NDA 3 is a 4.5 MW colocation data center built in 2009 served by a (4) chiller water cooled CHW plant and (22) chilled water air handlers with airside economizers. Taylor Engineers was original retained to investigate the root causes of a series of chiller failure events during the summer of 2017 on near design days. After identifying the mechanical and controls design flaws that led to the failures and providing recommendations to prevent recurrence, we were again engaged to (1) design a new control system and sequences of operation for the data center and (2) design a capacity expansion utilizing computer room air handlers (CRAHs). The new ALC control system utilizes an all ethernet topology to maximize network speed and a distributed failsafe control scheme to silo the impacts of any control device failure, thereby maintaining "N" devices under automated control in any single device failure scenario. To safely transition to the new controls while the data center remained in operation, we worked with the controls contractor to functionally test the sequences of operation on a simulator test bench. After thoroughly validating all sequences, the controls retrofit cutover happened one data hall at a time, followed at the end by the chilled water plant cutover. Field functional testing was phased to minimize risk, with testing of the first data hall occurring prior to cutover of the second. Since commissioning completion in October 2018, annualized data center PUE has dropped from 1.48 to 1.28, saving our client 5.4 million kWh per year.