EN 12845 design parameters for class OH1 — Ordinary Hazard Group 1: design density 5.00 mm/min, area of operation 72 m², water supply duration 60 minutes.
Which areas fall into this class?
OH1 examples from Table A.2: hospitals, hotels, restaurants, libraries (excluding book stores), schools and offices (see 6.2.2), data processing / computer rooms (excluding tape storage), cement works and sheet metal product 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) | 72 m² |
| Area of operation (dry / alternate) | 90 m² |
| Water supply duration (8.1.1) | 60 minutes |
| Theoretical minimum sprinkler flow | 360 L/min |
| Theoretical minimum water volume | 21.6 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 OH1 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, the OH1 area of operation rises from 72 m² to 90 m² for dry or alternate (wet/dry) installations; the density does not change. Pre-action installations use the same area as wet installations (72 m²). Dry pipe installations are 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 OH1 — Ordinary Hazard Group 1 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 OH1?
Under EN 12845 Table 3, 5.0 mm/min over an area of operation of 72 m², with a 60 minute water supply duration (8.1.1).
What minimum water volume does OH1 need?
A theoretical minimum of 21.6 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 the area of operation rises to 90 m² for dry or alternate installations; pre-action installations use the wet area. The 30% increase of NFPA 13 does not apply to an EN 12845 design.
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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.