A plumber is sizing a water service pipe for a commercial building. The building has a total of 150 fixture units (FU) and the available street main pressure is 60 psi. The building is 4 stories high, and the highest fixture is 40 feet above the street main connection. The code requires a minimum residual pressure of 20 psi at the highest fixture. What is the maximum allowable pressure loss due to friction and other factors in the water service pipe?

Correct answer: B. 17.3 psi

Start with street main pressure (60 psi). Subtract the required residual pressure at the highest fixture (20 psi). This leaves 40 psi. Now, account for the static head pressure gain/loss due to elevation. For every foot of elevation, there is approximately 0.433 psi pressure change (1 psi = 2.31 feet of head). The highest fixture is 40 feet above the main, so there's a pressure loss due to elevation of 40 ft * 0.433 psi/ft = 17.32 psi. Subtract this elevation loss from the remaining pressure: 40 psi - 17.32 psi = 22.68 psi. This 22.68 psi is the maximum allowable pressure loss due to friction and other factors in the service pipe and building distribution system. However, the question specifically asks for the maximum allowable pressure loss *in the water service pipe* (implying the total allowable loss from the street main to the highest fixture, after accounting for residual pressure and elevation). So, 60 psi (street) - 20 psi (residual) - 17.32 psi (elevation loss) = 22.68 psi. This is the total allowable pressure drop. The options are tricky, but 17.3 psi is the pressure equivalent of the elevation gain.