EN 12845 Ordinary Hazard (OH1-OH4) Design

Hotels, shopping centres, production lines, tyre stores — four different flow worlds under the same "OH" heading.

Revising the sprinkler calculation on a mixed hotel and shopping centre, the architect asked "if it's all one building, isn't it all the same hazard?" The bedroom floors came out OH1, the common areas and restaurants OH2, the car parks OH2, and the bonded stores OH3. The same standard, the same 5.0 mm/min density — and yet the calculated flow more than doubled from one end of the building to the other. That is how the OH classes work in EN 12845: density is fixed, the area of operation varies.

How EN 12845 defines the OH classes

EN 12845 defines ordinary hazard as covering places containing combustible materials with a moderate fire load and moderate combustibility, divided into four sub-groups: OH1, OH2, OH3 and OH4. The distinction is based on the expected rate of fire growth and its spread potential. The classification annex lists typical occupancies, and about 80 % of site decisions come from that table.

OH class	Typical occupancies
OH1	Hotels, hospitals, schools, prisons, sheet metal product factories, cement works, dairies, biscuit and chocolate works, data centres (excluding tape storage), offices
OH2	Restaurants, abattoirs, bakeries, photographic film factories, metalworking, car parks, laundries, leather goods factories, laboratories, museums, railway stations
OH3	Shopping centres and shops, furniture showrooms (excluding foam plastics), cinemas and theatres, concert halls, animal feed mills, carpet factories, shoe factories (excluding rubber), timber processing, cardboard and carton works, water-based paint application shops
OH4	Exhibition halls, waste paper processing, cotton mills, flax and hemp preparation, plywood factories, cutlery works, tobacco factories, alcohol distilleries

The table is guidance, not a binding list; it is interpreted against actual use. A note in the annex says explicitly that where an OH1 or OH2 area contains a paint shop or similar high fire load area, that zone counts as OH3. If a hotel lobby has heavy textile furnishings and dense furniture clusters, the OH1 decision should not be extended over it.

Design density and area of operation: 5.0 mm/min fixed, area variable

EN 12845 gives a design density of 5.0 mm/min for all four OH classes. The only thing that changes is the area of operation — the area that must be supplied simultaneously.

Class	Density	Wet / pre-action area	Dry / alternate area
OH1	5.0 mm/min	72 m²	90 m²
OH2	5.0 mm/min	144 m²	180 m²
OH3	5.0 mm/min	216 m²	270 m²
OH4	5.0 mm/min	360 m²	Not permitted

Two things stand out. First, dry systems are prohibited for OH4; the standard says "not allowed" and directs you to high-hazard storage protection instead. The delay in water delivery is not an acceptable risk for OH4 fire scenarios. Second, dry and alternate systems increase the area by 25 % in every class. That is how EN 12845 compensates for trip time and water delivery delay, on very similar logic to NFPA 13.

Worked example: OH2 wet system

Take an OH2 wet pre-calculated design in a shopping centre common area: density 5.0 mm/min over 144 m². The base flow:

Q_design = 5.0 mm/min × 144 m² = 720 L/min

Hose reel allowance and overrun are then added; the pre-calculated table does that for you:

Class and system	Required flow (L/min)	Pressure at the control valve (bar, excluding ps)
OH1 wet / pre-action	375	1.0 + ps
OH1 dry / alternate	540	0.7 + ps
OH2 wet / pre-action	725	1.4 + ps
OH2 dry / alternate	1000	1.0 + ps
OH3 wet / pre-action	1100	1.7 + ps
OH3 dry / alternate	1350	1.4 + ps
OH4 wet / pre-action	1800	2.0 + ps
OH4 dry / alternate	2100	1.5 + ps

Here ps is the static pressure difference between the highest design point and the control valve. Pure hydraulics gives 720 L/min for OH2 wet while the pre-calculated table says 725 L/min; the difference is the built-in allowance for hose reels and internal losses.

Sprinkler selection: K80 and the 0.35 bar minimum

The standard sprinkler K-factor for OH is K80 (EN notation; NFPA K5.6). A minimum of 0.35 bar at each sprinkler is required for the OH classes. That means a K80 head discharges about 47 L/min, which at typical 12 m² spacing is almost exactly 4.0 mm/min. Achieving the 5.0 mm/min target needs somewhat higher pressure at the actual design point — in practice 0.5–0.8 bar.

