In TS EN 12845+A2:2026 the pipe diameter comes out of two separate regimes: on pre-calculated systems everything downstream of the design point comes from tables and everything upstream from calculation; on fully calculated systems everything comes from hydraulic calculation. Beneath both regimes lies an unvarying floor: Table 36. Source: TS EN 12845+A2:2026, Clause 13.1, Clause 13.3, Clause 13.4.5 and Tables 27-36.
The unvarying floor: Table 36
Clause 13.4.5 (TS p.106 / EN p.103): the pipe diameter shall not be less than that shown in Table 36.
| Risk | Diameter |
|---|---|
| LH | 20 mm |
| OH and HH: horizontal and upright pipe feeding a single sprinkler with a K-factor not exceeding 80 | 20 mm |
| All other cases | 25 mm |
Clause 13.3.5.1 (TS p.92 / EN p.89) repeats this for high hazard: no pipe shall be of a nominal diameter smaller than 25 mm.
The three rules that follow in the same clause determine the topology of the installation:
- Pipe diameters on the installation side of the control valve set may only reduce in the direction of water flow; grid and loop configurations are the exception. (Clause 13.3.1.1 says the same for pre-calculated systems: pipe diameters shall not increase in the direction of water flow towards any sprinkler.)
- Upright sprinklers shall not be connected to any pipe of a diameter greater than 65 mm, or 50 mm where the pipe is lagged.
- Pendent sprinklers shall not be connected directly to any pipe of a diameter greater than 80 mm. For larger diameters, an arm pipe shall be fitted such that the distance from the sprinkler deflector to the side of the main pipe is not less than 1.5 times the diameter of that pipe.
Clause 3.12 defines an arm pipe: a pipe, other than the final section of a range pipe, feeding a single sprinkler and shorter than 0.3 m.
Which system falls under which regime?
Clause 13.1 (TS p.85 / EN p.82): pipe sizes are determined by one of two methods. The designer may choose between the two, but the fully calculated method shall always be used in the following cases:
- arrangements with intermediate level HHS sprinklers,
- gridded or looped arrangements.
Clause 13.3.1.3: all pipes upstream of each design point are sized by calculation as specified in Clause 13.3.3.2 for Light Hazard and in Clause 13.3.4.2 for Ordinary Hazard. Clause 13.3.1.4: risers and drops connecting distribution pipes to ranges, and pipes other than arm pipes feeding a single sprinkler, are regarded as distribution pipes and sized accordingly.
LH: Table 27
Clause 13.3.3.1 and Table 27 (TS p.90 / EN p.87):
| Pipes | Diameter | Maximum number of sprinklers |
|---|---|---|
| All range pipes and terminal distribution pipes | 20 mm | 1 |
| 25 mm | 3 |
Clause 13.3.1.2 narrows the scope: in Light Hazard, Table 27 determines only the pipes feeding the last three or four sprinklers on each range. Clause 13.3.3.1 also states that, where hydraulic calculation shows it to be possible, 25 mm pipe may be used between the design point and the control valve set, but that where the 2 sprinkler point governs, 25 mm pipe shall not be used between the 3rd and 4th sprinklers.
OH: Table 30 and Table 31
Clause 13.3.4.1 (TS p.91 / EN p.88): range pipe diameters shall comply with Table 30 and distribution pipe diameters with Table 31.
Table 30 - OH range pipes:
| Range pipes | Layout | Diameter | Maximum sprinklers fed |
|---|---|---|---|
| Last 2 ranges at the far end of all distribution pipes | 2-end-side | 25 mm | 1 |
| 32 mm | 2 | ||
| Last 3 ranges | 3-end-side | 25 mm | 2 |
| Last range | 32 mm | 3 | |
| All other layouts | 25 mm | 2 | |
| 32 mm | 3 | ||
| 40 mm | 4 | ||
| 50 mm | 9 | ||
| All other range pipes | All | 25 mm | 3 |
| 32 mm | 4 | ||
| 40 mm | 6 | ||
| 50 mm | 9 |
Table 31 - OH distribution pipes:
| Distribution pipes | Layout | Diameter | Maximum sprinklers fed |
|---|---|---|---|
| At the ends of the installation | 2-end-side | 32 mm | 2 |
| 40 mm | 4 | ||
| 50 mm | 8 | ||
| 65 mm | 16 | ||
| Others | 32 mm | 3 | |
| 40 mm | 6 | ||
| 50 mm | 9 | ||
| 65 mm | 18 | ||
| Between the design points and the control valve set | All | Calculated in accordance with Clause 13.3.4.2 |
The continuation of Clause 13.3.4.1 (TS p.92 / EN p.89): where range pipes run longitudinally beneath roofs with a slope greater than 6°, the number of sprinklers on a range pipe shall not exceed six.
