EN 12845 Light Hazard (LH) Design: 2.25 mm/min and 84 m²

Light Hazard is the most narrowly defined class in TS EN 12845+A2:2026: both the hazard definition and the list of examples are very short, yet the constraints it places on the system (the prohibition of dry installations, a single K-factor, a two row pipe table) are rather rigid. Source: TS EN 12845+A2:2026, Clause 6.2.2, Table 3, Table 6 and Annex A.

The short definition: Clause 6.2.2

Clause 6.2.2 (TS p.34 / EN p.31): LH covers occupancies with low fire load and low combustibility, with no single compartment larger than 126 m² and with a fire resistance of at least 30 minutes. Annex A is to be consulted for examples.

This definition closes two doors at once: even where the fire load is low, an area cannot be LH if the compartment exceeds 126 m² or if the separating construction does not resist for 30 minutes.

Annex A: the LH list is three rows long

Table A.1 (TS p.127 / EN p.124), headed "Light Hazard occupancies", gives only three items:

• Schools and other educational establishments (certain areas) - see Clause 6.2.2
• Offices (certain areas) - see Clause 6.2.2
• Prisons

The introductory sentence to Annex A states that these tables give the minimum hazard classification, that they are to be used as guidance for occupancies not specifically mentioned, and that they are to be read together with Clause 6.2.

To avoid a frequent mistake, it is worth remembering the OH1 column of Table A.2 (TS p.128 / EN p.125): Hospitals, Hotels, Libraries (excluding book storage), Restaurants, Schools (see 6.2.2) and Offices (see 6.2.2) appear in that column. Hospitals and hotels are therefore OH1 and not LH; schools and offices count as LH only in the certain areas that satisfy the compartment and resistance conditions of Clause 6.2.2.

Design criteria: Table 3

Table 3 (Clause 7.1, TS p.39 / EN p.36):

Clause 7.1 states that the design density shall not be less than this value with all the ceiling/roof sprinklers in the relevant room operating, or those in the area of operation, whichever is the fewer, plus any in-rack and supplementary sprinklers.

Simple arithmetic: 2.25 mm/min x 84 m² = 189 l/min. That is the theoretical water demand of the design; on pre-calculated systems the supply is sized to a higher figure, from Table 6.

The prohibition of dry installations: the dry/alternate column of the LH row in Table 3 reads "Not allowed - Use OH1". So where there is a frost risk in an LH area, either the installation stays wet and is protected by local heating/trace heating (Clause 11.1.2), or a small section becomes a subsidiary dry extension under Clause 11.5, or the whole area is designed to OH1 criteria.

Pre-calculated LH: Table 6 and Table 16

Table 6 (Clause 7.3.1, TS p.42 / EN p.39) is a single row for LH:

ps is the static head loss due to the height of the highest sprinkler in the relevant arrangement above the 'C' gauge of the control valve set. The maximum demand flow rate column is empty in the LH row; unlike the OH classes, LH has no second operating point.

Clause 4.4.3.2 b) 1) (TS p.28 / EN p.25) likewise requires the pressure losses between the control valve set and the design points to be summarized on pre-calculated drawings at a flow of 225 l/min for an LH installation.

On the pump side, Table 16 (Clause 10.7.1, TS p.65 / EN p.62) gives three height bands for LH:

NOTE 1: the pressures shown are measured at the control valve set.

On the water volume side, Table 9 (TS p.50 / EN p.47): for LH wet or pre-action, 9 m³ at h ≤ 15 m, 10 m³ at 15 < h ≤ 30 m and 11 m³ at 30 < h ≤ 45 m. Where a reduced capacity tank is used, the Table 11 floor (TS p.52 / EN p.49) is 5 m³. The duration under Clause 8.1.1 (TS p.45 / EN p.42) is 30 minutes, the shortest in the standard.

Sprinkler selection and layout

K-factor, Table 37a (TS p.107 / EN p.104): in LH the design density is 2.25 mm/min, the sprinkler types are conventional, spray, ceiling, flush, flat spray, recessed, concealed and sidewall, and the nominal K-factor is 57. LH is the only class in the standard restricted to a single K-factor.

Coverage area, Table 19 (Clause 12.2, TS p.73 / EN p.70):

Sidewall, Table 20 (TS p.74 / EN p.71): in LH the maximum area per sidewall sprinkler is 17.0 m², the spacing between sprinklers along the wall is 4.6 m and the distance from a sprinkler to the end wall is 2.3 m. Where the room width w ≤ 3.7 m a single row is used; where 3.7 < w ≤ 7.4 m and the room length ≤ 9.2 m, two rows in standard layout; where the length > 9.2 m, two rows in staggered layout.

