EN 12845 design parameters for class LH — Light Hazard: design density 2.25 mm/min, area of operation 84 m², water supply duration 30 minutes.
Which areas fall into this class?
Table A.1 lists only three types: certain areas of schools and other educational establishments, certain areas of offices (both only where no single compartment exceeds 126 m² and fire resistance is at least 30 min, clause 6.2.2) and prisons. Hospitals, hotels and libraries are OH1, not LH. See the Annex A article for the full list and edge cases.
Design parameters
| Parameter | Value |
|---|---|
| Design density | 2.25 mm/min |
| Area of operation (wet / pre-action) | 84 m² |
| Area of operation (dry / alternate) | Not allowed (use OH1) |
| Water supply duration (8.1.1) | 30 minutes |
| Theoretical minimum sprinkler flow | 189 L/min |
| Theoretical minimum water volume | 5.7 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 LH is 21.0 m² (Table 19). A sprinkler covering that area needs 47.2 L/min. Table 37a permits nominal K57 sprinklers in this class, and under 13.4.4 the pressure at the most unfavourable sprinkler may not be less than 0.70 bar.
| K-factor | Required flow (L/min) | Pressure for density (bar) | Table 37a | Pressure to use (bar) |
|---|---|---|---|---|
| K57 | 47.2 | 0.69 | Permitted | 0.70 |
| K80 | 47.2 | 0.35 | Not used in this class | — |
| K115 | 47.2 | 0.17 | Not used in this class | — |
| K160 | 47.2 | 0.09 | Not used in this class | — |
Where the pressure calculated for the density is below the 0.70 bar floor of 13.4.4, 0.70 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 LH; where one is needed, the design uses OH1 criteria (5.0 mm/min, dry/alternate area of operation 90 m²). Pre-action installations use the wet area (84 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 LH — Light Hazard 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 LH?
Under EN 12845 Table 3, 2.25 mm/min over an area of operation of 84 m², with a 30 minute water supply duration (8.1.1).
What minimum water volume does LH need?
A theoretical minimum of 5.7 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 LH; the design uses OH1 criteria (5.0 mm/min over 90 m²). Pre-action installations use the wet area.
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Download MEP Calc on the App StoreEN 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.