For the full breakdown, see our best ATX Power Supply Tester guide.
If you’re building a desktop tower, the ATX power supply (PSU) is the component that takes alternating current from your wall outlet and converts it into the direct current your motherboard, CPU, and drives actually use. The ATX standard defines both the physical size of the unit and the electrical rails it must deliver, which is why it fits almost every mid-tower and full-tower case on the market. Understanding what ATX means helps you buy the right unit and avoid a common mistake: assuming every “ATX” power supply is the same size.
The Physical Size of an ATX Power Supply
An ATX PSU’s width and height are standardized, but its depth is not. Depth, however, varies by model. A typical ATX PS/2 unit is 140 mm deep, but some models run longer to house higher-wattage components or quieter, larger fans.
This is the most common compatibility trap. A “standard” 140 mm unit slides into almost any ATX case, but a longer 180 mm or 200 mm unit may not fit a compact mid-tower without blocking drive cages or cable routing. Before you buy, measure the clearance in your case or check its spec sheet for maximum PSU depth.
Electrical Rails and the ATX Standard
An ATX power supply delivers power across multiple voltage rails, each with a specific job. The core rails are +3.3 V, +5 V, and +12 V, alongside -12 V and a +5VSB standby rail that keeps your PC’s wake-on-LAN and USB charging alive when the system is off. The +12 V rail does the heavy lifting for the CPU and GPU, which is why modern PSUs dedicate most of their wattage to it.
The electrical behavior of these rails is governed by design guides that define voltage tolerances and load behavior. It aligns with ATX Specification Version 3.0 and details how a compliant PSU must handle transient loads from high-end graphics cards.
ATX 3.0 vs. ATX 3.1: What Changed
The ATX 3.0 and 3.1 specifications are close relatives, but they are not identical. Intel documented ATX 3.0 in 2022, introducing stricter transient-load requirements and the 12VHPWR connector for high-power GPUs.
The 12V-2×6 connector is still a 16-pin plug, but it fixes a real problem. In the 12VHPWR design, the signal pins could lose contact if the plug wasn’t fully seated, causing overheating. ATX 3.1 recessed those signal pins and extended the power pins slightly so the connection is more reliable.
Choosing the Right ATX PSU
Beyond the standard, pick a unit whose wattage covers your components with headroom, and check the depth against your case. The table below summarizes what matters.
| Spec | Standard Value | Why It Matters |
|---|---|---|
| Width × Height | 150 mm × 86 mm | Fits the standard ATX bay in most cases |
| Depth | 140 mm typical, varies | Longer units need extra case clearance |
| Core Rails | +3.3 V, +5 V, +12 V, -12 V, +5VSB | Powers motherboard, CPU, and drives |
| GPU Connector | 12V-2×6 (ATX 3.1) | Replaces 12VHPWR for safer seating |
| Form Factor | Desktop only | Not compatible with laptop internals |
That’s a practical next step once you’ve settled on a PSU and want to confirm every rail is solid.
References & Sources
- Intel. “ATX Version 3.0 Multi Rail Desktop Platform Power Supply Design Guide.” Defines the ATX 3.0 electrical and mechanical requirements.
- Intel. “ATX12V Power Supply Design Guide.” Details the voltage rails and tolerances for ATX power supplies.