Single, duplex and N+1 arrangements; electric and diesel combinations; jockey pump sizing; and the jockey-versus-main pressure overlap error we see most often on site.
Arriving to commission a textile warehouse, we found two electric main pumps in the pump room with a jockey squeezed between them. The insurer rejected the report: under EN 12845, a duplicate supply cannot have both pumps driven electrically. The mechanical designer had not wanted to hear that for weeks. The clause is unambiguous — but because it goes unread on site, the pump room is where most errors are made.
EN 12845 treats the sprinkler pump set as a whole: pump selection, multiple pump arrangements, the pump room, valves and accessories, suction conditions and performance characteristics. This article covers what matters most in field practice — multiple pump arrangements and the jockey pump.
Single, two or three pumps
The multiple-pump rule fits in one paragraph, but everything that changes the calculation is in it:
- One pump: sufficient for a single water supply. It must meet the design flow and pressure, and deliver at least 70 % of duty head at 140 % flow for HHP and HHS. There is no redundancy; a failure takes the system down.
- Two pumps: each must be capable independently of providing the specified flows and pressures. That is genuine N+1 redundancy — either pump alone meets the commitment.
- Three pumps: each must deliver at least 50 % of design flow at design pressure. Two running together meet the full demand; the third is standby. In large warehouses where total flow reaches 7000–8000 L/min, three smaller pumps are often more practical than two large ones.
Pumps must have characteristics suited to parallel operation — matching curves and shut-off heads. Otherwise the larger pump backfeeds the smaller one and the smaller pump hunts continuously.
Duplicate supply: one electric, one diesel
EN 12845 defines superior and duplicate water supplies. A duplicate supply combines two independent water sources, each capable of feeding the system alone. The critical pump rule is this:
Where more than one pump is installed in a superior or duplicate water supply, no more than one shall be driven by an electric motor.
The logic is simple: loss of mains power is a single point of failure. With two electric pumps, a power cut stops both. So a duplicate arrangement must be electric plus diesel, or diesel plus diesel. Two diesels are safer but carry serious cost and maintenance burden; in practice most projects run one electric and one diesel.
The point that causes site arguments: even where a generator backs up the mains, EN 12845 does not count the generator as a second drive. A diesel is required in case the generator does not start one day. That is not a philosophical choice but a statistical one.
Pump characteristics for pre-calculated LH and OH
In pre-calculated systems, pump selection follows a table giving three characteristic points for each hazard class: the nominal point, an intermediate point and a peak point. The pump curve must pass above all three. For wet and pre-action systems with a height of 15 m or less, measured at the control valve set:
| Hazard | Nominal (bar / L/min) | Intermediate (bar / L/min) | Peak (bar / L/min) |
|---|---|---|---|
| LH | 1.5 / 300 | 3.7 / 225 | — |
| OH1 wet | 1.2 / 900 | 2.2 / 540 | 2.5 / 375 |
| OH2 wet | 1.4 / 1750 | 2.5 / 1000 | 2.9 / 725 |
| OH3 wet | 1.4 / 2250 | 2.9 / 1350 | 3.2 / 1100 |
| OH4 wet | 1.9 / 2650 | 3.0 / 2100 | 3.5 / 1800 |
In buildings taller than 15 m — multi-storey warehouses, atria — the pressures rise, and above 30 m the table moves to a higher band.
The site error here: selecting a curve that meets the nominal point and never checking the intermediate and peak points. Some pumps pass at nominal but collapse at 50–60 % flow. A selection should not be approved without overlaying the catalogue curve on all three points.
The jockey pump: small but dangerous
The jockey, or pressure maintenance pump, compensates for small leaks and pressure fluctuations from temperature change, so that the main pump does not start on every minor loss. Note that the standard sets no numerical limit:
The pressure maintenance pump shall be sized and arranged in such a way that it is not capable of providing enough flow and pressure for a single open sprinkler and thus of preventing the main pump(s) from starting.
