Source: TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clauses 6.2.3, 7.1, 7.3.1, 13.3.4, 13.4.4 and Tables 1, 3, 6, 9, 16, 19, 26, 30, 31, 37a.
The Ordinary Hazard (OH) class makes up the majority of EN 12845 projects and is where the pre-calculated method works most efficiently: pipe sizes are determined with two pipe tables and a single friction budget. This article builds the pre-calculated OH chain, from classification to the control valve set, using the standard's own figures.
OH classification: Clause 6.2.3
Clause 6.2.3 (TSE p.34 / EN p.31): OH covers occupancies where combustible materials with a medium fire load and medium combustibility are processed or manufactured, and is divided into four groups: OH1, OH2, OH3, OH4.
In occupancies classified as OH, storage is permitted only where three conditions are met together:
- a) the protection throughout the room is designed for at least OH3,
- b) the maximum stack heights shown in Table 1 are not exceeded,
- c) the maximum storage area of a single block does not exceed 50 m², with a clear space of at least 2.4 m around the block.
Where the area is classified as OH4, or where b) or c) cannot be met, the storage in the area shall be treated as HHS.
Table 1 (TSE p.35 / EN p.32) gives the maximum stack heights for OH3 protection:
| Storage category | Free-standing or block stacking ST1 (m) | Stacking arrangements ST2 - ST6 (m) |
|---|---|---|
| Category I | 4.0 | 3.5 |
| Category II | 3.0 | 2.6 |
| Category III | 2.1 | 1.7 |
| Category IV | 1.2 | 1.2 |
The hydraulic design criterion: Table 3
The OH rows of Table 3 (Clause 7.1, TSE p.39 / EN p.36):
| Hazard class | Design density (mm/min) | Area of operation, wet or pre-action (m²) | Area of operation, dry or alternate (m²) |
|---|---|---|---|
| OH1 | 5.0 | 72 | 90 |
| OH2 | 5.0 | 144 | 180 |
| OH3 | 5.0 | 216 | 270 |
| OH4 | 5.0 | 360 | Not permitted, use HHP1 |
The dry and alternate areas are given directly in the table; there is no separate "increase by 25 %" clause. (Such an increase appears only for HHS, as a recommendation in the notes to Tables 4 and 5.)
Multiplying the density by the area gives the demand of the area of operation: OH1 5.0 × 72 = 360 l/min, OH2 5.0 × 144 = 720 l/min, OH3 5.0 × 216 = 1 080 l/min, OH4 5.0 × 360 = 1 800 l/min.
Sprinkler layout and K-factor
The OH row of Table 19 (Clause 12.2, TSE p.73 / EN p.70):
| Hazard class | Maximum area per sprinkler (m²) | Standard layout S and D (m) | Staggered layout S (m) | Staggered layout D (m) |
|---|---|---|---|---|
| OH | 12.0 | 4.0 | 4.6 | 4.0 |
The OH row of Table 37a (TSE p.107 / EN p.104): at a density of 5.0 mm/min, conventional, spray, ceiling, flush, flat spray, recessed, concealed and sidewall types are used with K80 or K115. The K57 of LH does not apply in OH.
A practical check follows from these two tables. Taking the maximum area per sprinkler as 12.0 m², a density of 5.0 mm/min requires 5.0 × 12.0 = 60 l/min per sprinkler. With the formula Q = K√P of Clause 14.3 (TSE p.107-108 / EN p.104-105):
- pressure required for K80: (60/80)² = 0.5625 bar,
- pressure required for K115: (60/115)² = 0.272 bar.
Clause 13.4.4 (TSE p.105 / EN p.102) sets the minimum sprinkler discharge pressure in OH at 0.35 bar and says "whichever is the higher of the value required to achieve the density and this value". So with K80 the density requirement (0.5625 bar) governs; with K115 the 0.35 bar floor governs and the sprinkler delivers 115 × √0.35 = 68.0 l/min, which corresponds to a density of 5.67 mm/min over 12.0 m².
The design point: Table 26
Clause 13.3.2.3 (TSE p.89 / EN p.86): in OH and HH installations the design point lies downstream of the junction of the distribution pipes with the range pipes, in accordance with column 3 of Table 26.
| Hazard class | Number of sprinklers on one distribution pipe in a room | Junction of the range carrying the nth sprinkler, at which the design point lies | Range arrangement |
|---|---|---|---|
| OH | > 16 | 17 | two end-side |
| OH | > 18 | 19 | all other arrangements |
The same clause gives one further exception: where the number of sprinklers in an array, in a room or on a single distribution pipe is less than or equal to the number of sprinklers for which the distribution pipes are designed, the design point is downstream of the point at which the range or array hydraulically nearest the control valve set connects to the distribution pipe.
Pipe sizes: Tables 30 and 31
Clause 13.3.4.1 (TSE p.91 / EN p.88): range pipe sizes shall be in accordance with Table 30 and distribution pipe sizes in accordance with Table 31.
