Smaller Underground Than Fire Pump Discharge?

7/18/2025

Is there a velocity limitation or velocity direction provided for the design of private fire service mains?

Is it possible (or acceptable) to have an underground main smaller than the size of the discharge pipe required by NFPA 20?

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7 Comments

Pete H

7/18/2025 06:21:33 am

Not 100% sure but I think so.

Looking at NFPA 20 Table 4.27 on a 500 gpm pipe I have to have 5" suction/discharge piping for the pump, but I only need a 4" hose header supply. Additionally a pump can be fed from two separate water sources that would presumably be smaller (I've never seen it, but supposedly this happens)

So as long as the smaller service can provide the flow rate required to not cause cavitation and is code compliant, it's not ideal, but it can work.

Brett

7/18/2025 06:33:32 am

The minimum suction pipe size required by NFPA 20 only applies to a length of ten pipe diameters upstream of the pump. As long as the calcs work and the water supply can provide the flow needed then it's fine.

Brett

7/18/2025 07:22:28 am

I just realized that I read this wrong and that you're asking about the discharge side of the pump. The minimum discharge pipe size dictated by NFPA 20 only applies to NFPA 20's definition of discharge piping which is the piping from the pump discharge flange to the discharge control valve. So, the answer is still yes, as long as the calcs work provided that you have the appropriate sizing up to the discharge valve.

Dan Wilder

7/18/2025 08:22:02 am

It probably defeats the purpose of the pump to increase pressure and just lose it again for a restrictive pipe diameter (new install) however there is no requirements I am aware of.

In a retrofit situation, this would be a possible scenario as well.

Glenn Berger

7/18/2025 08:32:41 am

I believe that there used to be a maximum velocity allowed in underground (fire water) piping of 10 ft/sec. I could not find this requirement in the current version of NFPA 24.

It is possible to have an underground piping system that has a smaller pipe than the fire pump discharge. When this happens, someone should be prepare to raise questions.

Jack G

7/18/2025 09:27:16 am

There used to be a velocities limitation years and years ago. ( the 10 ft ) on the suction side was 10 fps. Velocity at discharge at 150% was 15 fps.
To burn up pressure even though it may work is not advised. It also may make it impossible to hit the points on the factory pump curve for your acceptance test.
The sizes on NFPA 20 s pump chart are based on velocity.

Phil W

7/18/2025 11:10:54 am

If you talking about the suction pipe, then you need to run a NPSH check on the diameter.

If its the diameter of the downstream below ground main you are talking about then I see no issue with a smaller diameter so long as its supported by your hydraulic calculations.

Delivery main velocities are not specifically restricted by NFPA as far as I know, although good engineering practice is to keep them at a sensible value (normally 10m/s would be considered a maximum for mains piping by most people in the UK).

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Smaller Underground Than Fire Pump Discharge?

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