I will be installing a R9 290x that requires both a 6 pin PCIe power connector and a 6 + 2 PCIe power connector.
Is it ok to use one PCIe 6 + 2 power cable from the PSU that splits into two 6 + 2 PCIe connectors, or is it better to use two separate PCIe 6 + 2 power cables from the PSU.
The PSU is a 750W Corsair CS750M.
Best Answer
In reality, the card itself has a 6 and an 8 :-) Your PSU is supplying 6+2s so they can be used as a 6 or an 8.
The real key to the electronics here is what does it connect? Check the wires and where they go, check also the layout of the "rails" on the PSU you're using.
The wiring general connects the card through more wires to the 12V and the ground 0V, to reduce the resistance via the wiring and connections. The extra 2 are oddly both extra grounds (check it), in most cases the +2 is not even necessary as 3 each 2 times of the 12V and 0V is enough wiring to get the amps and maintain the voltage to the cards. The extra ground +2 in everything I have seen and tested is connected direct to the same location. (It is not an added ground layer or clad.)
How the PSU circuits are wired then, the so called "Rails" be they from the same power conversion and just going through different safety circuits, or actually separate power conversion circuits, with or without safety circuits can be important. The safety circuits have a limit to the total amperage to keep problems of shorting out or the card overloading the power supply.
Separate whole power conversions can also be limited to how much power it is capable of delivering per rail.
The "rails" information is laid out on the side of the PSU and often also found in the manual for it. Depending on the layout of these sets of circuits or the One Big Fat 12V rail, the builder or upgrader can connect so various high use of the 12V can be distributed through the safety circuits, or separate power conversion areas.
If it is one fat 12V rail, just connect the wires and you're done, passing the power over "more metal" or more wiring, more connection, will reduce the resistance total (although it is not really critical). So a person could use an extra modular connection if they wanted to, or were overthinking it.
If it is multiple lower amperage rails, then you can see which one is going to the CPU power, modern CPU power regulation much of the CPU power comes from the 12V Motherboard 4-8 pin connection. Then get the power for the GPU off of a different rail.
Only when you get into multiple GPUs do you really have to worry much about that, and only when there are separate "rails" then it helps to run the CPU, and each GPU off the separate wiring, passing through separate safety circuits. On some PSUs, this is marked with numbers to make it easy to understand.
What would be real problems?
Connecting a Motherboard connector to the GPU when (remember) the GPU connections last 2 pins are both grounds, but the 4 and 8 pin connection to the motherboard do not use that layout.
Using Moduler wiring that did not come with that PSU, without checking how that is connected, not all modular connections are the same.
Notes: Some power can also be supplied via the PCI-E slot connections. Good wiring, making good connection and good distributions, could keep it so less power ever comes from these slots.