EN 12845 Pre-calculated LH Design: Table 27, Tables 28-29 and the 4 Sprinkler Rule

Source: TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clauses 6.2.2, 7.1, 13.3.3, 13.4.4, 14.3 and Tables 3, 6, 9, 16, 19, 25, 27, 28, 29, 37a.

The Light Hazard (LH) class is the shortest calculation chain in EN 12845: one density, one K-factor, a two line pipe table and a friction budget of two numbers. Even so, it is the place most often confused in practice, because the expression "4 sprinklers" is used in two different senses. This article builds the full chain of pre-calculated LH design using the standard's own figures.

What LH is: Clause 6.2.2

Clause 6.2.2 (TSE p.34 / EN p.31): LH covers occupancies with a low fire load and low combustibility, with no single compartment larger than 126 m² and with a fire resistance of at least 30 min. Examples are in Annex A.

Clause 5.4 (TSE p.33 / EN p.30) adds a derived rule: protection in concealed spaces shall be LH where the main hazard class is LH, and OH1 in all other cases.

The hydraulic design criterion: Table 3

Clause 7.1 (TSE p.38 / EN p.35): the design density shall be not less than the appropriate value given in this clause, with all ceiling/roof sprinklers in the relevant room or in the area of operation (whichever contains fewer sprinklers) operating, plus any in-rack and additional sprinklers.

The LH row of Table 3 (TSE p.39 / EN p.36):

Two derived figures follow. The demand of the area of operation: 2.25 mm/min × 84 m² = 189 l/min. The demand per sprinkler, using the maximum area given for LH in Table 19: 2.25 mm/min × 21.0 m² = 47.25 l/min.

The sprinkler: K57 and 0.70 bar

The LH row of Table 37a (Clause 14.2.1, TSE p.107 / EN p.104):

LH is the only class in which the standard permits K57.

Clause 13.4.4 (TSE p.105 / EN p.102): the pressure at the hydraulically most unfavourable sprinkler shall not be less than that required to achieve the density specified in 13.4.1 or 0.70 bar in LH, whichever is the higher.

Clause 14.3 (TSE p.107-108 / EN p.104-105) gives the formula: Q = K√P, with Q in l/min and P in bar. Accordingly a K57 sprinkler at 0.70 bar delivers 57 × √0.70 = 47.7 l/min. Divided by the 21.0 m² of Table 19 this gives 2.27 mm/min; in other words the 0.70 bar floor already achieves the 2.25 mm/min density of Table 3 at the maximum area of coverage.

Sprinkler layout: Tables 19 and 20

Table 19 (Clause 12.2, TSE p.73 / EN p.70), for sprinklers other than sidewall:

Where sidewall sprinklers are to be used, Table 20 (TSE p.74 / EN p.71) gives for LH 17.0 m² per sprinkler, 4.6 m between sprinklers along the wall and 2.3 m from the sprinkler to the end of the wall.

The distance to walls and partitions is limited by Clause 12.4.1 (TSE p.75 / EN p.72), and the smallest of the values listed in the clause is selected: 2.0 m for standard spacing; 2.3 m for staggered spacing; 1.5 m where the ceiling or roof is open-joisted or the rafters are exposed; 1.5 m from the open face of open-faced buildings; 1.5 m where the external walls are of combustible material; 1.5 m where the external walls are of metal (with or without combustible cladding or insulation); and half of the maximum distance given in Tables 19 and 20.

Clause 12.1.2 a) (TSE p.73 / EN p.70): in LH and OH a clear space of 0.3 m for flat spray sprinklers and 0.5 m in all other cases shall be maintained below the deflector of ceiling and roof sprinklers.

Clause 12.3 (TSE p.75 / EN p.72): sprinklers shall not be installed at intervals of less than 2 m; the exceptions are arrangements that prevent adjacent sprinklers from wetting each other (baffles of approximately 200 mm × 150 mm or intervening constructional features), intermediate sprinklers in racks, and escalators and stair wells.

The design point: Table 25 and what "4 sprinklers" means

Clause 13.3.2.2 (TSE p.89 / EN p.86): in LH installations the design point is downstream of the sprinkler specified in column 3 of Table 25.

