ATX 20 pin Main Power Cable
In 1996 PC makers started switching to the ATX standard which defined a new 20 pin motherboard power connector. It includes a 3.3 volt rail which is used to power newer chips which require a lower voltage than 5 volts. It also has a standby 5 volt rail which is always on even when the power supply is turned off to provide standby power to the motherboard when the machine is sleeping. The new connector also allows the motherboard to turn the power supply on and off rather than depend on the user to flip a power switch. This connector is polarized so it can only be plugged in pointing in the correct direction.
|Pins 1 through 10||Pins 11 through 20|
|Description||Wire color||Pin number||Pin number||Wire color||Description|
|+3.3 volts||orange||1||11||orange||+3.3 volts|
|+3.3 volts||orange||2||12||blue||-12 volts|
|PWR_OK||gray||8||18||white||-5 volts (optional)|
|VSB +5 volts||purple||9||19||red||+5 volts|
|+12 volts||yellow||10||20||red||+5 volts|
Some of the voltage lines on the connector may have smaller sense wires which allow the power supply to sense what voltage is actually seen by the motherboard. These are pretty common on the 3.3 volt line in pin 11 but are sometimes used for other voltages too. The -5 volt line on pin 18 was made optional in ATX12V 1.3 (introduced in 2003) because -5 had been rarely used for years. Newer motherboards virtually never require -5 volts but many older motherboards do. Most newer power supplies don’t provide -5 volts in which case the white wire is missing.
|Connector part numbers|
|Motherboard connector||Cable connector||Terminals||Maximum current per circuit|
|Molex 39-28-1203||Molex 39-01-2200||Molex 39-00-0168,
|Unofficial cable/connector maximum wattage delivery for main rails|
|Voltage rail||Number of lines||Maximum current||Maximum wattage|
|+3.3 volts||3||18 amps||59 watts|
|+5 volts||4||24 amps||120 watts|
|+12 volts||1||6 amps||72 watts|
You can plug a 20 pin ATX motherboard cable into a motherboard with a 24 pin ATX connector. A 20 pin ATX motherboard cable only fits into one end of a 24 pin motherboard connector so you can’t insert it incorrectly. The 24 pin motherboard connector is actually just the 20 pin connector with 4 extra pins added on the end. The original 20 pins were unchanged. The extra 4 pins are not separate rails. They’re just extra lines to provide more current to the same rails. On every 24 pin motherboard the extra lines for ground, 3.3, 5, and 12 are just connected to the other lines on the same rail from the 20 pin connector. As a result, you can plug a 20 pin ATX motherboard cable into a 24 pin motherboard and it will work just fine, but those extra 4 pins are there for a reason. When you plug a 20 pin ATX motherboard cable into a 24 pin connector you’re not providing the extra current carrying capacity which may be needed by the motherboard. If your motherboard’s current requirements are low enough then it will work properly with only a 20 pin power cabled plugged in. But if the motherboard draws enough current, then you can overheat the 20 pins you’re using on the 24 pin connector. Connectors really do get hot if you overload them so the safest thing to do is use a real 24 pin power supply on a motherboard with a 24 pin connector. Note that your 24 pin machine may work fine with a 20 pin power supply until you add a PCI Express card later on down the road. PCI Express cards can draw up to 75 watts through the motherboard connector so adding an expansion card can substantially increase power draw through the main power cable. Many PCI Express video cards which have the 6 pin or 8 pin PCI Express power cable still draw a substantial portion of their 12 volt load through the PCI Express slot. The extra 4 pins doubled the current capacity of the 12 volt rail so that one is easy to overload when only using a 20 pin main power cable.