Like many cryptocurrency miners, I bought a set of PCIe riser cards for my mining rig. I'm just a hobbyist miner, but I want to get the most out of my equipment. I want it to be reliable. I want it to last.
A PCIe riser card is a combination of a socket to plug your GPU into, a small PCB that plugs in to the motherboard's PCIe slot and a short cable to connect the two. Typically, it's a USB3 cable, although it has nothing to do with USB itself, it is just conveniently high-quality and can carry PCIe data at the required speeds.
What that USB cable doesn't do is pass the (up to) 75W of 12V power usually supplied to your GPU by the motherboard. This has to come from the existing PSU directly into the PCB you've connected your GPU to.
These PCIe risers come in a few flavours. Mine come with a standard PCIe 6-pin power connector on the riser itself plus a SATA power to 6-pin adaptor. The other common version I've seen has a SATA power connector built in to the riser.
This is where the problem and potential fire hazard arises. SATA power maxes out at 54W. Even if the SATA power adapter that comes with your PCIe card somehow does itself use wiring thick enough to be rated for 75W (a little more than 2A at 12V), the power supply cabling you're connecting it to is not.
The only possible outcome is that something is overheating. Either the cabling between your PSU and the PCIe riser, or the cabling and the internals of your PSU.
You'll likely be leaving your mining rig running unattended, 24/7 for days, possibly weeks at a time. The risk of something overheating a little bit too much for a little bit too long is vastly increased.
Absolutely do not buy PCIe riser cards that integrate a SATA power connector. If you buy the ones with a 6-pin PCIe power connector, do not be tempted to use a SATA power to 6-pin adaptor. Use a standard PCIe 6-pin power cable.
If your PSU has too few connectors for your intended use, you could also use a Molex to 6-pin adapter. Such an adapter needs two Molex connectors to provide enough power. If your PSU still has too few connectors to make this work, you need to buy a new (or a second) PSU. There is no other safe way of doing it!
Don't risk it. It's not worth the damage it could cause.