What are output rails?
A power supply takes AC from the wall and converts it into several fixed DC voltages. Each of those outputs is called a rail. The PSU's cables and connectors carry the rails to the motherboard, drives and expansion cards.
| Rail | ATX wire color | Main uses |
|---|---|---|
| +12 V | Yellow | CPU (through the CPU power connector), graphics cards, fans, 3.5-inch hard drive motors |
| +5 V | Red | USB ports, 2.5-inch drives and other drive electronics |
| +3.3 V | Orange | Motherboard chips, PCIe slots, M.2 SSDs |
| -12 V | Blue | Legacy serial ports and older cards; very little current |
| +5 V standby (+5VSB) | Purple | Powers the power button circuit and wake features while the PC is off but plugged in |
Ground wires are black. Older supplies also had a -5 V rail, dropped in later ATX revisions.
Why +12 V matters most
CPUs and graphics cards draw most of their power from +12 V, and voltage regulators on the motherboard and card step it down to what the chips need. That's why a PSU label lists the +12 V capacity separately, and why most of a typical desktop PSU's rated wattage is available on +12 V. The +3.3 V and +5 V rails usually share a smaller combined limit.
Single-rail vs. multi-rail +12 V
Some PSUs deliver +12 V as one large rail; others split it into several rails, each with its own over-current protection limit. Both designs work. With a single rail you don't need to think about which connector is on which rail. With multi-rail, spread high-draw loads such as graphics card cables across rails as the manual describes, or a rail's protection can trip and shut the PC off under load.
Testing the rails
- PSU tester: plug in the 24-pin and other connectors and read each rail's voltage and the Power Good signal.
- Multimeter: set it to DC volts and measure a rail's wire against a black ground wire, for example a yellow wire for +12 V.
- Tolerance: the ATX spec allows about 5 percent either way on the positive rails, so +12 V should read roughly 11.4 to 12.6 V. Readings outside that point to a failing PSU.
Never open a power supply: its capacitors can hold a dangerous charge even when unplugged.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.6 contrasts the PSU's 3.3 V, 5 V and 12 V outputs with its 110 to 120 VAC or 220 to 240 VAC input, and the Core 1 hardware list includes a multimeter and a power supply tester. Know which rail feeds what, and that symptoms like random shutdowns or no power (objective 5.1) can come from a rail that's out of tolerance.