EN 12845 design parameters for class OH4 — Ordinary Hazard Group 4: design density 5.00 mm/min, area of operation 360 m², water supply duration 60 minutes.
Which areas fall into this class?
Synthetic foam processing, paint manufacture and similar ordinary hazard areas with a high rate of spread.
Design parameters
| Parameter | Value |
|---|---|
| Design density | 5.00 mm/min |
| Area of operation (wet / alternate) | 360 m² |
| Water supply duration | 60 minutes |
| Theoretical minimum sprinkler flow | 1800 L/min |
| Theoretical minimum water volume | 108.0 m³ |
The theoretical minimum values come only from density × area. The real design flow is higher because of hydraulic imbalance and the minimum sprinkler pressure, and hose reel and hydrant flows are added on top.
Required pressure by K-factor
For a sprinkler covering 12 m², the required flow is 60.0 L/min. The pressure needed at different K-factors:
| K-factor | Required flow (L/min) | Required pressure (bar) |
|---|---|---|
| K57 | 60.0 | 1.11 |
| K80 | 60.0 | 0.56 |
| K115 | 60.0 | 0.27 |
| K160 | 60.0 | 0.14 |
Where the result falls below the minimum sprinkler pressure the standard requires, the minimum is used and the flow recalculated.
Dry and pre-action systems
Because water arrives late in dry and pre-action systems, the area of operation is increased. The increase depends on the standard chosen; NFPA 13 requires the design area to be increased by 30% for dry and pre-action systems.
Classification is a decision; the table is a consequence. Whether an area falls into class OH4 — Ordinary Hazard Group 4 is determined by assessing its actual activity, stored materials and fire load, and defended with a written justification in the design report. If the classification is wrong, every number here is invalid.
Checklist
- Has the area's activity and stored material been defined in writing?
- Have sections of the same building in different classes been calculated separately?
- Where there is storage, have the storage rules been applied as well?
- Have hose reel and hydrant flows been added to the total demand?
- Have the water supply duration and volume been verified?
- Is there any planned operational change that could alter the class?
Frequently Asked Questions
What is the design density for class OH4?
Under EN 12845, 5.00 mm/min over a 360 m² area of operation, with a 60 minute water supply duration.
What minimum water volume does OH4 need?
A theoretical minimum of 108.0 m³ (density × area × duration). The real volume is higher because of hydraulic imbalance, minimum sprinkler pressure and hose reel and hydrant flows.
Do these values change on a dry system?
The area of operation is increased. NFPA 13 requires the design area to be increased by 30% for dry and pre-action systems.
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 Store
MEP Calc — 86+ Engineering Calculators
MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 · NFPA 13 (2025). Table values are for information; the binding text is the current edition of the standard, and separate rules apply to storage.