EN 12845 Pre-Calculated OH Design

Part table, part calculation for OH1–OH4: when the pre-calculated route works, and where it jams.

At contract stage on a 4200 m² metal goods warehouse, the mechanical team asked: "For this OH3 area, do we produce a full hydraulic calculation or take it from the table?" A quick comparison of the hours involved and the error margin answered itself: in a single-storey, tree-layout, non-gridded OH installation, the EN 12845 pre-calculated method saves weeks of CAD-and-hydraulics iteration. But draw a layout that spills outside the table limits and the same method walks you quietly into under-design.

This article looks at the pre-calculated system clauses and the OH pipe size tables from a site perspective, covering sprinkler counts, K-factor, minimum pressure, range and distribution sizes, the wet-versus-dry difference and the NFPA pipe schedule equivalents.

The pre-calculated logic

EN 12845 recognises two design routes: pre-calculated (part table, part calculation) and fully calculated (every size hydraulically derived). The pre-calculated route is prohibited in three layouts:

• Gridded or looped pipe arrangements
• Rack protection containing intermediate in-rack sprinklers
• Mixed OH3/OH4 with high hazard areas, outside the specific mixing clause

For the classic tree-layout OH projects that remain, the table shortens the hydraulic cycle so much that roughly 70 % of OH installations across Europe and Turkey are still produced this way.

In summary: range pipe sizes and maximum sprinkler counts come straight from the range pipe table. Distribution pipe sizes come from the distribution table. Everything between the distribution design point and the control valve set is hydraulically calculated within a 0.5 bar friction limit at 1000 L/min. Branches from the table, the trunk from calculation.

Sprinkler counts and area of operation for OH

EN 12845 divides ordinary hazard into four groups, all at 5.0 mm/min density; what changes is the area of operation.

Hazard	Density	Wet area	Dry area (+25 %)	Pre-calc sprinklers
OH1	5.0 mm/min	72 m²	90 m²	about 6
OH2	5.0 mm/min	144 m²	180 m²	about 12
OH3	5.0 mm/min	216 m²	270 m²	about 18
OH4	5.0 mm/min	360 m²	450 m²	about 30 (usually fully calculated)

The counts assume a maximum coverage of 12 m² per sprinkler. In OH4, the number of sprinklers on a single line strains the pipe size limits, so designers generally move to fully calculated design; pre-calculated is not prohibited but is no longer optimal.

K-factor and minimum sprinkler pressure

The standard sprinkler for OH is K80. Delivering 5.0 mm/min over 12 m² needs 60 L/min per head. From Q = K√(10p), the mathematical minimum is p = (60/80)²/10 = 0.056 bar. EN 12845 sets a field safety floor of 0.35 bar minimum sprinkler pressure — absorbing static and friction losses along a horizontally and vertically routed range pipe, and preserving density if sprinklers outside the design area also operate.

In practice the end head is the calculation point; once 0.35 bar is achieved at the most remote design point, you work back with Hazen-Williams to find the pressure at the control valve set. The pre-calculated table gives you that figure directly: 1100 L/min at 1.7 + ps bar for OH3 wet.

Range and distribution pipe sizes

The range pipe table says two things: the last two branches at the remote end may be smaller because they feed fewer sprinklers; and inner branches, carrying more sprinklers, must step up in size.

Range pipe position	Size (mm)	Max. sprinklers
Last two branches at the remote end (two-end-side)	25	1
	32	2
All other branches	25	2
	32	3
	40	4
	50	9

The distribution pipe table:

Distribution pipe	Size (mm)	Max. sprinklers
Two-end-side at the end of the installation	32 / 40 / 50 / 65	2 / 4 / 8 / 16
All other layouts	32 / 40 / 50 / 65	3 / 6 / 9 / 18
Design point to control valve	Entirely hydraulic

One reminder that is easy to miss: where the range pipe slope exceeds 6°, no more than six sprinklers may sit on a single line. Under a pitched canopy roof, that is a common oversight.

Wet versus dry: 25 % more area, more than 25 % more cost

The area of operation increases by 25 % in dry and alternate systems. That is not simply more sprinklers; it multiplies through pump flow and head pressure. Taking OH3:

System type	Area	Sprinklers	Design Q	Max. demand Q
OH3 wet / pre-action	216 m²	about 18	1100 L/min at 1.7 bar	1350 L/min at 1.4 bar
OH3 dry / alternate	270 m²	about 23	1100 L/min at 1.7 bar	1350 L/min at 1.4 bar

The table leaving design flow unchanged for dry looks misleading, but the additional heads from the larger area push range pipe sizes up a step. With hose reel and alarm station allowances, the pump selection point for OH3 wet lands at around 1500 L/min. In cold stores, unsealed car parks and automated rack feed areas, a dry system is unavoidable; but where thermal insulation is possible, wet remains the preference.