Table 25 (TSE p.89 / EN p.86):

This table does not give the number of sprinklers assumed to open simultaneously; it gives the position of the design point. The assumption for sprinklers operating simultaneously comes, in fully calculated systems, from the definition of "the four adjacent sprinklers closest to each other" in Clause 13.4.1 (TSE p.101 / EN p.98); in pre-calculated LH the density is achieved indirectly through the supply requirements of Tables 6 and 16 (NOTE to Clause 7.1).

Pipe sizes: Table 27 and the 25 mm exception

Clause 13.3.1.2 (TSE p.88 / EN p.85): range pipe sizes and the maximum number of sprinklers each size may feed shall be determined in accordance with Table 30; in Light Hazard, however, Table 27 covers only the pipes feeding the last three or four sprinklers on each range.

Table 27 (Clause 13.3.3.1, TSE p.90 / EN p.87):

The second paragraph of Clause 13.3.3.1 contains one flexibility and one prohibition: a 25 mm pipe may be installed between the design point and the control valve set where hydraulic calculation shows this to be possible; however, where the 2 sprinkler point is decisive, no 25 mm pipe shall be installed between the 3rd and the 4th sprinkler.

Clause 13.3.1.1: pipe sizes shall not increase in the direction of water flow to any sprinkler. Table 36 (Clause 13.4.5, TSE p.106 / EN p.103) gives the absolute lower limit for LH: 20 mm.

Above the design point: Tables 28 and 29

Clause 13.3.3.2 (TSE p.90 / EN p.87): all pipework between the control valve set and the design point at each end of an array shall be sized by hydraulic calculation using the values in Tables 28 and 29.

Table 28 - maximum friction loss between the control valve set and any design point:

NOTE to Table 28: in multi-storey buildings the pressure loss may be increased by an amount equivalent to the static pressure between the level of the sprinklers concerned and the level of the sprinklers on the topmost storey.

Table 29 - pressure loss at the design flows in LH installations (mbar/m):

Clause 13.3.3.3 separates where the two columns are used: where there are more than two sprinklers on a range pipe, the pressure loss between the 2 sprinkler point and the distribution pipe shall be determined using the value in column 2 of Table 29, and the distribution pipe loss between that junction and the control valve set using the loss per metre in column 3.

A simple budget check: in an array with 4 or more sprinklers the budget is 0.7 bar = 700 mbar. Since a 32 mm distribution pipe loses 52 mbar per metre at 225 l/min, a 12 m run of 32 mm gives 12 × 52 = 624 mbar and stays within the budget; 14 m at the same diameter gives 728 mbar and exceeds it. This is the arithmetic of the table's own figures.

The supply side: Tables 6, 16 and 9

Clause 7.3.1 (TSE p.42 / EN p.39): the water supply shall be capable of providing not less than the flows and pressures specified in Table 6 at each control valve set. The friction and static head losses between the water supply and each control valve set shall be calculated separately.

The LH row of Table 6 (TSE p.42 / EN p.39): 2.2 + ps bar at the control valve set at a flow of 225 l/min. (ps is the static head loss arising from the height of the highest sprinkler in the relevant array above the 'C' gauge of the control valve set.) The maximum demand flow column is empty in the LH row.

Where a pump is used, the LH rows of Table 16 (Clause 10.7.1, TSE p.65 / EN p.62):

Where a storage tank is used, Table 9 (Clause 9.3.2.2, TSE p.50 / EN p.47):

Where a reduced capacity tank is used, Table 11 (Clause 9.3.4, TSE p.52 / EN p.49) gives the minimum effective capacity for LH as 5 m³; in that case the tank capacity plus the automatic inflow shall be sufficient to provide the full capacity of 9.3.2.

Clause 8.1.1 (TSE p.45 / EN p.42) gives the minimum supply duration for LH as 30 min; its note recalls, however, that for town mains, inexhaustible sources and all pre-calculated systems the duration is already covered by the other requirements of the standard.

The size of the installation

Clause 11.1.3 (TSE p.70 / EN p.67): the maximum area controlled by a single wet alarm valve, including sprinklers on any subsidiary extension, shall not exceed that shown in Table 17. The LH row of Table 17: 10 000 m². The standard sets no limit on the number of sprinklers per installation for LH.

