EN 12845 Reduced Capacity Tanks

Why and how far you can reduce tank volume using a town main plus automatic infill — the reduced capacity conditions translated to site practice.

On a logistics warehouse project the client said "we're casting a 160 m³ tank for OH3 and it doesn't fit". On site the main held 8 bar over long periods, the connection was DN200, and town water was available around the clock. In that situation EN 12845 offers a gift: the reduced capacity tank. Tank volume can drop to 50 m³, with the remainder taken from the main by instantaneous infill during the fire. But a designer who does not grasp the difference between "there is a main" and "there is a guaranteed main" is engineering a failure.

What the clause says

EN 12845:2015+A1:2019 permits sprinkler water to be provided from one of two arrangements:

• A full capacity tank — effective volume at least design duration times design flow.
• A reduced capacity tank — effective volume plus automatic infill together providing the same volume.

A reduced capacity tank relies on the main continuing to feed during a fire. The standard therefore imposes five conditions:

1. Infill comes only from the town main and is automatic, through at least two mechanical float valves. Failure of one valve must not compromise the required fill rate. The infill point must not disturb pump suction.
2. The tank's effective volume must not be less than the minimum in the table.
3. Tank volume plus infill rate must feed the system at full capacity for the full design duration.
4. The infill capacity must be verifiable — measurable in open flow when there is no fire.
5. The infill arrangement must be accessible for inspection.

Minimum effective volumes

How small can the tank be? These are the fixed figures — whatever the main delivers, you cannot go below them:

Hazard class	Minimum effective volume (m³)
LH — wet or pre-action	5
OH1 — wet or pre-action	10
OH1 — dry or alternate	20
OH2 — dry or alternate (also applied to wet)	30
OH3 — dry or alternate (also applied to wet)	50
OH4 — wet, dry or alternate	50
HHP and HHS	70, but never less than 10 % of the full volume

The footnote on HHP and HHS matters: if the full volume for a high-hazard storage system is 800 m³, 70 m³ is not enough — the 10 % rule takes over and the minimum becomes 80 m³. This table does not replace the full-volume tables; it is only the floor beneath them.

Volume calculation: full volume less the main's contribution

The basis of reduced capacity is this expression:

V_tank ≥ max( V_min ; V_full − Q_infill × t )

Where:

• V_full — the total water volume from the pre-calculated tables, or from maximum flow times duration for a fully calculated design.
• Q_infill — the guaranteed fill rate from the main through the float valves, taken from a pressure-flow curve measured during peak consumption.
• t — the design system duration: 30–60 min for LH and OH, 90 min for HHP and HHS.

Example: OH3 wet, height ≤ 15 m

The table gives a full volume of 140 m³ for OH3 wet, with a 60 min duration. The main test shows a guaranteed fill of 1500 L/min.

• Water the main delivers in 60 min: 1500 × 60 = 90 000 L = 90 m³
• Minimum tank required: 140 − 90 = 50 m³
• The OH3 floor from the table: 50 m³ — satisfied

Both conditions intersect, so the tank can be built at 50 m³. Casting 50 m³ instead of 140 m³ on that project saves 90 m³ of concrete and stainless volume.

Example: OH3 wet with a weak main

Same project, but the main test gives 600 L/min. The main's 60 min contribution is 36 m³, so the tank must be 140 − 36 = 104 m³. At which point the useful question becomes: is a 104 m³ tank simpler and safer than a 140 m³ full capacity tank? Usually yes — because the inspection and maintenance burden of reduced capacity loses its point once the volume gained shrinks.

Which hazard classes permit it?

The clause covers every hazard class from LH to HHS, so it is theoretically permitted throughout. But the duplicate supply clause imposes a limit: at most one reduced capacity tank may appear within a duplicate supply set. If the first supply is reduced capacity, the second must be full capacity.

