The long answer is: To power a GTX 1080 in a Z820 Workstation a single 6pin-to-8pin PCIe power adapter will not be sufficient. There has still been some discussion about if it might work because there isn't all that much information about the PSU and how it's configured internally. Sometimes a 6-to-8pin adapter can work with certain PSU's that exceed the normal specifications of the 6pin connector - perhaps to enable use of such adapters. On the Z820 and the 1125w PSU it doesn't work.
The system boots fine and is usable until you stress the GPU at which point it will promptly bluescreen. To be fair, on paper it should not work because a 6pin connector provides only 75w and the PCIe bus gives you another 75w - amounting to 150w. The GTX 1080, on the other hand, needs about 170-180w under full load. So, how does one even use a >150w card on the Z820? You will need something like this
A dual 6pin-to-8pin adapter, but unfortunately they are about as easy to find as pink unicorns (try eBay?). These provides you with 225w of power (75+75+75w) and will make your card run stable with 35-45w overhead. Nvidia does not recommend these adapters due to "grounding requirements" of these power hungry cards - as such you will not find these being sold by Nvidia nor will you get one with your GTX 1080 purchase. These adapters do work, so if you're out of options and are willing to risk a configuration that might not be covered under warranty then a dual 6pin-to-8pin adapter is pretty much your only option. These adapters are probably difficult to find because most people would buy a new PSU to fulfill the GTX 1080's requirements. As a Z820 owner, however, you don't have that luxury because of the non-standard PSU formfactor.
