Constantly changing computer specifications often cause power supply compatibility issues when you do a replacement or an upgrade. The frequent questions are how to connect a PC power supply with 20-pin main connector to a 24-pin motherboard or visa versa, and will a 20 pin plug work in 24 pin socket? This page provides detailed answers to these questions.
Just note that this guide applies only to standard ATX models based on Intel's guidelines. It does not apply to some branded cases (such as Power Mac's, HP and Dell), which use different non-standard wiring and may require an exactly-matching PSU.


Technically, a 20-pin ATX connector can be plugged straight into a 24-pin motherboard socket, leaving pins 11, 12, 23, and 24 unconnected. The four pins that you are leaving open are just redundant lines for higher current capability (see this guide to ATX connectors and pinouts). The power connector is polarized, so it's difficult to insert it wrong into the header unless you force it really hard. Nevertheless, double check that pin 1 goes into pin 1. However, power supply compatibility is not merely a mechanical issue. There was obviously a good reason to add extra four wires to the main cable-- it was done to support higher current requirements.
If you exceed current rating of a connector, it may overheat and even melt. Since the Molex power connector is rated at 6 amps per circuit, with 20-pin part you should not draw more than 12A from 3.3V, 24A from +5V and 6A from +12V (note that pin 11 is often used for 3.3V remote sense). The 4-pin 12V2 connector P4 is rated 8A/pin, so you can additionally draw up to 16A on +12V. A simple math shows that with 20-pin main and 4-pin 12V2 aux connectors your maximum power could be 355 watt. This is enough for most PCs. However, even if your system needs overall less than 355 watt, you still need to know the distribution of the currents on the motherboard and verify that you can get required amperage from each rail of your power supply. If your system needs more amps on any individual rail and the motherboard does not have an auxiliary 6-pin power connector (which has been removed from ATX 2.0 spec in 2003), you would be better off going with an ATX12V version 2 compliant power supply with 24-pin connector, even if your old PSU has sufficient overall power rating. Such v2.0 models are sometimes called ATX-II. You may use various online PC wattage calculators to estimate your requirements.
ATX pinout
24/20-pin power supply pinout (front view).

More specifically, if a motherboard requires either 3.3V current that exceeds 12 A or +5V current over 24 A, and it does not have an aux 6-pin connector, you should use a 24-pin PSU. Note that 20-to-24 pin adapter cables are totally useless since all the currents are still passing through a 20-pin power supply connector. These adapters actually make things worse because they add extra resistance between PSU and the load causing more voltage drop.
Another issue to keep in mind is PSUs built prior to 2003 may not have a serial ATA cable. In this case, you may try using an IDE to SATA adapter, which attaches to one of 4-pin peripheral connectors and converts it into SATA connector with missing 3.3V rail. It may work if your SATA device does not use 3.3V, which is often the case.

If you use high-end graphics card with an additional 6-pin or 8-pin power header, you may try to use an adapter that converts two 4-pin peripheral cables into a 6-pin PCI Express cable. For such an application, the PSU typically has to be >400W, or otherwise you may not have enough current rating on 12 volt output. Finally, an old PSU may cause a start up problem. Today's motherboards are using PWR_OK signal for proper timing. Upon initial power up it is first kept low. Then it should be asserted TTL logic high by the power supply within 100 to 500 ms after +12 VDC, +5 VDC, and +3.3 VDC outputs reach their regulation bands. Conversely, it has to switch to TTL low state when any of positive DC voltages falls below its under voltage threshold, or at least 16 ms after input power has been removed. An old cheap PSU may not have "Power OK" circuitry and might just connect pin 8 to +5V. Modern systems may have booting problems and may not work without properly functioning PWR_OK signal.


Many new ATX12V v2 compliant PC power supplies come with a dual 24/20 (sometimes called 20+4) connector for backward compatibility. It has the last four pins on a detachable section that slips out, so you can plug it into an older motherboard (see the diagram above). Just don't plug the remaining 4-pin piece anywhere- it is not compatible with any other circuits! If your PSU happens to have a solid 24-pin connector, you can still try to insert it into 20-pin slot with the last four pins hanging over the edge of the board's connector. Alternatively, you can buy a 24 to 20 pin adapter. To check compatibility of a specific model see Intel's database and selector of tested PSU.
The only potential problem with using a new PSU with an old computer is today's PSUs usually do not provide minus 5V output (they either don't have the white wire going to pin 20 at all, or they leave it not connected inside). Even some of ATX-I PSUs may not have "-5V", which was made optional in ATX12V version 1.2 spec as of 2002. The minus 5V rail was used by ISA cards. It was phased out when ISA were replaced by PCI. If you have an old computer with ISA slots, it will still need this buss. In this case, if you can't find a power supply with such a rail, you can derive it from -12V by adding a negative linear regulator such as MC79M05. Since minus 5V buss can draw up to 0.3A current according to ATX spec v.1, this regulator will dissipate up to (12-5)0.3=2.1W. This may be too much for a stand alone device, so may you need to attach it to a little heatsink.
Also see our tutorial on laptop batteries.

Note: The information in this site is provided AS IS and does not constitute a professional advice. Don't use it without independent verification (see complete Disclaimer linked below).