Your New PSU Can't Power Two Mining Cards
ATX 3.1 quietly gutted PCIe 8-pin counts — verified unit by unit. Plus the mining-card power socket that looks exactly like the connector that would destroy it.
Planning power for a 4090 plus two 250 W mining cards, I hit a supply-side change nobody seems to have announced: the ATX 3.1 generation traded away PCIe 8-pin connectors to pay for the new 12V-2x6 socket, and the marketing copy doesn’t mention it. Mining cards, datacenter cards, and older GPUs all drink exclusively from 8-pins.
I verified counts individually rather than trusting spec-sheet summaries, because the spec sheets summarize the cable family, not the box you’re buying:
| Budget ATX 3.x unit (2025) | PCIe 8-pin count |
|---|---|
| ASUS Prime 850W Gold | 3 |
| Corsair RM850e | 2 |
| SAMA G850 | 3 |
| ASRock Challenger CL-750B | 2 (non-modular) |
| MSI MAG A750GLS | 4 ✓ |
The cheapest widely-stocked 850 W Gold units cannot power two 8-pin mining cards, full stop. The cheapest unit that can — the A750GLS, four connectors at C$120 — delivers two of them via a “16-pin to dual 8-pin” split cable, which triggered a worry that dissolved under arithmetic: the split draws from the 450 W-rated 12V-2x6 socket across six conductors, so at 250 W per card it carries 3.5 A per conductor — less than a fully-loaded ordinary 8-pin at 4.2 A. The scary-looking cable is the strongest path in the box.
Two adjacent findings for anyone wiring this class of build:
Modular PSUs ship cables for the advertised connector count, not the socket count. The PCB serves a whole wattage family, so empty sockets outnumber included cables. Count cables in the box photos, not holes in the panel.
The mining card’s power socket is not a PCIe 8-pin, despite being its exact shape. The CMP’s inlet is EPS-keyed and expects NVIDIA’s own dongle (an EPS-male to dual PCIe-female splitter, 600 W-rated). Plug a standard PCIe cable into what looks like a standard PCIe socket and you apply 12 V to ground pins. And here’s the trap under the trap: no public source specifies pin-1 physical orientation for these Molex Mini-Fit shells — the pinout tables all describe the connector logically. The only safe verification is a continuity meter buzzing the actual cable against the actual card, which is how I spent a morning. (AllPinouts gets you the logical map; your meter gets you the truth.)
The budget arithmetic that reshaped my build: even with power limits applied, the tower — 4090, 3090, and two mining cards — computes to ~1,450 W against my 1,200 W supply — not a tuning problem, a topology problem. The mining pair moved to an external chassis with its own supply, and the in-tower pair got hard caps (350 W/300 W) that the thermal data justified anyway. One subtlety on choosing caps: prompt processing is compute-bound and collapses under aggressive power limits long before token generation does — tune your cap watching prefill throughput, or you’ll set it far too low and wonder why long prompts crawl.