I'm building my first PC and I'm confused about modular power-supply cables. If different PSUs can have different pinouts, how can the same CPU power connector work with any compatible motherboard, and the same PCIe connector work with different graphics cards? Why aren't all the pins simply wired straight through from one end of the cable to the other?
3 Answers
The CPU and GPU ends are also keyed differently and carry defined combinations of 12V and ground, which helps prevent plugging the wrong type of cable into a component. The PSU-side wiring is left to the manufacturer because the internal design and layout of the power supply can vary. A cable may physically fit another PSU while still having the wrong wires in the wrong places, potentially damaging hardware or creating a fire risk.
The device-side connectors are standardized, but the connectors that plug into the PSU are not. Motherboards expect the standard ATX/EPS pinout, and graphics cards expect the standard PCIe pinout. Each PSU manufacturer can arrange the modular-side pins differently, so you should only use the cables that came with that specific PSU.
Wiring every pin straight through would be simpler only if every PSU used the same internal layout. In practice, designers route voltage and ground connections wherever they work best with the PSU’s circuit board, connectors, current distribution, and protection features. Standardizing the component end gives hardware compatibility, while the PSU end remains proprietary.
That makes more sense. I was assuming the PSU’s modular connector had to mirror the component connector pin-for-pin, but it’s really just using a manufacturer-specific arrangement on that side.