I'm doing a workstation build, to include 4 GPU cards on a Asus X99 WS-E motherboard. The motherboard has a 4th power connector, a 6-pin "EATX-12V_1" with 6-pins. My understanding is that it is provide additional PCIE slot power when multiple graphics cards are used. 
 
The Supernova 1600-T2 doesn't have a cable for this. Anyone know if this is truly required, or if a PCIE 6-pin is the same pinout?
 
 

bcavnaugh
Your Power Supply has the cable you need, it is not missing.
Use one of the Normal GPU Cables that have a 6+2 Pin and just do not use the extra 2 Pins.
I would say required for more than 2 GPU yes. I use it for one GPU.
It reduces the Stress off of the 24-Pin Cable.

This is correct. If there is a 6-pin power connector near the PCIe slots then you would most likely want to use the 6+2 connector from the power supply and leave the +2 pins hanging off. As always, you would want to check with your motherboard manual but I have not seen a motherboard with an additional 6-pin socket (other than proprietary pre-builts) that is not a PCIe power connector. That additional 6-pin power is to help balance the load for multiple GPU's in the system. If you are only using one or two graphics cards, then you may not need it.

Even with only two graphics cards, if you have other add in cards it is a very good idea to hook up that supplemental 6 pin PCIE power. I've had my old X79 Dark fail to boot, and after multiple RMAs and troubleshooting with EVGA techs, we found out that it just needed that supplemental power hooked in.

The weird part was that the system had been working fine with two video cards and other add in cards, but one of the GPUs likely started drawing extra power on boot as it aged and messed things up when the system didn't have the required power on bootup.