pins 11, 12, 23, and 24 omitted. This means the newer 24-pin power supply is useful for motherboards that require more power, and therefore eliminates the need for ATX 12V power supplies to provide an auxiliary power cable (although some still may).
24-Pin & 20-Pin Compatibility
The additional four pins are usually detachable (see the lower part of the image above), allowing it to be used on a 20-pin motherboard connection. The extra pins simply hang over the motherboard connector—they don’t plug into another slot. Some motherboards allow the reverse: to use the older 20-pin power supply cable on a 24-pin motherboard connection.
