Yes, it is strictly mandatory. Under the National Building Code (NBC) 2016, Part 4, and Maharashtra Fire Services regulations, any building that requires a wet riser fire hydrant system must have an exclusive, static water storage tank (underground and/or overhead) dedicated solely to firefighting.
Sharing a tank with domestic supply is illegal because domestic usage could deplete the reserve needed during a fire. For ordinary hazard warehouses or industrial factories (Group G), a dedicated storage capacity of 100,000 to 200,000 liters is standard. This must be backed by a main electric pump (typically 2280 LPM), a standby diesel engine pump of equal capacity, and a jockey pump to maintain line pressure at 7 kg/cm².