When a transformer fails it can lead to an intense fire and violent explosion (feel free to check out one of the many videos online on exploding transformers). Transformers can hold anywhere between a few gallons to thousands of gallons. Transformers can be installed indoors or outdoors, but indoor transformers typically are not filled with oil while outdoor transformers often are.  Transformers should be protected using a water spray system for example by making provision  of  0.25 gpm/ft² [10.2 (L/min)/m²] of water to be discharged onto the envelope of the transformer itself and 0.15 gpm/ft² [6.1 (L/min)/m²]  on the surrounding area for exposure protection. The water supply for such a system needs to be able to keep up with the designed flow rate of the system as well as 250 gpm (946 L/min) for a hose for the duration of 1 hour.