There have been changes to the National Electric Code. As of August 1, 2019 there are 29 states that have completed the 2017 NEC update process. Odds are, the state that you work in is up to date on new NEC requirements or will be by 2020.
One of these changes regards critical power systems. According to NEC 700.3(F) it is now required to have an additional temporary power source for emergency systems. Essentially, there needs