Section 11.2.3/Pipe Sizing by Hydraulic Calculation
NFPA 13 Section 11.2.3 covers the hydraulic calculation method for pipe sizing based on flow, friction loss, and pressure at the water supply.
Sprinkler system piping must be sized to deliver the design density (gpm/sf) over the design area (sf) to the most hydraulically remote area of the system. The hydraulic calculation determines the flow rate and pressure required at each sprinkler, calculates the friction loss through the piping using the Hazen-Williams formula, and compares the system demand to the available water supply. The water supply must exceed the system demand at the required flow rate. Pipe schedules (pre-sized based on hazard) are permitted as an alternative for some system types, but hydraulic calculations are required for most commercial systems.
Why this section exists
A sprinkler system that cannot deliver adequate water to the most remote sprinklers will fail to control a fire. The hydraulic calculation method ensures every pipe segment is large enough to deliver the required flow without excessive friction loss. Unlike pipe schedules (which are based on rules of thumb), hydraulic calculations optimize pipe sizes to the actual layout and water supply, resulting in a system that is both reliable and economical.
What plan reviewers look for
Plan reviewers check the hydraulic calculation summary for the system demand (flow and pressure) versus the available water supply. They verify the design area and density match the hazard classification. They check the most remote area location and verify it is the hydraulically most demanding area. They review the node-by-node calculation for pipe sizes, flow rates, and pressure losses.