Table 30 - Range pipe sizes in OH installations (TSE p.91 / EN p.88):
| Range pipes | Arrangement | Diameter (mm) | Maximum number of sprinklers fed |
|---|---|---|---|
| Ranges at the remote end of all distribution pipes: last 2 ranges | 2-end-side | 25 | 1 |
| 32 | 2 | ||
| Last 3 ranges | 3-end-side | 25 | 2 |
| Last range | 32 | 3 | |
| All other arrangements | 25 | 2 | |
| 32 | 3 | ||
| 40 | 4 | ||
| 50 | 9 | ||
| All other range pipes | All | 25 | 3 |
| 32 | 4 | ||
| 40 | 6 | ||
| 50 | 9 |
Table 31 - Distribution pipe sizes in OH installations (TSE p.91 / EN p.88):
| Distribution pipes | Arrangement | Diameter (mm) | Maximum number of sprinklers fed |
|---|---|---|---|
| At the ends of the installation | 2-end-side | 32 | 2 |
| 40 | 4 | ||
| 50 | 8 | ||
| 65 | 16 | ||
| All other arrangements | 32 | 3 | |
| 40 | 6 | ||
| 50 | 9 | ||
| 65 | 18 | ||
| Between the design points and the control valve set | All | Calculated in accordance with 13.3.4.2 |
The last sentence of Clause 13.3.4.1 adds a geometric limit: where range pipes run longitudinally under roofs with a slope of more than 6 degrees, the number of sprinklers on a range pipe shall not exceed six.
Clause 13.3.1.1 (TSE p.88 / EN p.85): pipe sizes shall not increase in the direction of water flow to any sprinkler. Table 36 (TSE p.106 / EN p.103) gives the absolute lower limit for OH: 20 mm for horizontal and upright pipes feeding a single sprinkler with a K-factor not exceeding 80, and 25 mm in all other cases.
Above the design point: the 0.5 bar budget
Clause 13.3.4.2 (TSE p.92 / EN p.89): the pipe sizes between the design point in the most remote area of the installation and the control valve set shall be calculated so that the total pressure loss due to friction at a flow of 1 000 l/min does not exceed 0.5 bar. This may be modified by 13.3.4.3 and 13.3.4.4.
Clause 13.3.4.3: in multi-storey buildings, or where there are different levels such as platforms and canopies, the 0.5 bar loss at the design point may be increased by an amount equivalent to the static pressure arising from the height difference between the highest sprinkler point in the building and the design point of the remote area on the storey concerned. In that case the height difference between the level of the highest sprinkler and the installation pressure gauge, together with the required gauge pressure, shall be stated in the completion certificate.
Clause 13.3.4.4: where the same system contains both OH3 or OH4 and HHP or HHS areas, all connected to a common water supply, the maximum friction loss of 0.5 bar may be increased by 50 % of the additional pressure available. The standard's own OH3 example (TSE p.92 / EN p.89):
| Item | Value |
|---|---|
| Pressure required at the control valve set, excluding static (Table 6 for OH3) | 1.4 bar |
| Pressure arising from the height difference between the highest sprinkler and the control valve set | 1.2 bar |
| Pressure required at the control valve set | 2.6 bar |
| Pressure available at the control valve set at the appropriate flow for HH (example) | 6.0 bar |
| Additional pressure that may be used: 50 % × (6.0 - 2.6) | 1.7 bar |
| Maximum pressure loss for which the pipework may be sized: 0.5 + 1.7 × (1 000/1 350)² | 1.43 bar |
The supply side: Tables 6, 16 and 9
Table 6 (Clause 7.3.1, TSE p.42 / EN p.39), flows and pressures required at the control valve set:
| Hazard class | Flow (l/min) | Pressure at the control valve set (bar) | Maximum demand flow (l/min) | Pressure at the control valve set (bar) |
|---|---|---|---|---|
| LH (wet and pre-action) | 225 | 2.2 + ps | - | - |
| OH1 wet and pre-action | 375 | 1.0 + ps | 540 | 0.7 + ps |
| OH1 dry and alternate / OH2 wet and pre-action | 725 | 1.4 + ps | 1 000 | 1.0 + ps |
| OH2 dry and alternate / OH3 wet and pre-action | 1 100 | 1.7 + ps | 1 350 | 1.4 + ps |
| OH3 dry and alternate / OH4 wet and pre-action | 1 800 | 2.0 + ps | 2 100 | 1.5 + ps |
NOTE: ps is the static head loss (bar) arising from the height of the highest sprinkler in the relevant array above the 'C' gauge of the control valve set.
The row labels of the table combine two classes. An OH3 wet system is in the row 1 100 l/min @ 1.7 + ps bar; an OH3 dry system is one row down, at 1 800 l/min @ 2.0 + ps bar with a maximum demand of 2 100 l/min @ 1.5 + ps bar. Table 16 (TSE p.65 / EN p.62) combines rows in the same way for pump characteristics; for example the row "OH2 dry or alternate / OH3 wet or pre-action, h ≤ 15 m" gives nominal 1.4 bar - 2 250 l/min and characteristics 2.9 bar - 1 350 l/min and 3.2 bar - 1 100 l/min.