The standard defines an outcome rather than a limit: the jockey must be too small to feed one open sprinkler. Two working rules have grown out of that:
- Flow: roughly 1–2 % of the main pump's rated flow, typically 10–25 L/min. Insurers and guidance documents commonly treat 60 L/min as the practical upper bound — above that, a jockey can feed a K57 sprinkler at around 0.5 bar.
- Pressure: the jockey must be able to cut out slightly above the main pump's shut-off pressure. When a sprinkler opens and pressure falls, the jockey responds first and then the main pump — but the jockey must never meet sprinkler flow.
Field error — the jockey set to respond to the main pressure drop
The most common error: the jockey start pressure is set below the main pump start pressure. For example, with the system standing at 7.0 bar, the jockey is set at 6.5 bar and the main pump at 5.8 bar. When a sprinkler opens and pressure falls below 6.5 bar the jockey starts — but its flow is inadequate, so the system continues to 5.8 bar before the main pump starts, and the jockey can never reach its stop pressure, running continuously.
The correct sequence: jockey stop pressure > jockey start pressure > main pump 1 start pressure > main pump 2 start pressure > system static pressure, with at least 0.5–0.7 bar between each step. Otherwise pressure switch tolerance makes the pumps hunt. Main pumps are stopped manually; automatic stopping is prohibited under EN 12845 — one of the most frequently violated rules on site.
Basic pump room requirements
- Fire resistance: the pump compartment must be a separate space with at least 60 min fire resistance — preferably a separate building, otherwise attached to the protected building with an external entrance, and only as a last resort inside it.
- Sprinkler protection: the pump compartment is sprinkler protected; where it is remote from the control valve set, a branch with a flow detector is taken downstream of the pump discharge check valve.
- Temperature: at least 4 °C for electric motors and 10 °C for diesel engines.
- Ventilation: a diesel pump compartment is ventilated to the manufacturer's recommendations, with exhaust discharge, combustion air and heat rejection calculated separately.
- Water temperature: maximum 40 °C, or 25 °C for submersible pumps without a special motor rating.
Comparison with NFPA 20
NFPA 20 and EN 12845 look at the same problem but differ in detail:
- NFPA 20 prefers sizing the jockey so that it runs no more than ten times an hour; EN 12845 defines the outcome (cannot feed one sprinkler).
- NFPA 20 specifies Type 1 and Type 2 transfer switch requirements in detail for electric-plus-diesel pairs; EN 12845 states the simple rule that no more than one of multiple pumps may be electric.
- NFPA 20 defines the curve as shut-off no more than 140 % of rated, and at least 65 % of rated pressure at 150 % capacity; EN 12845 requires at least 70 % at 140 % flow for HHP and HHS — close but not identical.
Turkish context
BYKHY references TS EN 12845 for sprinkler installations. In pump arrangement terms, the reality is that most insurers operating in Turkey — typically local arms of global reinsurers — require a duplicate supply with an electric-plus-diesel combination. In HHS3 warehouses and above, an arrangement of two diesels plus a jockey has become the preferred solution after a number of rejected projects.
As field practice: mark the three table points on the pump curve in the selection report, demonstrate the "cannot feed one sprinkler" test for the jockey with a flow-pressure calculation, and present the transfer switch staging as a schematic. Those three documents prevent about 90 % of insurer objections.
Frequently Asked Questions
Can both pumps in a duplicate supply be electric?
No. Where more than one pump serves a superior or duplicate supply, no more than one may be electrically driven — loss of mains is a single point of failure.
Does a standby generator count as the second drive?
No. EN 12845 does not accept a generator in place of a diesel pump, because the generator itself may fail to start.
How is the jockey pump sized?
So that it cannot feed a single open sprinkler. In practice 1–2 % of main pump flow, typically 10–25 L/min, with 60 L/min treated as the practical upper bound.
Can a main pump stop automatically?
No. Under EN 12845 main pumps are stopped manually only — one of the most frequently violated rules on site.
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.
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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 StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.