Frequently Asked Questions

What are the design density and area of operation for LH in EN 12845?

Table 3 (Clause 7.1, TSE p.39 / EN p.36): for LH the design density is 2.25 mm/min and the area of operation is 84 m2 in wet or pre-action systems. Dry or alternate systems are not permitted in LH; OH1 is used instead.

Where is the design point in LH?

Table 25 (Clause 13.3.2.2, TSE p.89 / EN p.86): where there are 3 or fewer sprinklers on a range in a room, the design point is downstream of the 3rd sprinkler; where there are 4 or more, it is downstream of the 4th sprinkler. The table does not give a number of sprinklers assumed to operate simultaneously, it gives the position of the design point.

How much water does a K57 sprinkler deliver at the minimum pressure in LH?

Clause 13.4.4 (TSE p.105 / EN p.102) gives the minimum sprinkler discharge pressure for LH as 0.70 bar. Since the formula in Clause 14.3 (TSE p.107-108 / EN p.104-105) is Q = K sqrt(P), a K57 sprinkler at 0.70 bar delivers 57 x sqrt(0.70) = 47.7 l/min.

How are LH range pipe sizes selected?

Table 27 (Clause 13.3.3.1, TSE p.90 / EN p.87): for all range pipes and terminal distribution pipes, a 20 mm diameter feeds 1 sprinkler and a 25 mm diameter feeds 3 sprinklers. A 25 mm pipe may be installed between the design point and the control valve set where hydraulic calculation shows this to be possible; however, where the 2 sprinkler point is decisive, no 25 mm pipe shall be installed between the 3rd and 4th sprinkler.

What is the minimum water storage volume in an LH system?

Table 9 (Clause 9.3.2.2, TSE p.50 / EN p.47): in pre-calculated LH wet or pre-action systems, where the height h of the highest sprinkler above the lowest is h of 15 m or less the minimum water volume is 9 m3, for 15 < h of 30 m it is 10 m3, and for 30 < h of 45 m it is 11 m3.

How large an area may a single LH control valve set protect?

Table 17 (Clause 11.1.3, TSE p.70 / EN p.67): for LH the maximum protected area per control valve set is 10 000 m2. The standard sets no limit on the number of sprinklers for LH.

References

• TS EN 12845+A2:2026 Clause 5.4 (TSE p.33 / EN p.30), Clause 6.2.2 (TSE p.34 / EN p.31)
• TS EN 12845+A2:2026 Clause 7.1 (TSE p.38 / EN p.35), Table 3 (TSE p.39 / EN p.36)
• TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (TSE p.42 / EN p.39)
• TS EN 12845+A2:2026 Clause 8.1.1 (TSE p.45 / EN p.42)
• TS EN 12845+A2:2026 Clause 9.3.2.2, Table 9 (TSE p.50 / EN p.47), Table 11 (TSE p.52 / EN p.49)
• TS EN 12845+A2:2026 Clause 10.7.1, Table 16 (TSE p.65 / EN p.62)
• TS EN 12845+A2:2026 Clause 11.1.3, Table 17 (TSE p.70 / EN p.67)
• TS EN 12845+A2:2026 Clauses 12.1.2, 12.2, Table 19 (TSE p.73 / EN p.70), Table 20 (TSE p.74 / EN p.71)
• TS EN 12845+A2:2026 Clauses 12.3, 12.4.1 (TSE p.75 / EN p.72)
• TS EN 12845+A2:2026 Clauses 13.3.1.1, 13.3.1.2 (TSE p.88 / EN p.85)
• TS EN 12845+A2:2026 Clause 13.3.2.2, Table 25 (TSE p.89 / EN p.86)
• TS EN 12845+A2:2026 Clauses 13.3.3.1, 13.3.3.2, 13.3.3.3, Tables 27, 28, 29 (TSE p.90 / EN p.87)
• TS EN 12845+A2:2026 Clause 13.4.1 (TSE p.101 / EN p.98)
• TS EN 12845+A2:2026 Clause 13.4.4 (TSE p.105 / EN p.102), Table 36 (TSE p.106 / EN p.103)
• TS EN 12845+A2:2026 Table 37a, Clause 14.3 (TSE p.107-108 / EN p.104-105)

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