EN 12845 Town Main as a Water Supply

When connecting the sprinkler main directly to the town water supply is acceptable, and when it is nonsense — the clause and its field equivalent.

On a shopping centre project the client does not want to pay for a tank and pumps: "There's a town main, connect to that." The answer is not straightforward. EN 12845 recognises the town main as a legitimate water supply — but attaches signed numerical conditions to it. This article opens the town main provisions from a site perspective: under what conditions a direct connection is made, what must be obtained from the water authority, when backflow prevention comes into play, and where local utility practice sits within that framework.

What the clause says

The text is clear: the town main must be capable of meeting the pressure, flow and duration requirements, including manual firefighting needs such as hydrants and hose reels. Those three words — pressure, flow, duration — are the three legs of town main approval. Unless all three are met, a town main cannot be the sole water supply; a tank or booster pump is mandatory.

The clause additionally requires:

• A pressure switch that alarms if main pressure falls below a predetermined value.
• The switch mounted upstream of the check valve and fitted with a test valve.
• A strainer where water quality requires it — with a free area of at least 1.5 times the nominal pipe area, passing no particle larger than 6 mm.
• A note that connection to a town main generally requires the water undertaker's approval — the basis for the utility letter required in Turkey.

The flow test and what the utility letter must contain

Where a town main is chosen as one or both principal supplies, the standard lists what must be retained in the design file. From that comes the site check list:

• A tested pressure-flow characteristic curve. Not a single static pressure reading, but a curve drawn from at least two points — static plus residual under flow. The flow test should coincide with peak daily consumption.
• The test date and time. A measurement taken at 04:00 does not represent the afternoon reality.
• The position of the test point relative to the control valve set. A reading at the building entrance differs from the pressure at the sprinkler valve set; the loss between them must be calculated.
• An allowance for hydrant and hose reel flow. The authority sets that figure, and a separate commitment for fire service use may be required.

In practice the water utilities issue a fire water allocation letter or a main pressure-flow record. If that document does not state "at connection point X, Y bar static and W bar residual at a flow of Z L/min is guaranteed", do not accept the town main as the sole supply.

Where a town main is sufficient, and where it is not

For a practical decision table, set the EN 12845 pre-calculated reserve alongside real town main capacity. Network flow capacity depends not only on pipe size but on total consumption at that hour.

Class	Density	Duration (min)	Pre-calc reserve (m³)	Town main alone?
LH	2.25 mm/min	30	9	Usually yes
OH1	5.0 mm/min	60	55	Generally yes, with DN150 and good residual
OH2	5.0 mm/min	60	105	Marginal — a booster is needed
OH3	5.0 mm/min	60	135	Usually no
OH4	5.0 mm/min	60	High hazard protection	No
HHP1	7.5 mm/min	90	225	No — a tank is mandatory
HHP2	10.0 mm/min	90	275	No
HHS	≥ 7.5 mm/min	90	225–905	No

The general rule: a town main is never the sole supply for HHP and HHS. Two reasons: the instantaneous flow required exceeds the hourly capacity of an average DN200 main, and holding that flow steady for 90 minutes needs a network engineered not to lose pressure even under peak consumption — rare in Turkey. Even in OH classes, a town main plus booster combination is usually preferred.

The pressure switch and low pressure alarm

The clause asks not only for flow and pressure but for continuous monitoring. The pressure switch is placed upstream of the town main check valve, on the network side. If static network pressure falls below a predefined value — generally 10–15 % above the design residual — visual and audible alarms are triggered. That alarm must be monitored by the sprinkler operator or a permanently attended point, and continuous logging is normal field practice.

The detail that gets overlooked but is picked up at inspection: the switch must have a test valve, so that it can be tested at annual maintenance without dropping actual network pressure.

Backflow prevention

Water in the sprinkler system must not return to the potable supply. The reason is simple: water that has stood for weeks, dissolving zinc and copper oxides and supporting bacterial growth, entering the drinking water network is a serious health hazard.

EN 1717 defines contamination categories by use:

• Category 1: potable water direct
• Category 2: potable water with an aesthetic change but no health risk
• Category 3: may contain a low-toxicity substance
• Category 4: substances hazardous to health — sprinkler mains fall here
• Category 5: microbiological hazard — foam-additive systems

A sprinkler main is treated as category 4 by default, and the minimum protection is a type BA (reduced pressure zone, RPZ) backflow preventer. Systems containing foam or AFFF move to category 5, where only an AA or AB type air gap is accepted — the town main discharges into a tank and the sprinkler system draws from the tank, breaking physical continuity entirely.

Turkish water utilities require an RPZ valve in their fire water connection specifications, along with an annual test certificate for it.

Single, superior and duplicate — where a town main sits

EN 12845 classifies water supply reliability in three levels:

• Single supply: a town main alone or with a booster. Acceptable for LH and OH; inadequate for high hazard.
• Superior single supply: a town main fed from two ends, from two independent sources, with no single point of interruption on a shared trunk — in practice a ring-topology network.
• Duplicate supply: two independent single supplies, usually a town main plus a tank-and-pump set.

Ring networks that could qualify as superior single exist in a few older districts of Istanbul; most newer development areas and district centres are radial single-fed networks. That is why town main plus storage tank as a duplicate supply is the safest architecture even on moderate-risk projects outside HHP and HHS.

The reduced capacity tank — the combination solution

Where the town main exists but is inadequate, the standard offers an alternative: a reduced capacity tank. The town main continuously refills the tank through at least two float valves, the tank alone provides the minimum volume from the table, and the balance of capacity is met by town main inflow. For OH3 the tank alone must be at least 50 m³, with the 135 m³ total reserve made up by tank plus continuous infill.

This suits shopping centre and hotel projects very well: tank volume halves, while the RPZ and automatic tank fill arrangement guarantee pressure.

Comparison with NFPA

NFPA 13 covers the equivalent under public water supply, on the same logic. The flow test follows the NFPA 291 method, using residual pressure and a hydrant pitot reading. Backflow requirements vary by jurisdiction; whether a double check detector assembly or an RPZ is required differs by state in the US. The EN 1717 RPZ requirement aligns with the American high-hazard cross-connection approach.

The most common site error

Measuring the static pressure of the town main and concluding "there's 10 bar, that's plenty for sprinklers" is remarkably common. Static pressure means nothing on its own; pressure falls under flow. If a DN150 main drops from 10 bar to 3 bar residual at 1500 L/min, it is not adequate for OH3. If the authority's letter does not carry a guaranteed residual pressure under flow, it is only an estimate.

The second frequent error: placing the pressure switch downstream of the check valve. It then reads the reserve pressure in the sprinkler main rather than network pressure, and never alarms — a town main failure can go unnoticed for days.

Frequently Asked Questions

When can a town main be the sole water supply?

For light hazard and lower ordinary hazard classes, where the authority guarantees pressure, flow and duration under flow. It is never sufficient alone for high hazard process or storage.

Is static pressure enough to judge a town main?

No. Static pressure means nothing on its own — pressure falls under flow. The authority's letter must state a guaranteed residual pressure at the design flow.

What backflow protection does a sprinkler main need?

A sprinkler main is EN 1717 category 4, requiring at least a type BA (RPZ) preventer. Foam-additive systems are category 5 and require a physical air gap.

Where does the pressure switch go?

Upstream of the town main check valve, on the network side, with its own test valve. Placed downstream it reads system reserve pressure and never alarms on a network failure.

SprinkCalc — Fire Sprinkler Design Across Three Standards

SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.

Download SprinkCalc on the App Store

MEP Calc — 86+ Engineering Calculators

MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.

Download MEP Calc on the App Store