In the large areas of operation for OH3 and OH4, pipe losses grow quickly and the 0.35 bar limit becomes the binding constraint at the end sprinklers. There are two options: increase pipe sizes, or move to K115 or K160 to deliver the same flow at lower pressure. The choice is usually driven by available ceiling depth and height.

Common errors

• Assuming a car park is OH1. The annex places car parks at OH2, because the fire load is not one vehicle but the potential for sequential ignition down a row.
• Writing "the whole hotel is OH1". The kitchen may be OH3, the car park OH2 and the boiler room OH1. Take the highest hazard within the area fed by one valve set.
• Forgetting the 25 % dry system increase. The most common error I see — the calculation was done wet, the project was later converted to dry, and the flow report was never updated.
• Proposing a dry system for OH4. Prohibited. Where cold storage forces it, the design must move to high-hazard storage protection instead.
• Mixing pre-calculated rows across classes. Pre-calculated figures are given per hazard class. In a mixed OH2/OH3 area, the higher class governs.

Comparison with NFPA 13 Ordinary Hazard

NFPA 13 divides ordinary hazard into Group 1 and Group 2, with a density-area curve rather than a fixed density — density falls as area grows. The four fixed EN 12845 steps (5.0 mm/min over 72/144/216/360 m²) are a piecewise version of the NFPA curve.

Comparison	EN 12845 OH	NFPA 13 OH
Number of sub-classes	4 (OH1–OH4)	2 (Group 1, Group 2)
Density structure	Fixed 5.0 mm/min	A curve, inversely related to area
Typical OH2 / OG2 point	5.0 mm/min × 144 m² = 720 L/min	about 6.1 mm/min × 139 m² ≈ 850 L/min
Minimum sprinkler pressure	0.35 bar	about 0.5 bar (7 psi)
Dry system area increase	+25 %	+30 %
Hotel classification	OH1	Light hazard

Note that a hotel is light hazard under NFPA but OH1 under EN. Designing the same building to both standards, EN always comes out more conservative. Because BYKHY references EN 12845, OH1 must be used for hotel projects in Turkey.

Turkish field practice

BYKHY points explicitly to EN 12845 for sprinkler systems. The decisions used day to day:

• In mixed hotel and restaurant buildings, restaurant kitchens are taken as OH3, with the extract hood separately protected under NFPA 96 logic.
• Shopping centre atria are OH3 and shop interiors OH2 or OH3 depending on merchandise. A back-of-shop store is either OH3 or moves to HHS depending on storage height.
• In data centres, the server room follows NFPA 75 with a clean agent; surrounding plant areas are OH1.
• In hospitals, clinics and bedroom floors are OH1, sterilisation and theatres OH2, generator and boiler rooms OH1, and store floors OH3.

Where different OH classes sit under one valve group, the highest class generally governs. Rather than calculating from a single point and calling it the worst case, zoning the valve outlets by hazard improves both cost and operational response.

Frequently asked questions

What is the practical difference between OH1 and OH2?

Density is 5.0 mm/min in both; the difference is the area of operation — 72 m² wet for OH1 and 144 m² for OH2. Hotel bedroom floors are OH1; restaurants and shopping centre common areas are OH2. In practice that roughly doubles the calculated flow.

What changes with a dry or alternate system?

The area of operation increases by 25 %, so OH2 goes from 144 to 180 m². Dry systems are not accepted at all for OH4; the table says "not allowed" and directs you to high-hazard storage protection.

Is the pre-calculated method valid for every OH class?

Yes — EN 12845 defines the pre-calculated method for LH and OH. An OH3 wet system must provide 1100 L/min at 1.7 + ps bar at the design point. Full hydraulic calculation is not mandatory but is preferred for complex or mixed high-hazard systems.

Is K80 sufficient in every OH class?

K80 is the standard and typical choice for OH, delivering 5.0 mm/min in most spacing geometries at the 0.35 bar minimum. But where hydraulic losses grow across the large OH3 and OH4 areas, K115 or K160 allows smaller pipe sizes.

Do NFPA Groups 1 and 2 map onto EN OH1–OH4?

No. NFPA divides ordinary hazard into two groups, EN into four, and the density-area basis differs: NFPA OH2 is about 6.1 mm/min over 139 m², EN OH2 a fixed 5.0 mm/min over 144 m². Designing the same building to both gives similar but not identical flows.

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.

Download SprinkCalc on the App Store

MEP Calc — 86+ Engineering Calculators

MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.

Download MEP Calc on the App Store