Source: TS EN 12845+A2:2026 (EN 12845:2015+A2:2026), Clauses 4.4.4.3, 8.3, 8.5, 8.6.3, 9.1, 9.2, 9.3.4, 9.6, 9.7 and 10.7.4.
The town main is one of the four types of water supply accepted by EN 12845 and in most projects it is the cheapest option. The conditions under which the standard accepts a main, however, are far more concrete than saying "the pressure looks adequate": it requires a measured curve, a monitored pressure, a strainer and a correct connection arrangement. This article collects those conditions.
Its place as a type of supply: Clause 9.1
Clause 9.1 (TSE p.49 / EN p.46) lists water supplies under four headings:
- a) town mains in accordance with Clause 9.2,
- b) storage tanks in accordance with Clause 9.3,
- c) inexhaustible sources in accordance with Clause 9.4,
- d) pressure tanks in accordance with Clause 9.5.
Clause 9.2 in full
Clause 9.2 (TSE p.49 / EN p.46) is short but dense. It contains four provisions and two notes:
- The town main shall be capable of meeting the pressure, flow and duration requirements, taking into account any additional flow required for manual fire fighting purposes (hydrants, hose reels and the like).
- A pressure switch shall be installed to give an alarm when the supply pressure falls to a predetermined value.
- The switch shall be located upstream of any non-return valve and shall be provided with a test valve (see Annex I and H.2.5).
- In some cases the water quality requires a strainer to be fitted in all connections from the town main. The cross-sectional area of the strainers shall be at least 1.5 times the nominal area of the pipe and they shall not allow the passage of objects larger than 6 mm in diameter.
NOTE 1: the water demand for manual fire fighting is usually determined by the authority having jurisdiction; an additional flow for fire brigade purposes might need to be taken into account. NOTE 2: town main connections usually require the approval of the water authority.
Table I.1 (TSE p.158 / EN p.155) lists this alarm in the row "low pressure in the town main, Clause 9.2, type B"; that is, at technical fault alarm level.
Documenting the main: Clause 4.4.4.3
Where the town main is one of the supplies, or provides the inflow to a reduced capacity storage tank, Clause 4.4.4.3 (TSE p.31 / EN p.28) requires the following details:
- a) the nominal diameter of the main,
- b) whether the main is fed from both ends or is dead-ended; where it is dead-ended, the location of the nearest main fed from both ends to which it is connected,
- c) the pressure/flow characteristic graph determined by a test carried out during the period of maximum demand. At least three pressure/flow points shall be obtained. The graph shall be corrected for friction losses and static head difference between the place of the test and the 'C' gauge of the control valve or the suction tank inlet valve, as appropriate,
- d) the date and time of the test on the main,
- e) the location of the test point on the main relative to the control valve set.
Where the pipework is fully calculated, two further graphs are added:
- f) the pressure/flow characteristic graph showing the pressure available at each flow up to the maximum demand flow,
- g) the demand pressure/flow graph taken at the 'C' gauge of the control valve for the hydraulically most unfavourable (and where necessary the most favourable) area of operation of each installation.
Clause 8.5.1 (TSE p.47-48 / EN p.44-45) completes the measurement discipline: under f), at least three (3) pressure/flow readings shall be taken to establish a test curve, and in addition the static pressure shall be read with no flow. Under e), pressure readings shall be taken only with a test gauge with an accuracy of ±1.6 %; under c), the error of all flow measuring devices shall be within ±5 % of their maximum flow.
The site test: Clause 8.6.3
Clause 8.6.3 (TSE p.49 / EN p.46) gives the test method for town main, booster pump, elevated private reservoir and gravity tank supplies: the stop valves controlling the flow from the supply to the installation shall be fully opened. The automatic starting of the pump set shall be checked by reducing the system pressure. The drain and test valve shall be adjusted to give the flow specified in Clause 7. When the flow has stabilized, it shall be verified that the supply pressure measured at the 'C' gauge is at least the appropriate value specified in Clause 7.
Clause 8.6.1: the test arrangement specified in 8.5.2 shall be used; each supply shall be tested independently, with all other supplies isolated. In both pre-calculated and fully calculated installations the water supply shall be tested at least at the maximum demand flow of the installation.
