The first decision in fire pump selection is the drive type, and it affects everything from the tank room to the electrical design. There is no single right answer to electric or diesel; but where a superior or duplicate water supply has more than one pump, at most one may be electric.
Side by side
| Criterion | Electric motor | Diesel engine |
|---|---|---|
| Mains dependence | Total | None (independent on batteries) |
| First cost | Low | High (engine, fuel, exhaust, batteries) |
| Pump room area | Small | Large (ventilation, exhaust, fuel) |
| Ventilation | Ordinary | Combustion plus cooling plus radiator air |
| Room temperature | At least 4 °C | At least 10 °C |
| Fire resistance | REI 60 | REI 60 |
| Weekly test | 5 minutes at full speed | 20–30 minutes plus a manual restart |
| Maintenance burden | Low | High (oil, filters, batteries, fuel turnover) |
| Fuel management | None | Tank, turnover, water separation, level alarm |
| Noise | Low | High; silencer and insulation needed |
| Redundancy role | Duty pump | Standby pump (typical) |
The rule: only one electric pump on a duplicate supply
EN 12845 is explicit: where more than one pump forms part of a superior or duplicate water supply, at most one may be driven by an electric motor; the rest must be diesel. The reasoning is one sentence — a mains outage is a single point of failure. If two electric pumps are fed from the same transformer, both fall when that transformer does.
Feeding from two separate transformers is also not automatically two sources; two cables from one transformer are never two sources. Two independent medium-voltage feeds, or one transformer plus an independent generator, are required, and the cables must be run at least 3 m apart.
The hidden costs of diesel
- Ventilation free area. A 50 to 100 kW diesel pump typically needs 1.2 m² of net free area for combustion, cooling and radiator air. If that is not in the architectural plan from the start, a facade has to be cut open later.
- Exhaust. Directly outside, at high level, with a rain cap. Discharging into the room or an internal courtyard takes the room above 200 °C within minutes and destroys the batteries and sensors.
- Fuel turnover. Diesel has a shelf life; an annual turnover and water separation routine is needed. The tank is dedicated and may not be used for anything else.
- Test discipline. 20 to 30 minutes a week plus a manual restart. Where that routine slips, the diesel will not run at a real alarm — the most common failure I see on site.
The hidden risks of electric
- Supply independence. The pump supply must not be cut by any switching in the building; it is taken upstream of the main incoming switch or as a separate feeder.
- Cable class. Where a protected route cannot be achieved, E 90 / PH 90 fire-resistant cable becomes mandatory, and the cost difference is significant.
- Labelling. Every switch in the supply must carry a "do not switch off in the event of fire" label and be locked. It is the item most often sent back at inspection.
- Overload relay. If the panel was built from a general-purpose motor starter it contains an overload device, and that is a direct non-conformity.
The practical arrangement: on OH-2 and OH-3 projects, the most common solution is a single electric duty pump plus a diesel standby. That satisfies the one-electric rule and brings a second drive independent of the mains.
Frequently Asked Questions
Can I use two electric pumps on a duplicate supply?
No. EN 12845 permits at most one electric motor driven pump within a superior or duplicate supply; the rest must be diesel. A mains outage counts as a single point of failure.
How much ventilation does a diesel pump room need?
A 50 to 100 kW diesel pump typically needs at least 1.2 m² of net free area for combustion, cooling and radiator air. Take the exact figure from the engine manufacturer's table.
What temperature must the pump room be?
At least 4 °C for an electric motor pump and at least 10 °C for a diesel. The difference is for oil viscosity and battery performance.
Which cable is used for an electric pump?
Flame retardant cable is sufficient on a protected route (buried, in concrete, in the pump room). Where that cannot be achieved, E 90 / PH 90 fire-resistant cable becomes mandatory.
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Download MEP Calc on the App StoreNFPA 20 Standard for the Installation of Stationary Pumps for Fire Protection (2025). BS EN 12845:2015+A1:2019 pump clauses. EN 17451 (sprinkler pump sets). FM Global DS 3-7. Values are for information only.