07.02 How big should a day tank be?

Day tank sizes are restricted by regulation to a maximum capacity that may be allowed within a room of a given occupancy. Within that restriction, there is not a standard for day tank sizing.

The recommended approach is to take the maximum generator consumption per hour, which can range up to 200 gallons per hour for larger generators, and consider the day tank size for 1, 2, 4, and 8 hours.

Here is an example: If the building code requires 4 hours of run time on a life safety generator that consumes 50 gallons per hour, then you would have 200 gallon of consumption. Since the tank is not always full, NFPA requires a 1.33 factor adjustment (to compensate based on a 75% full tank) which would be 266 gallons. Then round up to a 300 gallon day tank and check to see if the volume is within the regulatory restrictions for the room.

Another example: If the day tank has a well designed re-fill system from a bulk tank, the codes are restrictive, and space is at a premium in the generator room, then a 100 gallon capacity day tank may be appropriate, even though this is only 30 minutes of run time on a large generator with 200 GPM consumption. If the facility has well trained operating personnel, and the fuel system is monitored by the BMS, then 30 minutes of time to react to a problem with the day tank, may be an appropriate measure.