Selecting a fire pump means determining the pump type, capacity, driver and control arrangement that will reliably meet the system's most demanding point of demand, in accordance with NFPA 20. This guide covers the seven steps from establishing the system demand to the commissioning test, including the 150% capacity rule, NPSH checks and jockey pump sizing, with worked numbers.
The Role of the Fire Pump
A fire pump is the pressure-boosting unit that comes into play when the available pressure of the water supply — tank, town main or well — cannot meet the demand pressure of the sprinkler or hydrant system. Correct selection eliminates both the risk of inadequate pressure, meaning failed protection, and of oversizing, which brings cost and churn pressure problems. NFPA 20 sets binding criteria for the performance curve, suction conditions, driver and control arrangement.
| Step | Decision | Key criterion |
|---|---|---|
| 1. Demand point | Q, P (sprinkler + hose) | Hydraulic calculation plus hose allowance |
| 2. Pump type | End-suction / split-case / vertical turbine | Suction condition |
| 3. Rated flow | Standard rating plus 150% check | At least 65% of rated pressure at 150% |
| 4. Head and NPSH | Total head, cavitation margin | NPSHa greater than NPSHr plus margin |
| 5. Driver | Electric or diesel | Reliability of the supply |
| 6. Jockey pump | About 1% of flow, above churn pressure | Pressure maintenance |
| 7. Controller and test | Listed controller plus acceptance test | NFPA 20 and 25 compliance |
Step 1 — Establish the System Demand Flow and Pressure
Pump selection starts with the most disadvantaged demand point from the sprinkler hydraulic calculation, to which the internal and external hose allowance is added. For the worked example:
- Sprinkler demand: 1,200 L/min at 4.8 bar (at the riser)
- Hose allowance: approximately 950 L/min
- Total pump demand: approximately 2,150 L/min, required head approximately 7.0 bar (the head the pump must add, after the static pressure of the supply is deducted)
Step 2 — Select the Pump Type
Pump type depends largely on the suction condition:
- Horizontal end-suction: for moderate flow systems with positive suction, from a pressurised main or a tank above the pump.
- Horizontal split-case: preferred at higher flows; double suction and high efficiency.
- Vertical turbine: mandatory where the water source lies below pump level (deep well, buried tank, canal), because it removes the suction lift problem entirely.
In the worked example a pressurised tank sits above the pump room, so a horizontal split-case pump is selected.
Step 3 — Rated Flow and the 150% Capacity Check
The demand flow (2,150 L/min) is rounded up to the next standard rating. NFPA 20 rated capacities run 250, 500, 750, 1000 gpm and so on, so a pump of roughly 2,270 L/min (600 gpm) class is selected. The three-point curve check then follows:
| Point | Flow | Pressure rule |
|---|---|---|
| Churn (closed valve) | 0% | Not more than 140% of rated pressure |
| Rated | 100% | Equal to rated pressure (at least 7.0 bar here) |
| Overload | 150% | At least 65% of rated pressure (at least 4.55 bar here) |
This rule provides a safety margin for the case where actual fire demand exceeds the rated flow; the pump must still deliver meaningful pressure at 150% of rating.
Step 4 — Total Head and Suction Conditions (NPSH)
The total head is the demand pressure plus pipe and fitting losses plus the elevation difference. A cavitation check then follows:
NPSHa must exceed NPSHr plus a safety margin of roughly 1 m
NPSHa, the available net positive suction head, is derived from atmospheric plus static pressure on the suction side, less friction and vapour pressure. NFPA 20 additionally requires that suction pressure does not fall below 0 psig at 150% of rated flow. Where the source lies below the pump, a vertical turbine pump removes the problem and is preferred.
Step 5 — Select the Driver (Electric or Diesel)
- Electric motor: preferred where a reliable supply meeting NFPA 20 criteria exists — a dedicated feeder, two independent sources or generator backup. It is simpler and needs less maintenance.
- Diesel engine: selected where the electrical supply is not reliable or where an independent backup is required. It runs independently of the grid and needs dual batteries and a fuel system.
- Dual arrangement: on critical and high-risk facilities two pumps are used, one electric and one diesel, as duty and standby.
Step 6 — Size the Jockey (Pressure Maintenance) Pump
The jockey pump keeps the system pressurised by making up small leaks and thermal pressure drops, preventing the main pump from starting unnecessarily. Sizing:
- Flow: approximately 1% of the main pump rated flow, so about 25–40 L/min for a 2,270 L/min pump.
- Pressure: slightly above the main pump churn pressure, so that when pressure falls the jockey starts first and the main pump only follows if the jockey cannot keep up.