A boosted main: Clause 10.7.4
Where the town main is boosted by a booster pump, Clause 10.7.4 (TSE p.66 / EN p.63) requires an additional test: it shall be demonstrated by a test that the unboosted supply provides the maximum demand flow plus 20 % at a pressure of at least 0.5 bar measured at the pump inlet. This test shall be carried out at the time of maximum demand on the main.
Clause 10.6.2.5 (TSE p.63 / EN p.60) describes the pressure maintenance pump option for mains with fluctuating pressure: a pressure maintenance pump may be installed to prevent the unnecessary running of one of the main pump sets, or to maintain the system pressure on the control valve sets where the supply is, for example, a town main with fluctuating pressure. This pump shall be sized and arranged so that it cannot provide the flow and pressure needed for a single open sprinkler and therefore cannot prevent the main pump sets from operating. The note to the clause warns that some water authorities might not permit a pressure maintenance pump on systems connected to the town main.
Is the main sufficient on its own: Clause 9.6
Clause 9.6.1 (TSE p.58-59 / EN p.55-56) lists the acceptable single water supplies:
- a) a town main,
- b) a town main with one or more booster pumps,
- c) a pressure tank (LH and OH1 only),
- d) a gravity tank,
- e) a storage tank with one or more pumps,
- f) an inexhaustible source with one or more pumps.
The only hazard class restriction in the list applies to the pressure tank in item c).
Clause 9.6.2 defines superior single water supplies, which are single supplies providing higher reliability. Item a) is for the town main and requires four conditions:
- the main shall be fed from both ends,
- each end shall be capable of meeting the flow demands of the system,
- the main shall be fed from two or more water sources,
- it shall be independent at any point of a single common main.
In addition: where only one end provides the required pressure, a single booster pump shall be installed; where neither end can provide the required pressure, two or more booster pumps shall be installed.
Clause 9.6.3 defines the duplicate supply: it consists of two single water supplies, each independent of the other and each conforming to the pressure and flow characteristics given in Clause 7. Any combination of single supplies (including superior single supplies) may be used, with two restrictions:
- a) more than one pressure tank shall not be used for OH systems,
- b) a storage tank of the reduced capacity type may be used (see 9.3.4).
Using the main to fill a tank: Clause 9.3.4
Where the town main is not sufficient as a direct supply, it can be used to fill a reduced capacity tank. Clause 9.3.4 (TSE p.51 / EN p.48) lists five conditions:
- a) the inflow shall be from a town main and automatic, provided by at least two mechanical ball valves. The inflow shall not adversely affect the pump set suction. The failure of a single ball valve shall not impair the required inflow rate;
- b) the effective capacity of the tank shall be not less than that shown in Table 11;
- c) the tank capacity plus the inflow shall be sufficient to supply the system at the full capacity specified in 9.3.2;
- d) it shall be possible to check the capacity of the inflow;
- e) the inflow arrangement shall be accessible for inspection.
Table 11 - minimum effective capacity of reduced capacity tanks (TSE p.52 / EN p.49):
| Hazard class | Minimum effective capacity (m³) |
|---|---|
| LH (wet or pre-action) | 5 |
| OH1 wet or pre-action | 10 |
| OH1 dry or alternate / OH2 wet or pre-action | 20 |
| OH2 dry or alternate / OH3 wet or pre-action | 30 |
| OH3 dry or alternate / OH4 wet or pre-action | 50 |
| HHP and HHS | 70 (but in no case less than 10 % of the full capacity) |
Where a full capacity tank is used, Clause 9.3.3 binds the filling rate: the water supply shall be capable of filling the tank in not more than 36 h, and the outlet of the supply pipe shall be at a horizontal distance of at least 2.0 m from the inlet of the suction pipe.
Taking water from the main for other services: Clause 8.3 and Table 8
Clause 8.3 (TSE p.46 / EN p.43) makes the supply of water to services other than the sprinkler system subject to four conditions: the connections shall be as specified in Table 8; the connection shall be made upstream of the control valve set(s), through a stop valve as close as possible to the point of connection to the supply pipe of the sprinkler system; the sprinkler system shall not be a high rise system; and the sprinkler system shall not protect a multi-storey building.
