When to Plan for 6-inch vs. 8-inch Fire Service?

6/26/2023

Is there a good dividing line for when to have a 6" vs 8" service main to a building?

Assuming light & ordinary hazard with 52,000 sf as an area limitation for a riser, is there a limit on number of risers to a 6" main?

What is the driver on how to predict that service main size?

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8 Comments

Pete H

6/26/2023 08:00:43 am

There is no limit as to the number of risers as far as I know.

If you have standpipe and wind up requiring a pump, typically larger pumps will require at least an 8" feed (see NFPA 20 and size your suction line according to pump size). While standpipe requirements found in IBC typically rate to height and/or building classification, I've known AHJ's to require standpipe beyond code in large enough buildings with enough entrances and exits a fire department could use.

Similarly an AHJ could see your unique set up and while realizing that a fire that requires multiple risers to supply water on a small amount of different floors is unlikely, just as he would ask for a standpipe that is beyond expectations of code, he could ask for either a minimum safety factor or an 8" feed beyond minimum code requirements.

Glenn Berger

6/26/2023 08:08:00 am

The number of risers typically does not affect the size of the incoming fire water service.

Make sure you consider the pressure loss in the underground piping when you do your calculations.

Franck

6/26/2023 08:38:31 am

The number of risers is not a factor. Sprinkler protection is based on the assumption there is no multiple fires at the same time.
Pipe size, as indicated by Glenn, is then related to the expected maximum flow and friction losses in the pipe. If incoming pressure us low, a larger diameter can help limiting the friction losses.

Dan Wilder

6/26/2023 11:17:10 am

My understanding is the length of the pipe and configuration (dead end vs loop) is one of the factors for underground sizing (depending on AHJ). Minimum sizing for pump intakes can play a factor, and building type is always a consideration (office vs warehouse).

As for number of risers, as many as the property owner or insurance feels comfortable (all eggs in one basket or not type thing). There is a cost benefit to additional risers to cut back bulk feeds to distant zones.

Alex

6/26/2023 11:44:54 am

The number of risers does not affect the size. You will need to run hydraulic calculations to determine the size

David Kendrick

6/26/2023 12:20:10 pm

The properties insurance underwriter may have input as to how many risers can be off a single incoming supply.

Jesse

6/28/2023 10:29:02 am

Basically, it's demand dependent. I was a high protected risk / loss control engineer for a large property carrier for a while. We didn't care about the number of individual risers fed from single 6" or 8" lead-in. We did however hydraulically calculate every single system.

There are some AHJs I deal with that require hydraulic calculations on underground fire-lines using both the IBC fire-flow requirement AND the AS demand.

Chris

7/13/2023 03:32:37 pm

No limit for the systems that I know of, but if you are working in a building that is going to have a bunch of systems they may be best served by multiple lead ins so you don't have thousands of feet of bulk pipe supplying systems far away from the riser. As long as you calc through the pipe and all is well it is ok, if you have a pump I would base it off of the minimum sized pipe for the supply to that pump though.

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When to Plan for 6-inch vs. 8-inch Fire Service?

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