How is the hazard class set in EN 12845?
EN 12845 groups protected areas by fire load and combustibility into three main families: Light Hazard (LH), Ordinary Hazard (OH1–OH4) and High Hazard. High hazard splits again into HHP1–HHP4 for production and process areas and HHS1–HHS4 for storage. The standard's Annex A links typical facility types to these classes, and its tables are a minimum classification: where the real use is more severe, choosing a higher class is the designer's responsibility.
The mappings in this calculator are checked, together with our Annex A article, against the Annex A tables (A.1, A.2, A.3) of BS EN 12845:2015+A1:2019; these tables are unchanged in the 2015+A2:2026 edition. The standard lists saw mills both as OH4 and as HHP2, so they appear as two separate entries. For occupancies not listed, the tables serve as guidance and the class is assessed for the specific facility.
Design points: density, area, duration
Unlike the NFPA 13 curves, each EN 12845 class has one fixed design point. The OH classes share the same density and differ only in area of operation; the HHP classes share the same area while the density rises.
| Class | Density | Area (wet) | Area (dry/alternate) | Duration | Minimum demand |
|---|---|---|---|---|---|
| LH | 2.25 mm/min | 84 m² | not permitted → OH1 | 30 min | 189 L/min |
| OH1 | 5.0 mm/min | 72 m² | 90 m² | 60 min | 360 L/min |
| OH2 | 5.0 mm/min | 144 m² | 180 m² | 60 min | 720 L/min |
| OH3 | 5.0 mm/min | 216 m² | 270 m² | 60 min | 1080 L/min |
| OH4 | 5.0 mm/min | 360 m² | not permitted → HHP1 | 60 min | 1800 L/min |
| HHP1 | 7.5 mm/min | 260 m² | 325 m² | 90 min | 1950 L/min |
| HHP2 | 10.0 mm/min | 260 m² | 325 m² | 90 min | 2600 L/min |
| HHP3 | 12.5 mm/min | 260 m² | 325 m² | 90 min | 3250 L/min |
| HHP4 | No numerical point — deluge / special protection | ||||
A handy identity: mm/min × m² = L/min. For water volume, multiply by duration: for OH3, 1080 L/min × 60 min = 64.8 m³. That is a theoretical minimum without overrun; in a real hydraulic calculation the remote-area sprinklers run above minimum pressure, so the flow grows. For tank sizing, use the fire water tank calculator.
Dry and alternate systems
For dry pipe and alternate (wet/dry) systems the area of operation is increased by 25%. There are two exceptions: LH does not permit dry systems, so at least the OH1 dry criterion (90 m²) applies; OH4 has no dry row, so the design moves up to the HHP1 dry criterion (7.5 mm/min over 325 m²). The calculator applies these upgrades automatically and shows them in the class tile, for example "OH4 → HHP1".
Coverage and spacing per sprinkler
For standard sprinklers the maximum area per sprinkler is 21 m² for LH, 12 m² for OH and 9 m² for HHP, with maximum spacing of 4.6 m, 4.0 m and 3.7 m respectively. Dividing the area of operation by that coverage gives the minimum number of sprinklers expected to open together. The scaled square in the visual shows the area of operation and those sprinklers; the dashed squares are the areas of the other classes.
Common classification mistakes
- Treating hotels and hospitals as LH: Annex A lists them as OH1. LH is only for certain school and office areas and prisons; if a compartment exceeds 126 m² or lacks 30 minutes' fire resistance, the class rises to OH1.
- Calling a car park light hazard: car parks are OH2; plastic vehicle components and fuel load rule out LH.
- Missing the paint zone: a paint shop or similar high fire-load area inside an OH1 or OH2 occupancy is protected as OH3.
- Solving HHP4 with the sprinkler table: for HHP4 processes such as fireworks manufacture the standard gives no numbers; a deluge or foam-water system is designed for the project.
If there is storage: Annex A classifies the facility type only. Stored goods are categorised through Annex B (material factor) and Annex C (alphabetical commodity list); if the height exceeds the OH limits, the area is designed separately as HHS. Where process and storage are openly connected, the higher class governs the whole zone.
Facilities not on the list
Annex A is also guidance for occupancies it does not name. Find the closest analogue, compare fire load and process conditions, and round up when in doubt. Recording the decision with clause and table references, together with the insurer's view, makes approval by the authority far smoother.
Frequently Asked Questions
What EN 12845 hazard class is a shopping centre?
Shopping centres, department stores and shops are OH3 in Annex A: 5.0 mm/min × 216 m² = 1080 L/min minimum sprinkler demand for 60 minutes (≈ 65 m³). If in-store storage exceeds the OH limits, that zone is designed separately as HHS.
Are hospitals and hotels LH or OH1?
In EN 12845 Annex A, hospitals, hotels, restaurants and libraries (excluding book stores) are on the OH1 list: 5.0 mm/min × 72 m² for 60 minutes. LH is reserved for certain school and office areas and prisons.
How does a dry system change the area of operation?
For dry and alternate systems the area grows by 25%: OH1 90 m², OH2 180 m², OH3 270 m², HHP 325 m². LH does not permit dry systems (use at least OH1 dry), and OH4 moves up to the HHP1 dry criterion.
Why are there no numbers for HHP4?
HHP4 covers processes with very fast fire growth, such as fireworks manufacture, and EN 12845 gives no density/area values for it. Protection is by deluge or foam-water, designed for the specific project with a risk assessment and authority approval.
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