Fire Pump Sizing Calculator (NFPA 20)

🚒 Fire Pump Sizing Calculator (NFPA 20)

Pump-curve points and required motor power from the rated duty.

Rated flow Q (L/min)

Rated head H (m)

Pump efficiency (0–1)

Motor margin

Churn (max 140%)

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150% Flow Point

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Head at 150% (≥65%)

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Motor Power

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⚠️ NFPA 20 acceptance points (churn ≤140%, 150% flow at ≥65% head). Power is hydraulic ÷ efficiency × motor margin; confirm against the certified pump curve and driver rating. Verify before professional use.

Standards & method

✓ Formula independently verified 12 July 2026

Governing standard

NFPA 20

Clauses applied

Churn ≤ 140% of rated pressure
150% flow at ≥ 65% of rated head
§4.7.6 driver non-overloading at every point on the curve

Independently verified

12 July 2026 — Formula re-derived from the standard and checked numerically against worked reference cases from the code book.

Why this matters: Brake power peaks at 150% flow, not at the rated point — so the driver is sized there. Sizing on the rated point alone trips the motor exactly when fire demand peaks.

Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.

NFPA 20 Fire Pump — Method

A fire pump is specified by its rated flow and rated total head. NFPA 20 then constrains the rest of the curve so the pump is stable and has reserve: at shut-off (churn) the head must not exceed 140% of rated, and at 150% of rated flow the head must be at least 65% of rated. The driver is sized from the hydraulic power P = ρ·g·Q·H divided by pump efficiency, with a margin so the motor is not overloaded at the 150% point.

Frequently Asked Questions

How do you size a fire pump per NFPA 20?⌄

Select the pump at its rated flow and head, then check the curve: churn (no-flow) head must be at most 140% of rated, and at 150% of rated flow the head must be at least 65% of rated. Size the driver from hydraulic power divided by efficiency with margin.

What is the 150% / 65% rule?⌄

At 150% of the rated flow, an NFPA 20 fire pump must still produce at least 65% of its rated total head. This guarantees reserve capacity for the fire demand.

What is churn pressure?⌄

Churn (shut-off) is the pressure at zero flow. NFPA 20 limits it to 140% of rated pressure so downstream piping and components are not over-pressured.

How do I calculate fire pump motor power?⌄

Hydraulic power P = density × g × flow × head. Divide by pump efficiency for shaft power, then add a margin (≈15%) so the motor is not overloaded at the 150% flow point. Convert kW to hp by dividing by 0.746.

What is a jockey pump for?⌄

A small jockey (pressure-maintenance) pump holds system pressure so the main fire pump does not start on minor leaks; it is sized for make-up flow, not fire demand.

Fire Pump Sizing (NFPA 20)

The fire pump is the heart of a sprinkler or hydrant system where the town main cannot meet the demand. NFPA 20 standardises both the duty point and the shape of the curve so the installation has guaranteed reserve and predictable behaviour.

The three acceptance points

Churn (0% flow) head ≤ 140% rated; rated (100%/100%); and overload (150% flow) head ≥ 65% rated. A pump that meets these three points across its curve is acceptable for fire service.

Driver power

Hydraulic power scales with flow × head. After dividing by pump efficiency and adding margin, the electric motor or diesel engine must not be overloaded anywhere on the curve, including the 150% point. Pair this with the pump head and pipe sizing tools for the full hydraulic picture.

Fire Pump Sizing Calculator

Fire pump sizing (NFPA 20) — Quick answer