Clause 13.3.4.2: the pipe diameters between the design point in the most remote area of the installation and the control valve set shall be calculated such that the total pressure loss due to friction at a flow of 1 000 l/min does not exceed 0.5 bar. Clause 13.3.4.3 states that this 0.5 bar may be increased by the static height difference in multi-storey buildings; Clause 13.3.4.4 states that where OH3/OH4 and HHP/HHS areas on the same system are connected to a common water supply, the maximum friction loss may be increased by 50 per cent of the excess pressure available, and gives an example.
Design point, Table 26 (Clause 13.3.2.3, TS p.89 / EN p.86):
| Hazard class | Number of sprinklers on one distribution pipe in the room | Design point at the connection of the range carrying the nth sprinkler | Range layout |
|---|---|---|---|
| OH | > 16 | 17 | two end-side |
| OH | > 18 | 19 | others |
| HHP and HHS | > 48 | 49 | all |
HH: four tables, two pairings
In high hazard, the choice of table depends on which block of Table 7 the water supply complies with and on the sprinkler K-factor.
Clause 13.3.5.2 (TS p.92 / EN p.89): on installations where the water supply complies with Table 7 (1) and the K-factor is 80, the range and distribution pipe diameters are taken from Table 32 and Table 33. Not more than four sprinklers shall be fitted to any range pipe; range pipes shall not be connected to distribution pipes of a diameter greater than 150 mm.
Table 32 - HH range pipes (Table 7 (1) or (2)):
| Range pipe | Layout | Diameter | Maximum sprinklers fed |
|---|---|---|---|
| Ranges at the far end of all distribution pipes | 2-end-side, last two ranges | 25 mm | 1 |
| 32 mm | 2 | ||
| 3-end-side, last three ranges | 25 mm | 2 | |
| 32 mm | 3 | ||
| All other layouts, last range only | 25 mm | 2 | |
| 32 mm | 3 | ||
| 40 mm | 4 | ||
| All other ranges | Any | 25 mm | 3 |
| 32 mm | 4 |
Table 32 contains only the 25, 32 and 40 mm rows; 50 mm and above do not appear in that table.
Table 33 - distribution pipes with Table 7 (1) (TS p.93 / EN p.90): at the ends of the installation, 32 mm 2, 40 mm 4, 50 mm 8, 65 mm 12, 80 mm 18, 100 mm 48 sprinklers.
Clause 13.3.5.3 (TS p.93 / EN p.90): on installations where the water supply complies with Table 7 (2) (or as modified by Clause 7.3.2.6) and the K-factor is 80, the sizes are determined from Table 32 and Table 34. Not more than four sprinklers are fitted to a range pipe; no range pipe is connected to a distribution pipe exceeding 150 mm; on 4-end-side systems, distribution pipes smaller than 65 mm are not used.
Table 34 - distribution pipes with Table 7 (2, 3 or 4) (TS p.94 / EN p.91): at the ends of the system, 50 mm 4, 65 mm 8, 80 mm 12, 100 mm 16, 150 mm 48 sprinklers.
Clause 13.3.5.4 (TS p.94 / EN p.91): on installations where the water supply satisfies the requirements of Table 7 (3) and the K-factor is 80, and on installations with a K-factor of 115 as shown in Table 7 (4), the range and distribution pipe sizes are determined from Table 34 and Table 35. Not more than six sprinklers are fitted to a range pipe in an end-side layout, and not more than four in a 2-end-centre layout.
Table 35 - HH range pipes (Table 7 (3) or (4), TS p.95 / EN p.92):
| Range pipes | Layout | Diameter | Maximum sprinklers fed |
|---|---|---|---|
| Ranges at the far end | End-side, last three ranges | 40 mm | 1 |
| 50 mm | 3 | ||
| 65 mm | 6 | ||
| Other ranges | 32 mm | 1 | |
| 40 mm | 2 | ||
| 50 mm | 4 | ||
| 65 mm | 6 | ||
| Ranges at the far end | 2-end-centre, last three ranges | 32 mm | 1 |
| 40 mm | 2 | ||
| Other ranges | 32 mm | 2 | |
| All ranges | 3 and 4 end-centre | 32 / 40 / 50 mm | 1 / 2 / 4 |
Clause 13.3.5.5: the pressure loss between the design points and the control valve set is determined by calculation. The pressure loss occurring at the flows in Table 7, plus the pressure required at the design point, plus the static pressure equal to the height difference between the highest sprinkler and the control valve set, shall not exceed the pressure available.
