Calc Residual Pressure for Pipe Schedule System?

I am doing an evaluation of a water supply for a fire sprinkler system. The system is an ordinary hazard wet pipe fire sprinkler system. It was designed as a pipe schedule system. Table 19.3.2.1 of NFPA 13 – 2019 Edition gives the water supply requirements for Pipe Schedule sprinkler systems.

Based on our buildings construction the table requires that for Ordinary Hazard we have a 1,500 gpm flow with a residual pressure of 20 psi. Section 19.3.2.6.1 then states, “The residual pressure requirement of Table 19.3.2.1 shall be met at the elevation of the highest sprinkler.”

My understanding of this requirement is that if I have a pipe schedule system where the highest sprinkler head is 20 feet above the finished floor which equates to roughly 8.7 psi of head I would need to have a water supply that provides a minimum of 28.7 psig residual water pressure flowing 1,500 gpm to the base of the riser to meet the requirement.

Am I calculating the required residual pressure correctly?

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