Pinout

ATX PSU 24-pin motherboard connector (ATX12V 2.x) pinout

Pins

1: +3.3V (orange)
2: +3.3V (orange)
3: GND (black)
4: +5V (red) — many older units will not regulate the other rails properly without a few hundred mA of load on +5V
5: GND (black)
6: +5V (red)
7: GND (black)
8: PWR_OK / Power Good (grey) — an output flag that goes high 100–500ms after every rail is in spec. Read it, never draw current from it
9: +5VSB standby (purple) — live whenever the PSU is plugged in and the rear switch is on, even with the machine off; usually only 2–3A
10: +12V (yellow)
11: +12V (yellow) — this pin plus 12, 23 and 24 are the detachable four-way block on a "20+4" cable
12: +3.3V (orange)
13: +3.3V (orange, often with a thin brown +3.3V remote-sense wire crimped into the same terminal — leave the sense wire connected or the 3.3V rail drifts out of tolerance)
14: −12V (blue) — low current only, typically 0.3–0.8A
15: GND (black)
16: PS_ON# (green) — short it to any GND pin to switch the supply on; open = off. This is the only pin you need to bench-start a PSU
17: GND (black)
18: GND (black)
19: GND (black)
20: −5V (white) — removed from the spec in ATX12V 2.01, so this position is empty on any modern supply
21: +5V (red)
22: +5V (red)
23: +5V (red)
24: GND (black) — the eight black pins share the whole return current, so if you are pulling real amps out of this connector spread the load over several grounds rather than one

Contacts	24 (2 × 12), older boards 20
Connector	Molex Mini-Fit Jr
Pitch	4.2 mm
Rails	+3.3 V, +5 V, +12 V, −12 V, +5 VSB
Tolerance	±5 % (+3.3, +5, +12 V, 5 VSB), ±10 % (−12 V)
+5 VSB	Always on when mains is on, typically 2–3 A
PS_ON#	Active low, short to GND to start
PWR_OK delay	100–500 ms after rails are in spec
Typical wire	18 AWG

These wire colors are a de-facto industry convention, not an enforced spec — some OEM (Dell/HP proprietary) supplies deviate. Verify by pin position and connector keying, not color alone, on anything non-standard.