A sprinkler system is only as strong as its water supply. The best layout and the most careful hydraulic calculation remain on paper if they are connected to an inadequate source. The water supplies part of NFPA 13 defines which sources may feed a system, how a source is evaluated and for how long the demand must be met. This article is part of the NFPA 13 Clause Guide series.
Acceptable types of water supply
NFPA 13 does not insist on one type of source; it requires a reliable supply that meets system demand for the required duration. Typical sources:
- Public mains: may be used directly if they provide adequate, reliable flow and pressure. In Turkey mains pressure varies through the day and across seasons, so mains alone are rarely accepted.
- Fire pumps: taking suction from a tank or the mains and installed to NFPA 20, which governs the pump itself.
- Tanks: above-ground, buried and elevated (gravity) tanks. Tank design is covered by NFPA 22.
- Pressure tanks: limited-volume solutions for small systems.
- Fire department connection: an auxiliary supply route, never a water supply in its own right.
The fire department connection is not a water supply. It lets the fire service pump extra water into the system. Showing it as the source in the hydraulic calculation means the system is dry until the brigade arrives.
Evaluating the supply: the flow test
For mains-fed systems, design rests on three values: static pressure, residual pressure while flowing and the test flow. Together they define the supply curve, and the hydraulic demand must fall below it. The test used for design is expected to be recent; many AHJs require data from within the last 12 months. The procedure is explained in our hydrant flow test article.
When evaluating the supply curve:
- Test close to peak demand hours, or adjust the results for daily and seasonal drops.
- Account for elevation and losses between the test point and the system connection.
- Leave a safety margin between the demand and the supply curve. NFPA 13 does not mandate a specific margin, but common practice is at least 0.7 bar (10 psi) or around 10%. Check local specifications.
Hose allowance and duration
For non-storage occupancies designed by the density/area method, inside and outside hose allowances are added to the sprinkler demand and the total must be available for a set duration. Recent editions give:
| Hazard class | Total hose allowance (inside + outside) | Duration |
|---|---|---|
| Light hazard | 380 L/min (100 gpm) | 30 minutes |
| Ordinary hazard | 950 L/min (250 gpm) | 60–90 minutes |
| Extra hazard | 1,900 L/min (500 gpm) | 90–120 minutes |
Where a range is given, the lower value is generally used when waterflow alarms are transmitted to a monitored station; otherwise use the upper value. Storage designs, ESFR and CMSA have their own duration and hose criteria. The inside hose allowance applies only if an inside hose system exists, and it forms part of the total rather than being added on top.
How big should the tank be?
Tank volume is roughly (sprinkler flow + hose allowance) × duration. The sprinkler flow here is the actual flow from the hydraulic calculation, not density × area; overpressure means sprinklers discharge more than the calculated minimum. With automatic refill, the refill rate may be deducted from the volume if the AHJ accepts it. For a first estimate use the fire water tank sizing calculator; for tank details read NFPA 22 fire water tank design.
Points to check on pumped systems
- The rated pump flow must cover system demand, and demand must not exceed the 150% point on the pump curve.
- Churn pressure plus suction pressure must stay within the component pressure rating.
- Check NPSH on the suction side; see the fire pump NPSH calculator.
- For pump selection read NFPA 20 fire pump sizing.
The practical picture in Turkey
In Turkey, mains pressure and continuity are rarely considered reliable enough to feed a sprinkler system on their own. The most common solution is therefore a fire water tank plus fire pump, with the mains used to refill the tank. NFPA 13's water supply rules then set volume and duration, NFPA 20 governs the pump and NFPA 22 the tank, while the national regulation adds its own minimums; where the two differ, the stricter applies. For the national pump and tank requirements see Turkish regulation on fire pumps and water tanks. Remember too that the usable volume is smaller than the gross volume because of sediment allowance, suction pipe position and the anti-vortex plate; always calculate on usable volume.
Common mistakes
- Designing on a flow test that is years old or was taken somewhere else on the network.
- Showing the fire department connection as the water supply.
- Sizing the tank on density × area and ignoring the actual hydraulic flow.
- Adding the hose allowance twice (inside and outside separately on top of the total) or not at all.
- Assessing mains pressure from a night-time reading and missing the daytime drop.
- Forgetting suction losses and anti-vortex requirements at the tank outlet.
Water supply checklist
- Is the flow test recent and adjusted to the connection point?
- Is there an adequate margin between demand and supply curve?
- Do hose allowance and duration match the hazard class?
- Was the tank sized on the actual hydraulic flow?
- Is there a plan for supply outages (maintenance, mains failure)?
For a side-by-side look at source types, read fire protection water supplies.
Frequently Asked Questions
Why is tank volume not calculated from density × area?
Because of pressure distribution, sprinklers in the hydraulic calculation discharge more than the minimum. The tank must be sized on the actual total flow from the calculation plus the hose allowance, or it may run dry before the duration is reached.
What is the water supply duration for light hazard?
For light hazard systems designed by the density/area method, 30 minutes with a total hose allowance of 380 L/min (100 gpm).
Does the fire department connection count as a water supply?
No. It is an auxiliary supply route. The system needs its own adequate and reliable water supply.
How recent must the flow test be?
Many AHJs require a test from within the last 12 months, and a newer one if the network has changed. Confirm the exact requirement with the AHJ.
How much safety margin should I leave?
NFPA 13 does not set one. Common practice is at least 0.7 bar (10 psi) or roughly 10%; some specifications and insurers ask for more.
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, Standard for the Installation of Sprinkler Systems (current edition), water supplies and hose allowance/duration criteria; NFPA 20; NFPA 22; NFPA 291 (flow testing). Confirm values against the edition in force and with the AHJ. The findings here are typical defect patterns, not an account of events at any particular site.