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?
OH4 examples from Table A.2: cinemas and theatres, concert halls, exhibition halls, film and TV production studios, tobacco factories, alcohol distilleries, waste paper processing, cotton mills, flax and hemp preparation plants, saw mills and plywood factories. See the Annex A article for the full list and edge cases.
Design parameters
| Parameter | Value |
|---|---|
| Design density | 5.0 mm/min |
| Area of operation (wet / pre-action) | 360 m² |
| Area of operation (dry / alternate) | Not allowed (use HHP1) |
| Water supply duration (8.1.1) | 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
The maximum area per sprinkler for OH4 is 12.0 m² (Table 19). A sprinkler covering that area needs 60.0 L/min. Table 37a permits nominal K80 / K115 sprinklers in this class, and under 13.4.4 the pressure at the most unfavourable sprinkler may not be less than 0.35 bar.
| K-factor | Required flow (L/min) | Pressure for density (bar) | Table 37a | Pressure to use (bar) |
|---|---|---|---|---|
| K57 | 60.0 | 1.11 | Not used in this class | — |
| K80 | 60.0 | 0.56 | Permitted | 0.56 |
| K115 | 60.0 | 0.27 | Permitted | 0.35 |
| K160 | 60.0 | 0.14 | Not used in this class | — |
Where the pressure calculated for the density is below the 0.35 bar floor of 13.4.4, 0.35 bar is used and the flow recalculated from Q = K√P.
Dry, alternate and pre-action installations
Under EN 12845 Table 3, dry or alternate (wet/dry) installations are not allowed for OH4; where one is needed, the design uses HHP1 criteria (7.5 mm/min, dry/alternate area of operation 325 m²). Pre-action installations use the wet area (360 m²). Dry pipe installations are in any case only installed where there is a risk of frost or the temperature exceeds 70 °C (11.2.1). NFPA 13 instead increases the design area by 30% for dry pipe and double interlock pre-action systems (19.2.3.2.6); the rules of the two standards are not interchangeable.
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 Table 3, 5.0 mm/min over an area of operation of 360 m², with a 60 minute water supply duration (8.1.1).
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?
Under EN 12845 Table 3 dry or alternate installations are not allowed for OH4; the design uses HHP1 criteria (7.5 mm/min over 325 m²). Pre-action installations use the wet area.
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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) · NFPA 13 (2025). Table values are for information; the binding text is the current edition of the standard, and separate rules apply to storage.