The town main rows of Table 8 (TSE p.47 / EN p.44):
| Type of water supply | Acceptable number, size and purpose of connections |
|---|---|
| Town main, main and supply pipe ≥ 100 mm | One connection, not exceeding 25 mm in diameter, for non-industrial use |
| Town main, main and supply pipe ≥ 150 mm | One connection not exceeding 40 mm in diameter for non-industrial use; or one connection not exceeding 50 mm in diameter for fire hose reels (to which a further connection of not more than 40 mm for non-industrial use may be made, with a stop valve close to the first connection and close to the supply end) |
In addition, the sprinkler system pumps shall be separate from the hydrant system pumps, unless a combined water supply in accordance with 9.6.4 is used.
The connection arrangement: Clause 9.7
Clause 9.7 (TSE p.60 / EN p.57): the connections between the water supplies and the sprinkler control valve sets shall be arranged so as to ensure:
- a) that the servicing of the main components such as strainers, pump sets, non-return valves and water meters is facilitated,
- b) that any problem occurring in one supply does not impair the operation of another source or supply,
- c) that maintenance work on one supply does not impair the operation of another source or supply.
Field note: the only concrete provision the standard gives for backflow prevention equipment is on the documentation side. Clause 4.4.4.1 (TSE p.30 / EN p.27) requires the water supply drawings to show the location and type of stop and non-return valves, pressure reducing valves, water meters, backflow preventers and connections supplying water to other services. The class and type of the backflow device is determined from sources outside EN 12845.
Frequently Asked Questions
How many measuring points are needed for the town main curve?
Clause 4.4.4.3 c) (TSE p.31 / EN p.28): the pressure/flow characteristic graph of the town main shall be determined by a test carried out during the period of maximum demand, and at least three pressure/flow points shall be obtained. Clause 8.5.1 f) (TSE p.48 / EN p.45) likewise requires at least three readings to establish a test curve, plus a static pressure reading with no flow.
Is an alarm mandatory on a town main connection?
Yes. Clause 9.2 (TSE p.49 / EN p.46): a pressure switch shall be installed to give an alarm when the supply pressure falls to a predetermined value. The switch shall be located upstream of any non-return valve and shall be provided with a test valve. Table I.1 lists this alarm as type B.
Can a town main be a water supply on its own?
Clause 9.6.1 (TSE p.58-59 / EN p.55-56) lists a town main, and a town main with one or more booster pumps, among the acceptable single water supplies. The only hazard class restriction in the list applies to the pressure tank: c) a pressure tank (LH and OH1 only).
What is the strainer requirement for town main water?
Clause 9.2 (TSE p.49 / EN p.46): in some cases the water quality requires a strainer to be fitted in all connections from the town main. The cross-sectional area of the strainers shall be at least 1.5 times the nominal area of the pipe and they shall not allow the passage of objects larger than 6 mm in diameter.
Which test applies to a boosted town main?
Clause 10.7.4 (TSE p.66 / EN p.63): a test shall demonstrate that the unboosted supply provides the maximum demand flow plus 20 percent at a pressure of at least 0.5 bar measured at the pump inlet. This test shall be carried out at the time of maximum demand on the main.
References
- TS EN 12845+A2:2026 Clause 4.4.4.1 (TSE p.30 / EN p.27), Clause 4.4.4.3 (TSE p.31 / EN p.28)
- TS EN 12845+A2:2026 Clause 8.3 (TSE p.46 / EN p.43), Table 8 (TSE p.47 / EN p.44)
- TS EN 12845+A2:2026 Clauses 8.5.1, 8.6.1, 8.6.2 (TSE p.47-48 / EN p.44-45)
- TS EN 12845+A2:2026 Clauses 8.6.3, 9.1, 9.2 (TSE p.49 / EN p.46)
- TS EN 12845+A2:2026 Clauses 9.3.3, 9.3.4 (TSE p.51 / EN p.48), Table 11 (TSE p.52 / EN p.49)
- TS EN 12845+A2:2026 Clause 9.6.1 (TSE p.58-59 / EN p.55-56), Clauses 9.6.2, 9.6.3 (TSE p.59 / EN p.56)
- TS EN 12845+A2:2026 Clause 9.7 (TSE p.60 / EN p.57)
- TS EN 12845+A2:2026 Clause 10.6.2.5 (TSE p.63 / EN p.60)
- TS EN 12845+A2:2026 Clause 10.7.4 (TSE p.66 / EN p.63)
- TS EN 12845+A2:2026 Table I.1 (TSE p.158 / EN p.155)
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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.