EN 12845 Water Supply Selection: Single, Duplicate and Combined

Single, superior single, duplicate and combined supplies — which belongs in which hazard class, and what goes wrong on site.

On a logistics warehouse project in Istanbul, the client was happy with the architectural team's proposal of "one pump plus a mains connection". I was not. The facility was HHS3, and the insurer's surveyor had written "no duplicate water supply" on the first page of the report. I had written the same note the day before. EN 12845 states the rules plainly — but without reading which supply arrangement is adequate for which hazard class, most projects get this wrong.

This article explains the four supply arrangements in a way that is usable on site: single, superior single, duplicate and combined. A decision table at the end sets out which suits which hazard class.

Water supply types

The standard lists the supply types: town main, pressure tank, gravity tank, storage tank with a pump set, and an inexhaustible source (lake, sea, river, deep borehole). Each may be used alone or in combination, but acceptability is set by the supply arrangement clauses. The type alone is not enough; the topology of the supply must suit the hazard class.

Single water supply

A single supply is one feed path with no redundancy. The standard accepts the following as single supplies:

• A town main
• A town main with a booster pump
• A pressure tank (LH and OH1 only)
• A gravity tank
• A storage tank with one or more pumps
• An inexhaustible source with one or more pumps

What a single supply means in practice: one pump room, one suction line, one tank. If the pump fails or the tank is taken out for maintenance, the system is defenceless. EN 12845 accepts it — but it is only sensible for lower hazard classes.

Superior single supply

It looks like a single supply but contains physical redundancy. Three forms are recognised:

• A main fed from both ends: a superior single supply if both ends can meet the system flow demand, both connect to independent water sources, and there is no single dependent point on the trunk main. Where only one end delivers the required pressure, a single booster pump is added; where neither does, two or more boosters are installed.
• A gravity tank without boosters, or a storage tank with two or more pumps: the tank must be full capacity, sealed against light and foreign matter, filled with clean water, and protected against corrosion to at least a ten-year maintenance interval.
• An inexhaustible source with two or more pumps.

My site observation: superior single is most often misread as "there are two pumps in the pump room, so we're fine". No. If two pumps are fed from a common suction line, there is a single point of failure — that is not superior single. What the standard wants is an independent backup for the supply itself.

Duplicate supply

The clause is explicit: duplicate water supplies consist of two single water supplies, each independent of the other. Both must be capable of meeting the pressure-flow characteristic alone. Take one out and the system still delivers design density.

Two restrictions apply:

• In OH systems, at most one leg of a duplicate supply may be a pressure tank.
• At most one reduced-capacity storage tank may be used within a duplicate supply.

Duplicate is often sold on site as a "two-pump package"; the correct version is two separate suction lines, two separate tanks (or one tank with two independent suction chambers — separate suction and settling compartments are mandatory for a duplicate supply), and two separate pump sets. The pump clause adds a hard rule: where more than one pump serves a superior or duplicate supply, no more than one may be electrically driven. The rest must be diesel. That is the sharpest counter to the "two electric pumps equals duplicate" fallacy.

Combined supply: sprinklers plus hydrants

Where one supply feeds both a sprinkler system and hydrants or hose reels, the combined supply clause applies. First a structural condition: a combined supply must be superior single or duplicate. An ordinary single supply cannot be used as a combined supply. Then four conditions:

1. The systems must be fully hydraulically calculated — the pre-calculated method is not accepted.
2. The supply must deliver the sum of the simultaneous maximum calculated flows, adjusted to the pressure required by the most demanding system.
3. The duration must be no less than that required by the most demanding system.
4. Duplicate pipe connections must be provided between the supply and the systems.

An important note: some countries do not permit a sprinkler system to share a supply with hydrants. In Turkey, BYKHY does not prohibit it outright, but insurers and large industrial projects prefer separated supplies with a separate pump room for each. Even where a combined supply is permitted, sprinkler pumps must remain separate from hydrant pumps.

