https://www.techpowerup.com/332019/nvidia-geforce-rtx-5090-runs-on-3x8-pin-pci-power-adapter-rtx-5080-not-booting-on-2x8-pin-configuration NVIDIA's flagship GeForce RTX 5090 demonstrated flexibility in power compatibility, while its sibling, the RTX 5080, struggled with stricter requirements. Recent tests by a German tech outlet,
ComputerBase, reveal that the RTX 5090 can operate with three 8-pin PCI power connectors instead of the recommended four, albeit with a performance trade-off. However, the RTX 5080 fails to boot when using only two 8-pin connectors. The RTX 5090, with a default TDP of 575 W, officially requires a 600 W 12V-2×6 connector or an adapter with four 8-pin PCI cables. However, tests on the ASUS ROG RTX 5090 Astral and Zotac RTX 5090 Solid show the GPU boots even with three 8-pin cables, capping its TDP at 450 W—matching the three connectors' 150 W-per-cable spec. Performance losses are modest: benchmarks indicate a 5% drop in average FPS at 450 W compared to full power.
In contrast, the RTX 5080's 360 W TDP proves less forgiving. Attempts to run the Founders Edition and Zotac RTX 5080 AMP Extreme Infinity with two 8-pin connectors (300 W total) resulted in failure: the screen remained blank, and the card refused to initialize. NVIDIA's firmware appears to lack a lower power-limit threshold for the RTX 5080, unlike the 5090, which automatically adjusts when detecting insufficient power delivery. This requirement forces users to adhere strictly to the three 8-pin or 12V-2×6 power connectors. While the RTX 5090 offers flexibility for users upgrading from older systems, the RTX 5080's limitations may frustrate owners of less powerful PSUs. For the RTX 5090, the 5% performance penalty at 450 W may be a reasonable trade-off for avoiding costly PSU upgrades, but RTX 5080 users have no such recourse. Verifying power supply compatibility, as underpowered setups risk instability or hardware damage, is a must, and when your $2000+ GPU runs, you should at least power it properly. This experiment is more a "for science" type of run.
In my opinion it is better to fully power these cards.