Every PC power supply has connectors that attach to the motherboard, providing power to the motherboard, processor, memory, chipset, integrated components (such as video, LAN, universal serial bus [USB], and FireWire), and any cards plugged into bus slots. These connectors are important; not only are these the main conduit through which power flows to your system, but attaching these connectors improperly can have a devastating effect on your PC, including burning up both your power supply and motherboard. Just as with the mechanical shape of the power supply, these connectors are usually designed to conform to one of several industry-standard specifications, which dictate the types of connectors used as well as the pinouts of the individual wires and terminals. Unfortunately, just as with the mechanical form factors, some PC manufacturers use power supplies with custom connectors or, worse yet, use standard connector types but with modified (incompatible) pinouts (meaning the signals and voltages are rearranged from standard specifications). Plugging a power supply with an incompatible pinout into a motherboard that uses a standard pinout (or vice versa) usually results in the destruction of either the board or the power supply—or both.

AT/LPX Power Supply Connectors
Industry-standard PC, XT, AT, Baby-AT, and LPX motherboards use the same type of main power supply connectors. AT/LPX power supplies feature two main power connectors (P8 and P9), each with six pins that attach the power supply to the motherboard. The terminals used in these connectors are rated to handle up to five amps at up to 250 V (even though the maximum used in a PC is +12 V). These two connectors are shown in the following figure.

ATX and ATX12V Motherboard Power Connectors
Power supplies conforming to the original ATX and ATX12V 1.x form factor standards or variations thereof use the following three motherboard power connectors:

•20-pin main power connector
•Six-pin auxiliary power connector
•Four-pin +12 V power connector

20-Pin Main Power Connector
The 20-pin main power connector is standard for all power supplies conforming to the ATX and ATX12V 1.x power supply form factors and consists of a Molex Mini-Fit Jr. connector housing with female terminals. For reference, the connector is Molex part number 39-01-2200 (or equivalent), and the standard terminals are part number 5556 (see the following figure). This is a 20-pin keyed connector with pins configured as shown in the next table. The colors for the wires listed are those the ATX standard recommends; however, to enable them to vary from manufacturer to manufacturer, they are not required for compliance to the specification. I like to show these connector pinouts in a wire end view, which shows how the pins are arranged looking at the back of the connector (from the wire and not the terminal end). This way, you can see how they would be oriented if you were back-probing the connector with the connector plugged in.