How to Calculate a Class 1 Horizontal Standpipe?

8/13/2025

I have a horizontal standpipe Class 1 in a 157,252 sqft warehouse that needs calc'd, but I'm trying to figure out how to go about that.

Per NFPA 14, 2016 edition, it seems to me that you would have the hydraulically most demanding hose valve flow 250-gpm @ 100-psi, and then flow three (3) more hose valves at 250-gpm until I get to a 1000-gpm total. However, Section 7.10.1.2.2 only says to do this up to 3 hose valves total.

What am I doing wrong?

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5 Comments

Brett

8/13/2025 07:07:23 am

It depends on your layout. A horizontal standpipe is defined as horizontal piping that supplies two or more hose connections on a floor. A branch line supplies maximum one hose valve. Branch lines can be 2 1/2" but horizontal standpipes are required to be minimum 4" and need an isolation valve.

If your layout creates at least two horizontal standpipes on one floor and at least one of them supplies three hose valves then you would calculate 750 gpm through one standpipe and 250 gpm through a second standpipe. I'm assuming that the building is fully sprinklered per NFPA 13.

7.10.1.2.2 is just saying that for horizontal standpipes you're required to calculate three hose valves instead of two. It doesn't change the total flow rate required.

Dan Wilder

8/13/2025 08:18:43 am

So there are a couple items coming into play on this one and without seeing the layout and valve configuration, we really can't give you a finite answer. This all assumes a fully sprinklered building as well.

7.10.1.1.1 - Minimum flow rate of 500 GPM from 2-2½" hose valves with a minimum pressure of 100 PSI.

7.10.1.1.2 - Horizontal standpipe requirement increases to 750 GPM when that standpipe supplies 3 or more hose valves.

7.10.1.1.3.1 - Floor areas greater than 80,000 ft² requires the next standpipe calculated to flow 500 GPM (not the typical 250GPM per 7.10.1.1.3)

So, you could have:
The remote standpipe flowing 750 GPM with the next standpipe flowing only 250 GPM

The remote standpipe flowing 500 GPM with the next standpipe flowing 500 GPM.

Curious

8/13/2025 08:29:47 am

A Class 1 horizontal standpipe in a 157,252 sqft warehouse sounds abnormal. Do mind sharing the code requirement for this so I can learn something new today?

Brett

8/13/2025 11:04:18 am

NFPA 1 (2018) and earlier editions require it in high piled storage occupancies where the storage is more than 200 ft from fire access doors. The requirement was removed in the 2021 edition. I'm not as familiar with the IFC off the top of my head. The fire AHJ could also simply state require it if they don't have a reliable way to fight fires in the building utilizing hydrants.

Jefferey M Byerly

8/13/2025 08:09:36 pm

This project in particular, is a 1,033,310 SF Costco Warehouse. It is fully sprinklered and the AHJ has requested the Class I standpipe. The standpipe is a single feed coming of the riser manifold and branching off to the north and south with 4" feed main to 2-1/2" drops to a single 2-1/2" hose valve at every exterior door.

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How to Calculate a Class 1 Horizontal Standpipe?

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