Matching GPU loads to PSU rails without wishful math

Forum shopping lists often add GPU nameplate watts, subtract a round number, and declare a PSU “fine.” In clinic we start from the opposite direction: what continuous load each rail can honestly carry, then how many cards fit under that ceiling with headroom for spikes and aging fans.

Write the envelope before the cart

List each card’s expected board power at your planned tune — not the marketing TDP from a gaming review. Add motherboard, risers, and fans as a fixed overhead. Then apply a headroom factor we usually keep between 20% and 30% for mining shelves that run for weeks.

If the total sits near the unit’s continuous rating rather than its brief peak claim, we ask you to drop a card or step up the PSU. Sitting on the edge is how quiet shutdowns appear at 03:00.

Rails are not a single bucket

Many builders treat wattage as one number. We still glance at how PCIe cables are distributed across connectors so one overworked lead does not become the weak link. During Fundamentals day one, learners redraw cable maps on paper before touching the frame.

A table you can copy

Columns we use: card model, planned watts, cable path, cumulative load, remaining headroom. Keep the sheet next to the shelf. When you add a fifth card months later, update the same page instead of trusting memory.

If you want this done with an instructor watching the arithmetic, book the Component Matching Clinic or the full GPU Mining Fundamentals block.