There are two ways to size the pipes on a commercial fire sprinkler system under NFPA 13: hydraulic calculation and pipe schedule. Almost every modern commercial installation uses the first one, and it’s worth understanding why.
Pipe Schedule: The Older, Simpler Method
Pipe schedule design uses fixed pipe sizing tables based on the number of sprinkler heads downstream, without calculating actual water flow and pressure at each point in the system. It’s simpler to design but tends to be conservative, often resulting in larger, more expensive piping than the system actually needs. NFPA 13 restricts pipe schedule design to smaller, lower-hazard systems.
Hydraulic Calculation: The Modern Standard
Hydraulically calculated systems size every pipe segment based on actual water demand, factoring in the building’s hazard classification, sprinkler head layout, and available water supply pressure and flow. This produces a more efficient, often less expensive system, since pipes are sized to actual demand rather than a conservative table.
Why It Matters for Your Project
If your water supply is marginal, hydraulic calculation is also what determines whether a fire pump is needed to meet system demand. This is one of the reasons early water supply testing is such a standard first step in fire sprinkler system design.