EN 12845 Pre-Calculated Design for HHP and HHS

The narrow corridor of the high-hazard pre-calculated method: when you can trust the table, and when fully calculated design is unavoidable.

On a sprinkler revision at a chemical packaging plant, the designer had drawn the HHP2 area pre-calculated, taken pipe sizes from the tables and closed the calculation on a single sheet. On site, the goods were being racked — which meant we were approaching HHS3 with ST4 racking, and close to needing in-rack protection. The moment the table ran out, so did the method. This article defines that boundary: how narrow a box the EN 12845 pre-calculated method is for high hazard, and under what conditions you can open its lid.

What the clause covers

The pre-calculated method is set out generally, with a sub-clause written specifically for HHP and HHS. The parenthesis in its heading sets the boundary: except intermediate level sprinklers. Any system with in-rack sprinklers is outside the pre-calculated method, and fully calculated design becomes mandatory.

The logic of pre-calculated design is simple: range pipe sizes and the distribution pipe sizes below the design point come from tables in the standard, while the section between the design point and the control valve set is calculated hydraulically. Part table, part calculation.

Four parameters that set the sizing

Pipe size selection depends on:

• Design density (HHP1: 7.5 mm/min, HHP2: 10.0 mm/min, HHP3: 12.5 mm/min)
• Sprinkler spacing and layout arrangement
• K-factor — K80 or K115 for pre-calculated design, no higher
• The pressure and flow characteristic of the water supply

If any one of these steps outside the pre-calculated box, you must move to fully calculated design. No pipe may be smaller than 25 mm nominal, which sets the lower bound of the tables.

Matching table to water supply

This is the point most often confused on site. The standard defines four water supply characteristics, and the choice of pre-calculated table is tied entirely to that row:

Water supply	K-factor	Range pipe	Distribution pipe
Supply type 1	K80	Table 32	Table 33
Supply type 2 (or modified)	K80	Table 32	Table 34
Supply type 3	K80	Table 35	Table 34
Supply type 4	K115	Table 35	Table 34

Choose the wrong pair and the result is undersized distribution pipework: pressure losses above 0.5 bar per metre at 1000 L/min, and then a pump pressure requirement with no corresponding capacity. The first error caught in a site review is usually this mismatch.

Range pipe sizes for supply types 1 and 2

Range position	Layout	Size (mm)	Max. sprinklers
At the end of every distribution pipe	Two-end-side, last two ranges	25	1
		32	2
	Three-end-side, last three ranges	25	2
		32	3
	Other layouts, last range only	25	2
		32	3
40		4
All other ranges	Any	25	3
		32	4

A hard rule: no range pipe may carry more than four sprinklers. And a range pipe may not connect to a distribution pipe larger than 150 mm.

Distribution pipe sizes (supply type 1)

Position	Size (mm)	Max. sprinklers
Distribution pipes at the end of the installation	32	2
	40	4
	50	8
	65	12
	80	18
	100	48
Design point to control valve	Hydraulic calculation

Distribution pipe sizes (supply types 2, 3 and 4)

Position	Size (mm)	Max. sprinklers
Distribution pipes at the end of the installation	50	4
	65	8
	80	12
	100	16
	150	48
Design point to control valve	Hydraulic calculation

Note that in four-end-side systems, distribution pipe may not be smaller than 65 mm — so the 50 mm row applies only to layouts other than end-side.

Range pipe sizes for supply types 3 and 4

Range position	Layout	Size (mm)	Max. sprinklers
At the end of the distribution pipe	End-side, last three ranges	40	1
		50	3
		65	6
Other ranges	End-side	32	1
		40	2
		50	4
		65	6
At the end of the distribution pipe	Two-end-centre, last three ranges	32	1
		40	2
Other ranges	Two-end-centre	32	2
All ranges	Three- and four-end-centre	32	1
		40	2
		50	4

The end-side versus end-centre distinction is critical: end-side permits six sprinklers per range, while two-end-centre drops the ceiling to four. On site, this detail causes undersized ranges through a wrong layout selection.