The OH rows of Table 9 (Clause 9.3.2.2, TSE p.50 / EN p.47), minimum effective water volume for pre-calculated systems:
| Group | h ≤ 15 m | 15 < h ≤ 30 m | 30 < h ≤ 45 m |
|---|---|---|---|
| OH1 wet or pre-action | 55 m³ | 70 m³ | 80 m³ |
| OH1 dry/alternate or OH2 wet/pre-action | 105 m³ | 125 m³ | 140 m³ |
| OH2 dry/alternate or OH3 wet/pre-action | 135 m³ | 160 m³ | 185 m³ |
| OH3 dry/alternate or OH4 wet/pre-action | 160 m³ | 185 m³ | 200 m³ |
| OH4 dry or alternate | Use HH protection |
Clause 8.1.1 (TSE p.45 / EN p.42) gives the minimum supply duration for OH as 60 min.
The size of the installation
Table 17 (Clause 11.1.3, TSE p.70 / EN p.67): for OH, including LH sprinklers within it, the maximum protected area per control valve set is 12 000 m² (except as permitted in Annex D and Annex F).
When pre-calculation is not allowed
Clause 13.1 (TSE p.85 / EN p.82) makes full calculation mandatory in two cases: layouts with HHS intermediate sprinklers and gridded or looped layouts. Clause 11.1.1 (TSE p.69 / EN p.66) completes this: only wet pipe installations shall be used in grid and loop systems.
Frequently Asked Questions
What are the design densities and areas of operation for the OH groups in EN 12845?
Table 3 (Clause 7.1, TSE p.39 / EN p.36) gives a density of 5.0 mm/min to all four groups. The wet or pre-action areas of operation are OH1 72 m2, OH2 144 m2, OH3 216 m2 and OH4 360 m2. In dry or alternate systems they are OH1 90 m2, OH2 180 m2 and OH3 270 m2; dry systems are not permitted in OH4, where HHP1 is used.
What is the friction budget between the design point and the control valve set in OH?
Clause 13.3.4.2 (TSE p.92 / EN p.89): the pipe sizes between the design point in the most remote area of the installation and the control valve set shall be calculated so that the total pressure loss due to friction at a flow of 1 000 l/min does not exceed 0.5 bar. This is a total budget, not a value per metre.
Where is the design point in OH?
Table 26 (Clause 13.3.2.3, TSE p.89 / EN p.86): where there are more than 16 sprinklers on one distribution pipe in a room, the design point in a two end-side arrangement is at the junction of the range carrying the 17th sprinkler; where there are more than 18 sprinklers, in all other arrangements it is at the junction of the range carrying the 19th sprinkler.
What is the minimum sprinkler operating pressure in OH?
Clause 13.4.4 (TSE p.105 / EN p.102): 0.35 bar in OH. This value is compared with the pressure required to achieve the density and the higher of the two applies.
Does the 0.5 bar limit change where OH3/OH4 and HH areas share a system?
Yes. Clause 13.3.4.4 (TSE p.92 / EN p.89): where the same system contains both OH3 or OH4 and HHP or HHS areas connected to a common water supply, the maximum friction loss of 0.5 bar may be increased by 50 percent of the additional pressure available.
How many sprinklers may be connected to a range pipe in OH?
Table 30 (TSE p.91 / EN p.88) gives a limit per diameter; the largest value is 9 sprinklers for a 50 mm pipe. In addition, Clause 13.3.4.1 (TSE p.92 / EN p.89): where range pipes run longitudinally under roofs with a slope of more than 6 degrees, the number of sprinklers on a range pipe shall not exceed six.
References
- TS EN 12845+A2:2026 Clause 6.2.3 (TSE p.34 / EN p.31), Table 1 (TSE p.35 / EN p.32)
- TS EN 12845+A2:2026 Clause 7.1, 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)
- TS EN 12845+A2:2026 Clause 10.7.1, Table 16 (TSE p.65 / EN p.62)
- TS EN 12845+A2:2026 Clause 11.1.1 (TSE p.69 / EN p.66), Clause 11.1.3, Table 17 (TSE p.70 / EN p.67)
- TS EN 12845+A2:2026 Clause 12.2, Table 19 (TSE p.73 / EN p.70)
- TS EN 12845+A2:2026 Clause 13.1 (TSE p.85 / EN p.82), Clause 13.3.1.1 (TSE p.88 / EN p.85)
- TS EN 12845+A2:2026 Clause 13.3.2.3, Table 26 (TSE p.89 / EN p.86)
- TS EN 12845+A2:2026 Clause 13.3.4.1, Table 30, Table 31 (TSE p.91 / EN p.88)
- TS EN 12845+A2:2026 Clauses 13.3.4.2, 13.3.4.3, 13.3.4.4 (TSE p.92 / EN p.89)
- 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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Download MEP Calc on the App StoreTS EN 12845+A2:2026 (EN 12845:2015+A2:2026) Fixed firefighting systems — Automatic sprinkler systems — Design, installation and maintenance; TS EN 12845-2:2025 (EN 12845-2:2024) ESFR and CMSA sprinkler systems. Every figure in this article is taken from the published standard text; clause, table and page references are listed under References. General information only, not a substitute for the standard or for a design review.