Power supply inside a Sierra is a Seasonic Model (X-850) SS-850KM Active PFC F3 power supply rated at 850 Watts.  There are two.  One supply goes to the left PCIE power inputs of all 3 boards and the other goes to the right PCIE power inputs of each board.  I will post some photos later on today, since after verifying that it mined, I took it apart to see how everything was connected.