Long time lurker first time poster.... Apologies but wasn't sure if this was GPU or PSU related...
So i've built quite a few systems in my time, but believe it or not, today is the first time that i have had to build a system with a GPU that requires PCI-E external power.
I took delivery of a 7870 LE today and it has 8+6 pin external power, fine.
My psu is a 500w silverstone strider and has modular cables, one of which contains a 6+2 and a 6 pin. The thing i wonder is these are just daisy chained, with only a 6 pin connector back into the PSU. I can't find anything to tell me if i'm OK using these two together, or should just use the 6+2 and run a molex > 6pin connector for the other one.
Splitting an 8 pin into an 6+2 and 6 is essentially dividing 150W up into what should be 225W of power delivered. In most cases, that doesn't matter (I can overclock the exact same "daily driver" OC on my 680 with both dedicated 8 and 6 pins and on a splitter cable), but the ideal config is a dedicated cable for each connector.
Thanks for the replies. That's what i thought. Although its a single rail PSU I guess it's still better to split them up, and seeing as i have a molex to 6 pin i should probably use that!
This isn't an issue of rails, it's an issue of the cables. the 8-pin PCIe cable only have 3 +12V lines, at 4.2 Amps per (double the 2.1 on 6 pin), you have a little over 150W of power that can be delivered. Normally, you would have that 8 pin capable of 150W, and the 6 pin, capable of 75W, delivering up to 225W. When you split the 8 pin PCIe into an 8 pin and a 6 pin, you are still only delivering 150W, so if your card needs more, it can't get it.