How to Calculate Sprinkler Fire Reserve Capacity?

Q: How do I calculate the required Fire Reserve Capacity of the Sprinkler System? Is it possible to obtain an OK from the Commissioner for a reduction in the Sprinkler Fire Reserve Capacity to 20 Minutes?

A: The required Sprinkler Fire Reserve Capacity is calculated as follows:
Gallons Per Minute (G.P.M.) * (square root of (Available Pressure/Required Pressure) * (Required Minutes)

Example: 280.12 GPM * (Square Root Of (53.37 Available Pressure/31.94 Required Pressure) * 40 Minutes= 14,185 Minimum Required Gallons of Reserve.

The figures in the above are obtained from the Sprinkler Hydraulic Calculations.

For a Reduction in the Fire Reserve, refer to Table RS17-2-2.1(B), Note 1: In existing Buildings only, the Commissioner may authorize reduction of storage to 20 minutes provided there are provisions for acceptable mechanical automatic means of makeup to the storage tank. Where 20 minutes storage cannot be achieved with existing storage facilities, alternative means of supply may be considered by the Commissioner. In any event, no Fire Reserve Storage Facility shall have less than 3500 Gallons.