Sprinkler count and the pressure budget

The HHP area of operation is 260 m². With a maximum spacing of 9 m² (HHP) or 7.5 m² (HHS), the protected area generally holds 30–35 sprinklers per 260 m². Pre-calculated design is most applicable in a single-storey building with a flat ceiling below 12 m, with goods unracked or at most in ST1 block stacks. With K80 at 7.5 mm/min we expect at least 1.1 bar at the most remote head; for HHP3 at 12.5 mm/min that rises to about 2.4 bar. Because the distribution section is always fully calculated, the K selection affects pump pressure directly: moving to K115 cuts the end-sprinkler pressure to about two thirds at the same flow.

When fully calculated design becomes mandatory

• Where in-rack sprinklers exist: excluded by the clause heading. The in-rack structure of three sprinklers per level across three racks does not fit the table.
• HHS3, HHS4 and ST5/ST6 shelving: the upper bands fall outside the assumptions of the pre-calculated tables.
• Gridded or looped arrays: the tables are written for end-side and end-centre branched layouts; the two-way flow balance of a gridded array does not fit.
• K200 and above: pre-calculated design is written only for K80 and K115. High-K systems such as ESFR are fully calculated under EN 12845-2.
• Multi-storey buildings or varying levels: there is no high-hazard equivalent of the parallel rule for OH, so fully calculated design is preferred.
• HHP4 with foam or deluge: the hazard class table already requires a project-specific hydraulic calculation.

Comparison with the NFPA 13 schedule method

The schedule method still exists in NFPA 13 for extra hazard, but hydraulic calculation has dominated practice for years. The EN 12845 pre-calculated method is stricter: fewer permitted K-factors, hard limits on sprinklers per range, gridded arrays prohibited. On the other hand, NFPA 13 assigns schedule sizes by sprinkler count alone, while EN 12845 also makes the water supply characteristic decisive. So EN 12845 can propose different pipe sizes for the same hazard under two different pump capacities — the consequence of sizing pump and installation together.

Common field errors

• Leaving a plan drawn as HHP2 on the original tables when the warehouse has become HHS3. If the hazard class changes, the supply row changes and the table pair must be renewed.
• Jumping to the K115 table because K115 sprinklers are used; without the corresponding supply type, K115 does not entitle you to pre-calculated design.
• Seeing 50 mm distribution in a four-end-side layout, where 65 mm is the minimum.
• Trying to fit five or six sprinklers per range. Six is permitted only on end-side layouts; two-end-centre is limited to four.
• Merging pre-calculated output with a gridded main. The gridded model is closed to pre-calculated design.

Turkish context

BYKHY leaves the choice between NFPA 13 and EN 12845 open. On facilities insured through European programmes — logistics, automotive suppliers — EN 12845 is generally preferred. Pre-calculated design may get formal approval quickly, but most modern Turkish warehouses are racked and tall enough to need in-rack protection. I have lost count of projects that started pre-calculated and ended fully calculated. In most reports, the section beyond the design point is hydraulically solved anyway, so the only gain from pre-calculated design is the range pipe sizing — a small gain against the risk. In modern warehouses we prefer to start fully calculated.

Frequently asked questions

Can pre-calculated design always be used for HHP and HHS?

No. It is permitted with K80 or K115, with a maximum of four sprinklers per range (six for three- and four-end-side layouts), on flat-ceiling, unracked configurations without intermediate in-rack sprinklers. Everything else requires fully calculated design.

How are the pipe size tables selected?

By the water supply characteristic. Supply type 1 uses Tables 32 and 33; type 2 uses 32 and 34; types 3 and 4 use 34 and 35. Choosing the wrong pair produces undersized distribution pipework.

Why can't a warehouse with in-rack sprinklers use pre-calculated design?

Because the clause heading excludes intermediate level sprinklers. The in-rack circuit is calculated on a hydraulic balance of three sprinklers per level across one to three racks, which the pre-calculated tables cannot model. Above 50 in-rack heads, a separate control valve set is required anyway.

Is a gridded pipe arrangement suitable for pre-calculated HHP design?

No. The tables are written for end-side and end-centre branched ranges. A gridded array requires two-way flow sharing, which breaks the tables' assumptions.

Is the NFPA 13 schedule method the same as EN 12845 pre-calculated design?

The logic is similar — sizes from a table — but EN 12845 permits it within a much narrower corridor: K80 and K115 only, limited layouts, no gridded arrays, and the water supply characteristic changing the table. In NFPA schedule design, supply and pipe size are not directly linked.

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