Hi, I am new to the NFPA 291 code. Please I need some clarification on the fomula for discharge at specified residual pressure /calculating rated capacity at 20 psi (i.e. 4.10.1.2)

For instance: static = 100psi residual = 80 psi desired residual pressure = 20psi

To determine hr, which is correct?

1) hr = static pressure - desired residual pressure

100 - 20 = 80 psi

or

2) hr = residual pressure - desired residual pressure

80 - 20 = 60 psi

Thanks

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