Step 7 — Controller and Commissioning Test
A fire pump is operated by a listed (UL/FM) fire pump controller, never a standard motor starter panel. The electric controller provides automatic start, manual stop, phase protection and alarm transmission; the diesel controller manages dual battery charging, automatic start and engine alarms. Pressure switches are set so that the jockey and main pump start in sequence.
The commissioning test is performed at three points: churn, rated flow and 150% of rated flow. Measured flow and pressure are compared against the manufacturer's curve.
Common Mistakes
- Ignoring churn pressure: an oversized pump can exceed the system pressure limit at churn.
- Neglecting NPSH: cavitation causes impeller damage and loss of performance; always leave a margin.
- Using a non-listed panel: a fire pump must be operated by a listed controller.
- Sizing the motor at the rated point: the motor must also cover the highest power demand at 150% flow, so a non-overloading selection is required.
Frequently Asked Questions
What is the first step in selecting a fire pump?
Establishing the demand point - flow and pressure - for the most disadvantaged operating scenario. That value comes from the sprinkler hydraulic calculation, with the internal and external hose allowance added to give the total the pump must meet. The pump is then selected so it meets that demand point within the efficient region of its performance curve. Nothing else can be decided until the demand point is fixed.
What is the 150% capacity rule in NFPA 20?
NFPA 20 defines three check points on the pump curve. The pump must deliver its rated pressure at rated flow; at 150% of rated flow it must still deliver at least 65% of rated pressure; and at churn, meaning zero flow against a closed valve, the pressure must not exceed 140% of rated. The rule provides a safety margin for the case where real fire demand exceeds the rated flow.
How do I choose between end-suction, split-case and vertical turbine pumps?
The choice depends largely on the suction condition. Horizontal end-suction and split-case pumps are used where the source is above the pump or pressurised, giving positive suction, with split-case preferred at higher flows. A vertical turbine pump becomes mandatory where the source lies below pump level - a deep well, buried tank or canal - because it removes the suction lift problem.
What is NPSH and why does it matter for a fire pump?
NPSH, net positive suction head, is the margin by which the pressure at the pump suction exceeds the vapour pressure of the water. If the available NPSH falls below what the pump requires, cavitation occurs, causing loss of performance, noise and impeller damage. A margin of roughly 1 m above the required value is used for safe design, and NFPA 20 additionally requires suction pressure not to fall below 0 psig at 150% flow.
Should I choose an electric or a diesel fire pump?
An electric pump is preferred where a reliable supply meeting NFPA 20 criteria exists, such as a dedicated feeder, two independent sources or generator backup; it is simpler and needs less maintenance. Where the supply is not reliable or the risk is high, a diesel pump is chosen because it runs independently of the grid. On critical facilities a duty and standby arrangement uses one of each.
What does the jockey pump do and how is it sized?
The jockey pump keeps the system pressurised by making up small leaks and thermal pressure drops, so the main pump does not start unnecessarily. Its flow is typically about 1% of the main pump rated flow - for a 2,000 L/min pump, roughly 20 to 40 L/min. Its discharge pressure is set slightly above the main pump churn pressure so the pressure switches start the jockey first and the main pump only if the jockey cannot keep up.
What should I look for in a fire pump controller?
It must be a listed (UL/FM) controller made specifically for fire pumps; a standard motor control panel cannot be used. An electric controller provides automatic start, manual stop, phase protection and alarm outputs. A diesel controller manages battery charging, dual batteries, automatic start and engine alarms. The controller must transmit trouble, running and power loss signals to the building fire alarm system.
What does the fire pump commissioning test involve?
The field acceptance test measures flow and pressure at three performance points - churn, rated flow and 150% of rated flow - and compares them with the manufacturer's curve, using a test header or flow meter. It also verifies automatic start, the minimum run timer, alarm transmission and, on diesel units, the fuel and battery arrangements. NFPA 25 then governs the weekly, monthly and annual repetition of these tests in service.
What if a single fire pump cannot meet the whole demand?
Pumps can be arranged in parallel, where flows add at the same pressure, or in series, where pressures add at the same flow. Parallel suits a high flow requirement; series suits a high pressure requirement, such as vertical pressure zoning in a tall building. Each pump has its own NFPA 20 compliant controller and the starting sequence is staged through the pressure switch settings.
How is the fire pump motor power determined?
The shaft power follows from flow, head, water density and pump efficiency. However the motor must also cover the highest power demand at 150% flow, which can exceed the demand at the rated point. NFPA 20 therefore requires a non-overloading selection: the motor must be capable of the maximum brake power anywhere along the curve, from churn through to 150% of rated flow.
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Download MEP Calc on the App StoreNFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection (pump types, the 150% capacity curve, controllers and drivers). NFPA 25, Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems (pump performance testing). Standard pump shaft power and NPSH relationships.