Approving a sprinkler system takes two sets of documents: working plans showing how the system will be installed, and hydraulic calculations proving there is enough water. The plans and calculations part of NFPA 13 defines what these documents contain and the core rules of the calculation method. The aim is that another engineer can reproduce the calculation and check it.
What this part of the standard covers
- Information required on working plans
- Water supply data and test information
- Calculation method: friction loss, elevation, fitting equivalent lengths
- Output format: summary sheet, graph sheet, detailed worksheets
- The hydraulic design information sign on the system
Working plans
Working plans must be to scale and usable on site on their own. The main information expected:
- Building data: occupancy, hazard class, ceiling heights and types, fire walls
- Sprinkler data: type, K-factor, temperature rating, response, manufacturer and model
- Pipe data: material, size, lengths, elevations, joining method
- Valves, drains, test connections, fire department connection, alarm devices
- Hanger, seismic brace and restraint locations with typical details
- Location of hydraulic design areas and node numbers
- Water supply data and test location
- For storage: commodity class, storage arrangement and maximum storage height
Wherever the drawings are silent, decisions get made on site, and site decisions are usually missing from the calculation.
Calculation method
Friction loss: Hazen-Williams
For water-filled systems NFPA 13 uses the Hazen-Williams formula for friction loss. In SI units:
p = 6.05 × 10⁵ × Q^1.85 / (C^1.85 × d^4.87)
where p is loss per metre (bar/m), Q is flow (L/min), d is internal diameter (mm) and C is the roughness coefficient. In US units the same formula uses a constant of 4.52 with psi/ft, gpm and inches. Special cases such as antifreeze systems may need the Darcy-Weisbach method.
| Pipe and system | C-factor |
|---|---|
| Black steel – wet system | 120 |
| Black steel – dry and pre-action | 100 |
| Galvanised steel – wet system | 120 |
| Copper and CPVC | 150 |
| Cement-lined ductile iron | 140 |
Example: 500 L/min through DN50 steel pipe with a 52.5 mm bore. With C = 120 the loss is about 0.036 bar/m, or about 0.71 bar over 20 m. The same pipe on a dry system (C = 100) loses about 0.050 bar/m, or about 1.0 bar over 20 m. The C-factor alone makes a 0.3 bar difference on this run, which shows why getting C right on dry systems matters.
Fitting equivalent lengths
Losses through elbows, tees and valves are added as equivalent pipe length. The tables are based on C = 120, and a multiplier is applied for other C values (about 0.71 for C = 100 and about 1.51 for C = 150). Forgetting the correction gives wrong answers, especially with plastic and copper pipe.
Elevation
Each metre of height is roughly 0.098 bar (0.433 psi/ft). In tall buildings and high-bay warehouses this term can exceed friction loss.
Junction balancing
At a node fed by two paths, the pressures from each path should generally balance within 0.03 bar (0.5 psi). Gridded and looped systems reach this by iteration; software does it automatically, but the result should appear in the report.
Water supply and graph
The calculation ends by plotting system demand (flow and pressure) on the same graph as the water supply curve. For a town main, the curve comes from a flow test giving static pressure, residual pressure and the flow at that residual. For method see hydrant flow test procedure. Where a pump is used, its curve and suction conditions are assessed together; the fire pump NPSH calculator helps with that check.
NFPA 13 does not impose a universal safety margin figure, but many authorities, insurers and clients require a margin between the supply curve and the demand point. It protects against seasonal and long-term drops in mains pressure.
Calculation output and the hydraulic data sign
| Document | Content |
|---|---|
| Summary sheet | Project, hazard class, density/area or number of sprinklers, total flow and pressure, hose allowance |
| Graph sheet | Supply curve and system demand (on N^1.85 scale) |
| Detailed worksheets | Node-by-node flow, pressure, size, length, fittings, C and loss |
| Hydraulic design information sign | At the riser: location of design area, density/area or number of sprinklers, flow and pressure required at the base of the riser, hose allowance |
The hydraulic sign may be the only information an engineer has when modifying the system years later. A missing or wrong sign is one of the most common problems when modifying existing systems.
Common mistakes
- Using C = 120 on a dry system.
- Skipping the C correction on equivalent lengths.
- Placing the design area in a "typical" location rather than the most demanding one; see the hydraulically most demanding area.
- Calculating from an old flow test, or one taken at a different point.
- Adding the hose allowance at the wrong point in the calculation.
- Pipe routes changed on site but not reflected in an as-built calculation.
How to check a calculation: look first at the graph, then at the pressure at the most remote sprinkler, then at the three pipe sections with the largest losses. Most errors show up there. For a step-by-step method see the sprinkler hydraulic calculation guide; the unit converter helps with conversions.
Frequently Asked Questions
Which friction loss formula does NFPA 13 use?
Hazen-Williams for water-filled systems. In SI units p = 6.05 × 10⁵ × Q^1.85 / (C^1.85 × d^4.87) bar/m, with Q in L/min and d in mm. Special cases such as antifreeze systems may need Darcy-Weisbach.
Why is the C-factor lower for dry systems?
Dry system pipe corrodes faster inside because of moisture and oxygen, so roughness increases. Black steel in dry and pre-action systems therefore uses C = 100.
How closely must pressures balance at junctions?
At a node fed by two paths, pressures should generally balance within 0.03 bar (0.5 psi).
What goes on the hydraulic design information sign?
The location of the design area, the density and area or number of design sprinklers, the flow and pressure required at the base of the riser, and the hose allowance. The sign is permanently fixed to the riser.
Does NFPA 13 require a water supply safety margin?
The standard does not impose a universal margin figure, but authorities, insurers or clients often require one. Settle this at the start of the project.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13 (current edition) – plans and calculations; NFPA 291 – fire flow testing practice. Numeric values are general criteria; confirm against the edition in force and the authority having jurisdiction for each project. The findings here are typical defect patterns, not an account of events at any particular site.