This study deals with the fire suppression capability and cooling characteristics required to prevent spontaneous re-ignition in a power transformer room. A water mist system was considered as a possible alternative to a gaseous suppression system. High- and low-pressure water mist systems were examined. The power transformer examined in this study occupied about 7% of a 10 m × 10 m × 10 m test room. Full-scale suppression tests were performed for six different fire scenarios: two spray fires, three pool fires, and one cascade fire. Three reduced-scale models were used to estimate the cooling characteristics required for the power transformer and to calculate the necessary discharge time for the water mist to prevent spontaneous re-ignition. The fire suppression test results demonstrated that the high-pressure system was superior to the low-pressure system, especially considering oxygen depletion and the ambient temperature distribution. In the cooling tests, the discharge time required to prevent spontaneous reignition was estimated to be about 12 h for both the high- and low-pressure systems.