What is hazard classification in fire sprinkler design?

IS 15105 and NBC classify occupancies into three hazard groups: Light Hazard (offices, hospitals, hotels), Ordinary Hazard Group 1 and 2 (retail, light manufacturing, warehouses with moderate fire risk), and High Hazard (chemical storage, paint shops, heavy manufacturing). Hazard classification determines sprinkler density (L/min/m²) and the design area for hydraulic calculation.

How many sprinklers can operate simultaneously in a design?

IS 15105 requires hydraulic design based on the most hydraulically remote area. For Light Hazard: design area of 84 m² (typically 4 sprinklers). For Ordinary Hazard: 144 m² (typically 6–9 sprinklers). For High Hazard: 260 m² (typically 12–18 sprinklers). All sprinklers in the design area are assumed to operate simultaneously.

What is the minimum residual pressure at a sprinkler head?

IS 15105 requires a minimum residual pressure of 0.5 bar (50 kPa / 5 m WC) at the most hydraulically remote sprinkler head. At the pump outlet, the pressure must be sufficient to deliver the design flow at the required residual pressure at the most remote head, accounting for all pipe friction losses.

How long must the fire water supply last?

IS 15105 and NBC India require the fire water tank to provide water for a minimum of 30 minutes of sprinkler operation at the design flow rate, plus simultaneous hose reel / hydrant demand if applicable. For high-hazard occupancies or large buildings, 60-minute storage is often specified.

What is the difference between a wet pipe and dry pipe sprinkler system?

In a wet pipe system, pipes are always filled with pressurised water — the fastest response on head activation. In a dry pipe system, pipes contain pressurised air or nitrogen and water fills them only after activation — used in areas subject to freezing. Wet pipe systems are simpler, more reliable, and preferred for most commercial buildings in India.

Fire Sprinkler System Design — Water Demand and Tank Sizing

Automatic fire sprinkler systems are the most effective life-safety measure in modern commercial buildings. When correctly designed, they control or suppress 96% of fires before the fire brigade arrives. But designing a sprinkler system involves far more than placing heads on a layout drawing — it requires a rigorous hydraulic calculation to determine water demand, pump sizing, and fire water tank capacity as per IS 15105 and the National Building Code (NBC) of India.

Step 1 — Hazard Classification

Every sprinkler design begins with classifying the occupancy hazard. IS 15105 defines three hazard categories, each with different design densities and areas:

Hazard Class	Typical Occupancies	Design Density (L/min/m²)	Design Area (m²)
Light Hazard (LH)	Offices, hotels, hospitals, schools	2.25	84
Ordinary Hazard Gp 1 (OH1)	Retail, light assembly, libraries	5.0	72
Ordinary Hazard Gp 2 (OH2)	Warehouses, machine shops, stage areas	5.0	144
High Hazard Gp 1 (HH1)	Paint spraying, foam plastics	7.5–10	260
High Hazard Gp 2 (HH2)	Chemical storage, flammable liquids	12.5+	260

Step 2 — Sprinkler Head Layout

Standard sprinkler coverage areas per IS 15105:

Light Hazard: Maximum 21 m² per head, maximum spacing 4.6m between heads
Ordinary Hazard: Maximum 12 m² per head, maximum spacing 4.0m
High Hazard: Maximum 9 m² per head, maximum spacing 3.7m

Sprinkler heads must also be positioned within 2.0m of walls in ordinary hazard and within 1.5m in high hazard occupancies. Installation must comply with IS 15105 Chapter 7 for correct deflector-to-ceiling distances (75–150mm for standard pendant heads).

Step 3 — Design Water Demand Calculation

The design flow is calculated from the most hydraulically remote area (typically the highest floor, furthest corner from the pump). All sprinklers within the design area are assumed to operate simultaneously.

