NFPA 24 Private Fire Service Mains and Underground Piping
Underground rules
NFPA 24 covers private fire service mains from the public supply to the building riser. Pipe must be listed for fire service, buried below frost depth (and at least 30 in. cover unless the standard permits otherwise), and protected against corrosion. Thrust blocks or restrained joints are required at bends, tees, reducers, and dead ends. Gate valves must be accessible and indicating. Hydrants need proper drainage, clearance, and nozzle height.
Testing and flushing
- Flush mains at a velocity of at least 10 ft/s before connecting sprinkler piping.
- Hydrostatically test at 200 psi for two hours, or 50 psi above the working pressure where the working pressure exceeds 200 psi.
- Test before backfilling, with thrust blocks cured, and document gauge readings.
- Fire department connections must be visible, accessible, and marked.
Worked example
An 8-in. main has a working pressure of 175 psi. Test at 200 psi for two hours. If the working pressure were 250 psi, test at 300 psi (50 psi above working). A 100-ft run with a 2-psi gauge drop from 200 to 198 psi passes the visual and pressure-hold check if no leakage is found; a 20-psi drop fails.
Traps
Testing before thrust blocks cure; failing to flush; omitting thrust restraint at a dead end; using domestic water pipe; and setting hydrants so the nozzle is below grade or the barrel does not drain.