Recommendation by hazard class

The standard does not say "duplicate is mandatory for HHP"; it lists the types and leaves the choice to the designer. But the annex covering high-hazard storage with saving stock states clearly that the system must have at least one superior single supply. Insurer supplements make duplicate a contractual condition for HHP2 and above. My working decision table:

Hazard	Minimum (EN 12845)	Recommended in practice
LH	Single	Single is adequate; pressure tank accepted
OH1	Single	Single adequate; pressure tank only at OH1
OH2	Single	Superior single (two pumps, one diesel)
OH3	Single	Treat superior single as mandatory
OH4	Single	Superior single; duplicate where insured
HHP1	Single	Superior single, preferably duplicate
HHP2 / HHP3	Single (per the standard)	Duplicate, as an insurance condition
HHS1–HHS4	At least superior single	Duplicate

Supply isolation

Choosing the supply is not enough; the connection topology must satisfy the isolation requirements. Three conditions: main components such as strainers, pump sets, check valves and water meters must be serviceable; a fault on one supply must not prevent the other from operating; and one supply must be able to continue working while the other is under maintenance. Build a duplicate supply and run it through a common isolating valve and the duplication is meaningless.

Common errors

• The "two electric pumps equals duplicate" fallacy. Where more than one pump serves a superior or duplicate supply, at most one may be electric. The rest must be diesel.
• A common suction line. Two pumps fed from one tank through one suction pipe is not a duplicate supply; separate suction and settling compartments are mandatory.
• Using the pre-calculated method on a combined supply. Fully calculated design is explicitly required.
• Omitting the flow summation on a combined supply. It is not sprinkler flow with hydrant flow appended; it is the sum of the simultaneous maximum flows, adjusted to the highest required pressure.
• Using a pressure tank on an HHP or HHS leg. Pressure tanks are permitted only at LH and OH1.
• Claiming superior single for any two-ended main. If the two ends do not come from independent sources, or there is a single dependent point on the trunk main, it is not superior single.

Comparison with NFPA 13

NFPA 13's water supply chapter does not draw the single/duplicate distinction as sharply as EN 12845. It requires the supply to be reliable and of adequate capacity, pressure and duration, leaving reliability to the authority having jurisdiction. NFPA 20 solves the secondary supply question at the pump set through the diesel-plus-electric combination. The advantage of EN 12845 is that it takes the decision out of individual judgement and puts it into readable rules. In field practice, an EN 12845 duplicate supply is equivalent to the combined effect of NFPA 13, NFPA 20 and the relevant FM Global data sheet.

Turkish context

BYKHY uses the general phrase "adequate capacity and reliability" without detailing supply topology. Filling that gap falls to EN 12845 or NFPA. Because large logistics, automotive and chemical investments in Turkey generally work with international insurers, contracts reference EN 12845 plus the insurer's data sheet — making a duplicate supply non-negotiable for HHP2 and above. For mid-sized warehouses and production facilities in industrial zones, superior single is widely accepted at OH3 and OH4.

Decision flow

1. Fix the hazard class (LH, OH1–4, HHP1–3, HHS1–4).
2. Check whether the insurer or authority imposes a specific condition. If so, it takes precedence.
3. Measure the town main characteristic — fed from both ends? how many bar, how many L/min? If it can be made into a superior single arrangement, evaluate it for OH.
4. For HHP and HHS, plan a duplicate supply: separate tank, separate suction, separate pump room, at least one diesel pump.
5. If hydrants share the supply with sprinklers, add the four combined-supply conditions to the design.
6. Valve the system for proper isolation; leave no common isolating valve.

Water supply selection is the most expensive decision in sprinkler design. A wrong choice cannot be unpicked on site; planning a duplicate supply from the start is always cheaper than converting to one later.

Frequently Asked Questions

Does two pumps make a duplicate supply?

No. Two pumps on a common suction line share a single point of failure. A duplicate supply needs two independent suctions, tanks and pump sets — and at most one may be electrically driven.

What is a superior single supply?

A single supply containing physical redundancy — a main fed from two independent sources with no single dependent point, a full-capacity gravity tank, or an inexhaustible source with two or more pumps.

Can hydrants share the sprinkler supply?

Only where the supply is superior single or duplicate, both systems are fully calculated, the supply meets the sum of simultaneous maximum flows, and duplicate pipe connections are provided.

Where can a pressure tank be used?

Only at LH and OH1, and at most as one leg of a duplicate supply in OH systems.

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