Hello,
 
I just finished my first PC build. Everything is brand new except the GPU.
 
EVGA - 650W Gold PSU ( I have ECO mode turned on)
Gigabyte H370 HD3 Mobo
Corsair 8gb DDR4 RAM x 2- I believe it is Vengeance
I5 - 8500K Coffee Lake CPU
MSI ATI R7950 HD TwinFrozr 3GB GPU
 
The GPU has two 6 pin receptacles on it, so I ran one 6+2 x 2 cable from the PSU to the GPU and used the two ends of the split cable to power it. Computer powers on (all fans/lights on) but won't output any video signal. I removed the GPU and the computer boots to Windows just fine (originally had to install windows without GPU installed because of this). I decided to run one cable from VGA1 on PSU to the GPU and one cable from VGA2 from the PSU to the GPU and the computer booted up. The Mobo has a red light on the CPU, but I have read that a custom bios from Gigabyte might be needed for that issue and I believe it to be unrelated to the GPU issue (maybe?).
 
Everything I have read online says that the GPU should be able to run with one power cable, and if it was under-powered it would present different problems and probably only under heavy load. However, mine won't even boot up with only one installed. Is this PSU related, GPU related, or is this not a concern at all? It just seems odd that it won't output a display at all with just the one cable.
 
Thank you very much for taking the time to read and respond.
 

When I attached the second PCIe power cable the computer booted up.

Ah, sorry.
I used the second 6+2 x2 cable (from VGA2 in the PSU) and plugged both ends into the GPU to see if it would work (this is after I got the PC running with two cables). I did not take the second cable from VGA2 and put it into VGA1 to try it though.. So, one single cable from VGA1 failed, as did one from VGA2.
 
Sorry for my misunderstanding.