403.3.3 Secondary on-site water supply.
BC § 403.3.3
A secondary on-site water supply equal to the hydraulically calculated sprinkler demand, including the hose stream requirement, shall be provided for high-rise buildings in Seismic Design Category C or D as determined by this code, and in any high-rise building with occupied floors located more than 500 feet (152 m) above the lowest level of fire department vehicle access. The secondary on-site water supply shall have a duration of not less than 30 minutes as determined by the occupancy hazard classification in accordance with NFPA 13 as modified by Appendix Q of this code.