Pump characteristics

The pump table gives the pressure-flow characteristic for pre-calculated OH by sprinkler height. For OH3 wet with h ≤ 15 m: 1.9 bar at 2650 L/min, 3.0 bar at 2100 L/min and 3.5 bar at 1800 L/min — three points the pump supplier must demonstrate on test, or the design is not approved. In practice those figures are met by an electric or diesel pump of around 75 kW.

Comparison with the NFPA pipe schedule

NFPA 13's pipe schedule method carries the same logic: a size-versus-sprinkler-count table, a minimum head pressure and a hydraulic bypass. The only slot where both meet head on is ordinary hazard groups 1 and 2; group 3 and above requires fully hydraulic design on the NFPA side.

Item	EN 12845 pre-calculated	NFPA 13 pipe schedule
Scope	LH, OH1–OH4	LH and OH only, and limited for new buildings
Min. sprinkler pressure	0.35 bar	0.5 bar (about 7 psi)
Sprinkler K-factor	K80	K80 (US K5.6)
32 mm two-end-side branch	2 sprinklers	2 sprinklers
40 mm branch	4 sprinklers	4 sprinklers
50 mm branch	9 sprinklers	10 sprinklers
Gridded layout	Prohibited	Prohibited

NFPA is slightly bolder on the schedule sizes; EN 12845 is bolder on the lower pressure limit and on keeping the pre-calculated door open for OH4. On a project running both, apply both tables to the same layout and find the bottleneck — usually the limits bind about 5 % sooner under NFPA.

The five errors I see most often

• Treating the table limit as an average. Nine sprinklers on a 50 mm branch is a maximum, not a target. Exceed it and you move to fully calculated design automatically.
• Forgetting the hydraulics between distribution and control valve. The 0.5 bar per 1000 L/min limit is the part of design approval most often overlooked.
• Raising sizes a lot and sprinkler counts a little in a dry system. The 25 % area increase shifts both together.
• Skipping the sloped roof caveat. Above 6°, a maximum of six sprinklers per range.
• Insisting on pre-calculated design for OH4. Thirty sprinklers over 360 m² pushes the main above 100 mm on most projects; fully calculated is more efficient.

Turkish context

BYKHY accepts EN 12845 and NFPA 13 equally for sprinkler design. The pre-calculated route dominates OH1 (car parks, light manufacturing, empty office stores) and OH2 (general manufacturing, supermarket sales areas) projects in Turkey. It remains valid for OH3 — furniture manufacture, paper processing, plastic goods storage up to 4 m — but above about 1500 m², the pipe size optimisation from a hydraulic calculation pays for itself in material. OH4 goes straight to fully hydraulic design in field practice.

Frequently asked questions

Can pre-calculated design be used on every OH project?

No. EN 12845 requires fully hydraulic calculation for gridded or looped layouts and for installations with intermediate high-hazard storage sprinklers. It is the practical choice for pure tree-layout OH1 to OH3; OH4 is technically permitted but usually goes fully calculated.

Why is the OH3 wet pump flow 1350 rather than 1100 L/min?

The table defines two points for OH3: design flow of 1100 L/min at 1.7 + ps bar and maximum demand of 1350 L/min at 1.4 + ps bar. The pump characteristic must satisfy both, and selection is made at the maximum demand point — about 1500 L/min with the hose allowance.

Why does a dry system increase the sprinkler count?

Because the area of operation grows by 25 %. OH3 goes from 216 to 270 m², and at 12 m² per sprinkler the count rises from 18 to 23.

Why is the minimum sprinkler pressure 0.35 bar?

The mathematical minimum for 5.0 mm/min over 12 m² with a K80 head is 0.056 bar, but EN 12845 sets 0.35 bar as an absolute field safety floor — a buffer guaranteeing density at every head and absorbing pipe losses.

What is the main difference from the NFPA pipe schedule?

About 95 % of the limit values overlap. The critical difference: the NFPA schedule is open only to LH, OH1 and OH2, requiring fully hydraulic design from OH3 upward, while EN 12845 keeps the pre-calculated door open including OH4.

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