Clearance below the deflector, Clause 12.1.2 a) (TS p.73 / EN p.70): in LH and OH a clear space of at least 0.3 m for flat spray sprinklers, and at least 0.5 m in all other cases, is left below the deflector of roof and ceiling sprinklers.

Distance from walls, Clause 12.4.1 (TS p.75 / EN p.72): the maximum distance from walls and partitions to the sprinklers is the smallest of the following: 2.0 m for standard spacing; 2.3 m for staggered spacing; 1.5 m where the ceiling/roof has open joists or where the rafters are exposed; 1.5 m from the open face of open fronted buildings; 1.5 m where the external walls are of combustible material or of metal; half the maximum distance given in Table 19 and Table 20. In LH, half of the 4.6 m of Table 19 is 2.3 m, so 2.0 m governs in a standard layout.

Pipe sizing specific to LH

LH is the only class in the standard with its own pipe table, and that table has two rows.

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

Clause 13.3.1.2 (TS p.88 / EN p.85) clarifies the scope of this table: in Light Hazard, Table 27 determines only the pipes feeding the last three or four sprinklers on each range.

Design point, Table 25 (Clause 13.3.2.2, TS p.89 / EN p.86):

Clause 13.3.3.1 allows one relaxation: where hydraulic calculation shows it to be possible, 25 mm diameter pipe may be used between the design point and the control valve set. However, where the 2 sprinkler point governs, 25 mm pipe shall not be used between the 3rd and 4th sprinklers.

Pressure loss limit, Table 28 (Clause 13.3.3.2, TS p.90 / EN p.87):

NOTE: in multi-storey buildings the pressure loss may be increased by an amount equivalent to the static pressure between the relevant sprinkler level and the sprinkler level on the top floor.

Unit losses, Table 29 (TS p.90 / EN p.87):

Clause 13.3.3.3: where there are more than two sprinklers on a range pipe, the pressure loss between the 2 sprinkler point and the distribution pipe is determined with the value in Table 29 column 2, while the loss between that connection and the control valve set is determined with the per metre value in column 3.

Worked example (arithmetic using the values of the standard): on an LH range with four or more sprinklers, the total friction loss permitted by Table 28 is 0.7 bar = 700 mbar. Had the whole run been in 25 mm, at 198 mbar/m from Table 29 column 3 only 700 / 198 = 3.5 m could be covered; if the same run is in 50 mm, at 7.8 mbar/m it reaches 700 / 7.8 = 89 m. Those two figures show why diameters grow quickly in LH.

Minimum diameter, Table 36 (Clause 13.4.5, TS p.106 / EN p.103): the minimum pipe diameter in an LH risk is 20 mm.

Minimum pressure and the other LH limits

• Clause 13.4.4 (TS p.105 / EN p.102): the pressure at the most unfavourable sprinkler shall not be less than that required to provide the density in Clause 13.4.1, or 0.70 bar in LH, whichever is the greater.
• Table 17 (Clause 11.1.3, TS p.70 / EN p.67): the maximum protected area controlled by a single wet alarm valve in LH is 10 000 m².
• Table 39 (Clause 15.4, TS p.111 / EN p.108): the minimum diameter of the drain valve and pipe draining an LH installation is 40 mm.
• Clause 20.1.4 a) (TS p.121 / EN p.118): at least 6 spare sprinklers are kept on site for LH installations.
• Table 15 (TS p.65 / EN p.62): on pumps with a suction lift, the priming tank for LH is at least 100 litres and the priming pipe at least 25 mm.
• Clause 9.5.3 (TS p.57 / EN p.54): the minimum water volume in a pressure tank used as a single supply is 15 m³ for LH. Clause 9.6.1 c) accepts a pressure tank as a single supply only in LH and OH1.
• Clause 5.4 (TS p.33 / EN p.30): protection in concealed spaces is designed to LH where the main hazard class is LH, and to OH1 in all other cases.
• Clause F.4 (TS p.146 / EN p.143): quick response sprinklers are used where Annex F applies; standard 'A' and special response may be used only in rooms with an area of at least 500 m² or a height of at least 5 m.