The practical distribution on site:

• LH and OH1/OH2 — the most common application. The tank is small anyway, and main assistance gives a clear saving.
• OH3 and OH4 — the largest gain, in logistics and production facilities. Half of a 140–200 m³ tank can be transferred to the main.
• HHP1/HHP2 — applicable, but guaranteeing a steady main flow over 90 minutes is risky, so most designers prefer full capacity.
• HHP3 and HHS — technically permitted, but between the 10 % floor and the long duration the saving narrows sharply, and insurers usually decline to approve it.

Common field errors

1. "There is a main" is not "there is a guaranteed main"

The most frequent error is treating the existence of town water as an infill guarantee. The clause requires the main's characteristic to be documented by a pressure-flow test during peak consumption. A design measuring 1500 L/min on a summer afternoon, then getting half that in a fire when line pressure has dropped, has a collapsed calculation. The test data, its date and the distance from the test point to the control valve set must all be reported.

2. A single float valve or a single infill line

The clause requires two mechanical float valves, with failure of one not compromising the fill rate. In practice, both valves must be arranged in parallel and each sized for the full rate. "Two small valves that suffice when both are working" is an error; when one fails, the fill falls short.

3. Solving the equation and forgetting the floor

With a main delivering 2400 L/min, the OH3 equation gives 140 − 144 = negative, and someone concludes "no tank required". Wrong. The floor is 50 m³ and no main, however strong, takes you below it. The tank lets the system stand alone for minutes if the main fails; it is designed as a buffer, not as insurance.

4. Never measuring the infill rate

The clause requires infill capacity to be verifiable. A common omission on site is that no control drain or flow meter is provided to measure water passing the float valves. Without it, the actual fill rate cannot be verified at the annual test. A test drain behind the manifold is sufficient in most cases.

5. Freeze protection on the main line

The standard requires the tank to be insulated against freezing — but with reduced capacity, the part of the main line outside the tank is also exposed to winter temperatures. A frozen infill line is functionally identical to a fully blocked strainer. The line must be either buried below the frost line or in a heated enclosure.

Comparison with NFPA

NFPA 13 and NFPA 22 recognise a comparable limited water supply concept, but without a fixed minimum volume table. On the NFPA side the supply is sized as system duration times design flow, and where main infill is used, authority approval is required — with fire departments generally conservative about pressure drop. EN 12845 takes a more mechanical approach, on condition that infill is measurable.

Turkish context

BYKHY directs sprinkler tank volume to EN 12845 and NFPA 13. Town mains in many Turkish industrial zones are physically DN150–DN200; but designers who document the required peak-hour pressure-flow test in coordination with the fire service or water authority are in the minority. As a result, most reduced capacity tank designs in Turkey are wrongly sized — the tank has been reduced against an assumed main flow rather than a measured one. During a fire, a neighbouring facility using hydrants or hose reels shares the main; the test must therefore be run under a peak consumption scenario, not at a single quiet point.

Conclusion — when to do it and when not to

A reduced capacity tank makes sense under three conditions: a documented peak-hour test on the main; a parallel infill manifold with two float valves that can actually be built; and the ability to measure and record the infill rate at annual maintenance. Miss any of the three and go back to a full capacity tank — the post-loss cost of a 50 m³ tank cast on the assumption that "there is a main", when the system fails to run at full flow on the day, is not measured in the tank volume you saved.

Frequently Asked Questions

How small can a reduced capacity tank be?

5 m³ for LH, 10–20 m³ for OH1, 50 m³ for OH3 and OH4, and 70 m³ for high hazard — but never less than 10 % of the full volume.

What infill arrangement is required?

Automatic filling from the town main through at least two mechanical float valves, each sized for the full rate, positioned so as not to disturb pump suction.

Can a strong main eliminate the tank entirely?

No. However strong the main, the minimum effective volume from the table still applies. The tank is a buffer that keeps the system running if the main fails.

Why must the infill rate be measured?

Because the calculation depends on a guaranteed rate. Without a test drain or flow meter, the actual fill rate cannot be verified at annual maintenance and the design is unproven.

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