The mathematics of the calculation
Clause 13.2.1 (TS p.85-86 / EN p.82-83): pipe friction loss calculations shall not be less than those derived from the Hazen-Williams formula:
p = 6.05 x 10⁵ x L x Q^1.85 / (C^1.85 x d^4.87)
where p is in bar, Q in l/min, d in mm (mean internal diameter), C is a constant depending on the type and condition of the pipe, and L is the equivalent length of pipe and fittings (m). Velocity pressure loss may be neglected.
Table 22 - values of C (TS p.86 / EN p.83): cast iron 100; ductile iron 110; mild steel 120; galvanized steel 120; spun cement 130; cement lined cast iron 130; stainless steel 140; copper 140; glass fibre reinforced 140.
The exponent d^4.87 in the formula shows the effect of a change in diameter. Table 29 of the standard itself (LH, 225 l/min column) gives the numerical equivalent: 198 mbar/m at 25 mm against 52 mbar/m at 32 mm, so going up one size reduces the loss to roughly a quarter.
Clause 13.2.4 and Table 23 (TS p.87 / EN p.84) give the losses at fittings and valves as equivalent lengths; where both the direction and the diameter change at an elbow, tee or cross, the equivalent length and pressure loss are determined on the basis of the smaller diameter.
Diameter rules on the material and workmanship side
- Clause 17.1.2 (TS p.114 / EN p.111): steel pipes of nominal diameter equal to or less than 150 mm shall have a minimum wall thickness complying with ISO 65 where they are threaded, grooved or otherwise machined. Where the pipe ends are formed without significant reduction in wall thickness (e.g. roll-grooving or end preparation for welding), they shall have a minimum wall thickness complying with ISO 4200 range D. Galvanized steel should preferably be used on dry, alternate or pre-action installations.
- Clause 17.1.3 (TS p.114 / EN p.111): pipes and fittings of a diameter smaller than 50 mm shall not be welded on site unless the installer uses an automatic welding machine.
- Clause 17.1.9 (TS p.116 / EN p.113): copper pipes may be used only on LH, OH1, OH2 and OH3 wet systems, downstream of steel pipe.
- Clause 17.2.2 (TS p.116 / EN p.113): supports are spaced at not more than 4 m on steel pipe and not more than 2 m on copper pipe; on pipes larger than 50 mm these distances may be increased by 50 per cent under stated conditions. The distance from any terminal sprinkler to a support shall not exceed 0.9 m on 25 mm pipe and 1.2 m on pipe larger than 25 mm.
- Clause 15.6 (TS p.112 / EN p.109): flushing connections shall be the same size as the distribution pipe; on pipes larger than DN40, a DN40 flushing connection may be used provided that it is connected to the underside of the distribution pipe.
- Table 39 (Clause 15.4, TS p.111 / EN p.108): minimum drain valve diameters: LH installation 40 mm; OH, HHP or HHS installation 50 mm; subsidiary installation 50 mm; zone 50 mm; dead end distribution pipes of diameter ≤ 80 mm 25 mm and those > 80 mm 40 mm; dead end range pipes 25 mm.
- Clause 17.3.3 (TS p.118 / EN p.115): on pre-calculated systems, the diameter of distribution pipes feeding sprinklers both inside and outside a concealed space shall not be less than 65 mm.
- Clause 11.6 (TS p.72 / EN p.69): water spray extensions may be connected to a sprinkler installation provided that the connection is not greater than 80 mm.
- Clause 16.1.3 (TS p.113 / EN p.110): the pipe to the water motor alarm shall be 20 mm diameter galvanized steel or non-ferrous metallic material.
The second check on the diameter: pressure and velocity
The pipe diameter has to pass not only the table but two upper limits as well:
- 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, or the greater of the following: 0.70 bar in LH, 0.35 bar in OH, 0.50 bar in HHP and HHS (excluding in-rack), 1.00 bar for K115 in-rack sprinklers and 2.00 bar for K80 in-rack sprinklers.
- Clause 13.2.3 (TS p.86 / EN p.83): the water velocity shall not exceed 6 m/s at any valve, flow monitoring device and/or strainer, nor 10 m/s at any other point in the system.
Field note: because all of the tables are written in terms of a "maximum number of sprinklers", a single sprinkler added to a range later in the project can push that range up one diameter; on revisions, looking at the table before the calculation saves time.
