Power supply units (PSU) are the most common type of rack load in a data center, and as such they must remain stable as the scale of the data center grows to several MW. In the past, Facebook has experienced data center power system instability as discussed at the Open Compute Project Data Center Facility Project Call (https://www.youtube.com/watch?v=aUk_-ufhR2Y&feature=youtu.be). The objective of this paper is to show the analytical process to establish system specifications for the PSU to remain stable within large scale data centers, which are characterized by having a large number of regulated power supply units as rack loads. The essential characteristic to derive system specifications is the impedance of the PSU. The paper will also give methods to calculate and measure the PSU impedance in practice.