A consistency check (arithmetic): if the maximum area covered by one sprinkler in LH is 21.0 m² and the design density is 2.25 mm/min, the required flow is 2.25 x 21.0 = 47.25 l/min. With the relationship Q = K√P in Clause 14.3 and K = 57, the pressure this flow requires is (47.25 / 57)² = 0.69 bar; the 0.70 bar floor of Clause 13.4.4 sits just above that value, which means the minimum pressure rule governs in LH even at full coverage area.

Frequently Asked Questions

What is the Light Hazard design criterion in EN 12845?

Table 3 (Clause 7.1, TS p.39 / EN p.36): for LH the design density is 2.25 mm/min and the area of operation 84 m² (wet or pre-action system). The product of the two is 189 l/min. The same row does not permit a dry or alternate installation in LH and requires a move to OH1.

Under what conditions does an area qualify as LH?

Clause 6.2.2 (TS p.34 / EN p.31): LH covers occupancies with low fire load and low combustibility, with no single compartment larger than 126 m² and with a fire resistance of at least 30 minutes. Table A.1 (TS p.127 / EN p.124) lists only three items: schools and other educational establishments (certain areas), offices (certain areas) and prisons.

Are hospitals and hotels LH?

No. The OH1 column of Table A.2 (TS p.128 / EN p.125) contains Hospitals, Hotels, Libraries (excluding book storage), Restaurants, Schools and Offices. Table A.1, which is the LH list, contains only certain areas of schools and offices plus prisons; hospitals and hotels are not on that list.

What is the minimum sprinkler discharge pressure in LH?

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

How are pipe diameters determined in LH?

Table 27 (TS p.90 / EN p.87) has only two rows: for all range pipes and terminal distribution pipes, a 20 mm diameter feeds 1 sprinkler and a 25 mm diameter feeds 3 sprinklers. Everything upstream of the design point is sized by hydraulic calculation using the values in Table 28 and Table 29 in accordance with Clause 13.3.3.2. Clause 13.4.5 and Table 36 give the minimum diameter in LH as 20 mm.

References

• TS EN 12845+A2:2026 Clause 4.4.3.2 (TS p.28 / EN p.25)
• TS EN 12845+A2:2026 Clause 5.4 (TS p.33 / EN p.30), Clause 6.2.2 (TS p.34 / EN p.31)
• TS EN 12845+A2:2026 Clause 7.1, Table 3 (TS p.38-39 / EN p.35-36)
• TS EN 12845+A2:2026 Clause 7.3.1, Table 6 (TS p.42 / EN p.39)
• TS EN 12845+A2:2026 Clause 8.1.1 (TS p.45 / EN p.42)
• TS EN 12845+A2:2026 Table 9 (TS p.50 / EN p.47), Table 11 (TS p.52 / EN p.49)
• TS EN 12845+A2:2026 Clause 9.5.3, Clause 9.6.1 (TS p.57-59 / EN p.54-56)
• TS EN 12845+A2:2026 Table 15 (TS p.65 / EN p.62), Clause 10.7.1, Table 16 (TS p.65 / EN p.62)
• TS EN 12845+A2:2026 Clause 11.1.3, Table 17 (TS p.70 / EN p.67)
• TS EN 12845+A2:2026 Clause 12.1.2, Clause 12.2, Table 19 (TS p.73 / EN p.70), Table 20 (TS p.74 / EN p.71), Clause 12.4.1 (TS p.75 / EN p.72)
• TS EN 12845+A2:2026 Clause 13.3.1.2 (TS p.88 / EN p.85), Clause 13.3.2.2, Table 25 (TS p.89 / EN p.86)
• TS EN 12845+A2:2026 Clauses 13.3.3.1, 13.3.3.2, 13.3.3.3, Table 27, Table 28, Table 29 (TS p.89-90 / EN p.86-87)
• TS EN 12845+A2:2026 Clause 13.4.4 (TS p.105 / EN p.102), Clause 13.4.5, Table 36 (TS p.106 / EN p.103)
• TS EN 12845+A2:2026 Clause 14.3, Table 37a (TS p.107 / EN p.104)
• TS EN 12845+A2:2026 Clause 15.4, Table 39 (TS p.111 / EN p.108)
• TS EN 12845+A2:2026 Clause 20.1.4 (TS p.121 / EN p.118)
• TS EN 12845+A2:2026 Table A.1 (TS p.127 / EN p.124), Table A.2 (TS p.128 / EN p.125)
• TS EN 12845+A2:2026 Clause F.4 (TS p.146 / EN p.143)

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