The LWF Blog

Fire Safety Engineering for Design – Sprinkler – Water supplies – Part 256

October 13, 2025 10:02 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 255, LWF discussed duplicate water supplies and how to ensure your sprinkler system has sufficient water for any foreseen eventuality. In part 256, we will continue to talk about water provision for sprinkler systems, starting with the NFPA design guide.

NFPA 13 is a US-based standard which is widely used. The method used to calculate water supply for a sprinkler system is somewhat simpler than the UK standards discussed in the last few blogs.

The minimum required tank capacity is based on the density/area calculation, multiplied by the duration. An allowance for hose streams may be added to this requirement. Water supply durations are as follows:

Light hazard – 30 minutes
Ordinary hazard – 60 or 90 minutes
Extra hazard – 90 or 120 minutes

Lower duration/hazard values may be used in cases where the system is supervised and monitored.

In the case of suction lift pumps, it is important that full priming facilities, including a priming tank and pipework should be provided. Separate suction pipes should be provided for each pump and the size of the pipes may be larger than usual as a result of a decreased velocity limit.

Use of suction lift fire pumps with priming arrangements is not possible under NFPA standards. Vertical turbine pumps should be used in this case.

An addition to the main sprinkler pumps might be a “jockey” pump. A jockey pump has a smaller capacity and is designed to make up small shortfalls in the trunk main, to prevent operation of the main pumps. The jockey pump is automatically switched off when the pre-determined cut out pressure is obtained.

Common water supplies should be considered of importance where water is a valued commodity. When a site has a number of buildings requiring protection from sprinklers, consideration should be given to providing a water supply common to all buildings. The supply should be capable of furnishing the required flows and pressures required by the building with the highest level of risk. Potential issues regarding the responsibilities of ownership, service and maintenance should be considered in advance and mitigated wherever possible.

In part 257 of LWF’s series on fire engineering we will talk about the commissioning and testing of sprinkler systems. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.