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?
Offices, classrooms, hospital patient rooms, hotel bedrooms and library reading rooms — spaces with a low fire load and slow rate of spread.
Design parameters
| Parameter | Value |
|---|---|
| Design density | 2.25 mm/min |
| Area of operation (wet / alternate) | 84 m² |
| Water supply duration | 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
For a sprinkler covering 12 m², the required flow is 27.0 L/min. The pressure needed at different K-factors:
| K-factor | Required flow (L/min) | Required pressure (bar) |
|---|---|---|
| K57 | 27.0 | 0.22 |
| K80 | 27.0 | 0.11 |
| K115 | 27.0 | 0.06 |
| K160 | 27.0 | 0.03 |
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 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, 2.25 mm/min over a 84 m² area of operation, with a 30 minute water supply duration.
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?
The area of operation is increased. NFPA 13 requires the design area to be increased by 30% for dry and pre-action systems.
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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.