A fire pump is selected at its rated point (rated flow and rated pressure), but a real system may ask for more water than that. Two numbers define how far you can go: 150% of rated flow, and the pressure the pump must still deliver there, 65% of rated pressure.
The two rules
- Demand limit. NFPA 20 (Section 4.10.1) requires that the largest single system demand does not exceed 150% of the pump’s rated flow.
- Pressure at 150%. UL 448 requires a pump to provide at least 65% of rated pressure at 150% of rated flow. FM Approval Standard 1311 has the same minimum.
Why 150%
Listed pumps are tested and certified to operate safely up to 150% of rated capacity, and not beyond it. Staying inside that tested envelope avoids overloading the pump, starving the protected systems, and problems such as overheating, cavitation or mechanical instability.
Working it through
| Question | Calculation | Result |
|---|---|---|
| Largest system demand is 3,000 gpm. Smallest rated flow? | 3,000 ÷ 1.5 | 2,000 gpm rated pump |
| Rated flow 2,500 gpm. Highest demand the pump may serve? | 2,500 × 1.5 | 3,750 gpm |
| Rated pressure 100 psi. Minimum pressure at 150% flow? | 100 × 0.65 | 65 psi |
The examples are arithmetic illustrations of the two rules. They are not selections; actual pumps are chosen from the listed performance curve of the model.
How to use it when selecting
- Find the largest single system demand, including hose allowance where it applies, and the pressure needed at the pump.
- Pick a rated flow so the demand is no more than 150% of it.
- On the listed curve, confirm the pressure at the demand point still meets the system requirement, and that it is not below 65% of rated pressure at 150% flow.
Next step
The Pumpsense UL / FM fire pump selector finds listed models for your flow and pressure. For complete packages, see our 2500 gpm skid and container packages, the fire pumps page and the fire pump controllers page.