Design Flow (Q_design): Q = Density × Design Area Q = 2.25 L/min/m² × 84 m² (Light Hazard example) Q = 189 L/min = 3.15 L/s Individual Sprinkler Flow (q): For 9 sprinklers in 84m² design area: q_avg = 189 / 9 = 21 L/min per head Minimum Flow per Head (IS 15105): q = K × √P Where: K = K-factor of sprinkler head (typically K=80 for standard heads) P = Operating pressure at head (bar) For q = 21 L/min: P = (q/K)² = (21/80)² = 0.069 bar = 6.9 kPa Minimum pressure = 0.5 bar per IS 15105 Design at 0.5 bar minimum → q = 80 × √0.5 = 56.6 L/min per head Revised total design flow (at 0.5 bar): Q = 9 heads × 56.6 = 509 L/min = 8.5 L/s

Step 4 — Hose Reel / Hydrant Demand (Simultaneous)

NBC and IS 15105 require that sprinkler water demand is supplemented by simultaneous hose reel or hydrant demand during a fire event:

System	Simultaneous Demand
Hose reels (light hazard)	400–600 L/min total
Internal hydrants (OH)	900 L/min (2 hoses × 450 L/min)
External hydrants	As per NBC / local fire authority

Total System Flow: Q_total = Q_sprinkler + Q_hose Light Hazard Example: Q_total = 509 L/min (sprinkler) + 450 L/min (2 hose reels) = 959 L/min = 15.98 L/s ≈ 16 L/s

Step 5 — Fire Pump Sizing

Pump Flow: Q = 16 L/s = 57.6 m³/h Pump Head (TDH): Static head (ground to topmost sprinkler): 30m Pipe friction losses: ~10m (estimated) Residual at topmost head: 5m (0.5 bar) TDH = 30 + 10 + 5 = 45m Pump Motor Power: P = (1000 × 9.81 × 0.016 × 45) / (0.70 × 0.90 × 1000) = 7063 / 630 = 11.2 kW Select: 15 kW fire pump (next standard size above 11.2 kW) Also provide: - Jockey (pressure maintenance) pump: 1–2 L/s - Diesel-engine backup fire pump (for generator-independent operation)

Step 6 — Fire Water Tank Sizing

IS 15105 requires a minimum of 30 minutes of fire water storage at the design flow rate. For buildings with both sprinkler and hydrant systems, the tank must supply both simultaneously for the required duration.

Fire Water Tank Volume: V = Q_total × Duration (minutes) × 60 seconds / 1000 V = 959 L/min × 30 min / 1000 = 28,770 litres ≈ 29 m³ Add 20% for pump inlet submergence and dead storage: V_total = 29 × 1.20 = 34.8 m³ → Select 35,000 L tank Note: - NBC Table 20 specifies tank volumes by occupancy and floor area - For buildings >15m height: minimum 50,000L per NBC Part 4 - Tank must be exclusively dedicated to fire use - Provide overflow, vent, drain, and level gauge

Use the Sprinkler System Calculator to compute design flow and head counts, and the Fire Water Tank Calculator to determine your minimum tank volume — both per IS 15105 and NBC.

Types of Sprinkler Systems

System Type	Pipe Condition	Best For
Wet Pipe	Always filled with water	Most commercial buildings in India
Dry Pipe	Filled with compressed air	Cold storage, unheated areas
Pre-action	Air — water on detection signal	IT rooms, museums, libraries
Deluge	Open heads, water on demand	High-hazard, chemical plants

Common Fire Sprinkler Design Mistakes

Sizing for density only, ignoring minimum head pressure: IS 15105 requires a minimum 0.5 bar at every operating head. Many designs compute flow from density but do not verify head pressure — leading to under-pressure remote heads that cannot deliver adequate flow.
Not including hose reel demand in tank sizing: The fire water tank must supply sprinklers + hose reels simultaneously. Sizing only for sprinklers will result in an undersized tank.
Sharing the fire tank with domestic water supply: IS 15105 is explicit — the fire reserve must be dedicated and cannot be consumed by domestic use. Use separate compartments or separate tanks with a non-return valve.
Skipping the hydraulic most-remote-area check: Always identify and calculate the most hydraulically remote area — typically the top floor, furthest corner. This governs pump head, not the nearest area to the pump.

Conclusion

Designing a fire sprinkler system that meets IS 15105 and NBC requirements involves four key steps: classify the hazard, layout sprinkler heads correctly, calculate design water demand hydraulically, and size the fire pump and tank for the combined sprinkler and hose demand over the required duration. A correctly designed system saves lives and property — and ensures building approval from fire authority having jurisdiction (AHJ).

Use the MEPMate Sprinkler System Calculator and Fire Water Tank Calculator for fast, code-compliant fire system design on every project.