Frequently Asked Questions
What is the minimum pipe diameter in EN 12845?
Table 36 (Clause 13.4.5, TS p.106 / EN p.103): 20 mm for an LH risk; 20 mm for horizontal and upright pipe feeding a single sprinkler with a K-factor not exceeding 80 in OH and HH; 25 mm in all other cases. In addition, Clause 13.3.5.1 states that no pipe in high hazard shall be of a nominal diameter smaller than 25 mm.
May the pipe diameter increase in the direction of flow?
No. Clause 13.3.1.1 (TS p.88 / EN p.85) states that on pre-calculated systems the pipe diameters shall not increase in the direction of water flow towards any sprinkler. Clause 13.4.5 states that pipe diameters on the installation side of the control valve set may only reduce in the direction of water flow, grid and loop configurations being the exception.
Up to what diameter may upright and pendent sprinklers be connected directly?
Clause 13.4.5 (TS p.106 / EN p.103): upright sprinklers shall not be connected to any pipe of a diameter greater than 65 mm, or 50 mm where the pipe is lagged. Pendent sprinklers shall not be connected directly to any pipe of a diameter greater than 80 mm. For larger diameters, an arm pipe is fitted such that the distance from the sprinkler deflector to the side of the main pipe is not less than 1.5 times the diameter of that pipe.
How many sprinklers may be connected to a range pipe in OH?
Table 30 (TS p.91 / EN p.88): on all other range pipes, 25 mm feeds 3, 32 mm 4, 40 mm 6 and 50 mm 9 sprinklers. On the last two ranges at the far end of all distribution pipes (2-end-side layout), 25 mm feeds 1 and 32 mm 2; on the last three ranges in a 3-end-side layout, 25 mm feeds 2 and the last range in 32 mm feeds 3; in all other layouts, 25 mm feeds 2, 32 mm 3, 40 mm 4 and 50 mm 9 sprinklers.
Which tables apply where K115 sprinklers are used in HH?
Clause 13.3.5.4 (TS p.94 / EN p.91): on installations where the water supply satisfies the requirements of Table 7 (3) and the K-factor is 80, and on installations with a K-factor of 115 as shown in Table 7 (4), the range and distribution pipe sizes are determined from Table 34 and Table 35. Table 35 gives the range pipes and Table 34 the distribution pipes. Not more than six sprinklers are fitted to a range pipe in an end-side layout, and not more than four in a 2-end-centre layout.
References
- TS EN 12845+A2:2026 Clause 3.12 (TS p.19 / EN p.16)
- TS EN 12845+A2:2026 Clause 11.6 (TS p.72 / EN p.69)
- TS EN 12845+A2:2026 Clause 13.1, Clause 13.2.1 (TS p.85-86 / EN p.82-83)
- TS EN 12845+A2:2026 Table 22, Clause 13.2.3 (TS p.86 / EN p.83), Clause 13.2.4, Table 23 (TS p.87 / EN p.84)
- TS EN 12845+A2:2026 Clauses 13.3.1.1, 13.3.1.2, 13.3.1.3, 13.3.1.4 (TS p.88 / EN p.85)
- TS EN 12845+A2:2026 Clause 13.3.2.3, Table 26 (TS p.89 / EN p.86), Clause 13.3.3.1, Table 27, Table 29 (TS p.90 / EN p.87)
- TS EN 12845+A2:2026 Clause 13.3.4.1, Table 30, Table 31 (TS p.91 / EN p.88)
- TS EN 12845+A2:2026 Clauses 13.3.4.2-13.3.4.4, Clauses 13.3.5.1, 13.3.5.2, Table 32 (TS p.92 / EN p.89)
- TS EN 12845+A2:2026 Table 33, Clause 13.3.5.3 (TS p.93 / EN p.90), Table 34, Clause 13.3.5.4 (TS p.94 / EN p.91)
- TS EN 12845+A2:2026 Table 35, Clause 13.3.5.5 (TS p.95 / EN p.92)
- 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 15.4, Table 39 (TS p.111 / EN p.108), Clause 15.6 (TS p.112 / EN p.109)
- TS EN 12845+A2:2026 Clause 16.1.3 (TS p.113 / EN p.110)
- TS EN 12845+A2:2026 Clause 17.1.2, Clause 17.1.3 (TS p.114 / EN p.111), Clause 17.1.9, Clause 17.2.2 (TS p.116 / EN p.113), Clause 17.3.3 (TS p.118 / EN p.115)
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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.