Minimum Sprinkler Service Pipe Size for NFPA 13?

4/21/2020

Do sprinkler service entry sizes have a minimum mandated size for an NFPA 13 system?

If I do not have a hydrant on an incoming line and can prove hydraulically that a 3-inch service entry works, is that acceptable under 13 or only permissible in 13R or 13D projects?

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9 Comments

Franck

4/21/2020 08:58:30 am

(To be confirmed by US resident, more used than I am to US requirements)

I don't think there is a minimum requirement for the in-coming pipe size. As long as you can achieve your goals and prove it by hydraulic calculations, I don't even see a reason for not accepting it.
The only issue may be related to the size of your Alarm Check Valve if you need one (which could be necessary for a large system).
For small systems, I often see a simple valve and a flow switch (with an alarm reported to a constantly attended location).
I had the case is some metal working facilities where the only requirements for sprinkler protection was the hydraulic cellar and a 3" pipe was largely sufficient for a few sprinkler heads, even though the area was considered as an extra hazard occupancy.

Jonathan Sullivan

4/21/2020 10:20:50 am

Franck - you are correct.

I was just having this conversation with a developer the other day. They felt 6" was to large based on their previous experience, but since the only project was land development, the civil engineer was not performing the hydraulic calculations. They only could plan for what was allowed by the un-calculated minimum.

That being said, 6" is the minimum for a main without hydrants unless hydraulically calculated. There are a few other restrictions such as system and standpipe type and technical limitations you pointed out. Check out NFPA 13-17 24.1.3.1 for more information.

MIKE

4/21/2020 10:13:28 am

Refer to NFPA 24.
4".

Jonathan Sullivan

4/21/2020 10:27:36 am

Mike - can you provide a section reference? I had thought that the Size of Fire Mains sections matched between the two standards.

Mike

4/21/2020 03:36:10 pm

My mistake. I can't find a reference but I remember it from somewhere or maybe it's just my imagination.

A larger point is that why would you want to squeeze the underground piping from 4" to 3", especially with steel, considering it's corrosion history?

Also, There is not PVC listed below 4". I don't know about 3", though.

James Phifer

4/21/2020 10:39:37 am

Check Out NFPA 13 2016 ed. chapter 24 section 24.1.3

seems to indicate that for mains that do not supply hydrants, the fire main can be as large as the riser.

Thomas Jones III

4/21/2020 10:48:40 am

No minimum size requirements for incoming lines that I know of. However, if it is a combined domestic/fire line you do not have to include the domestic demand in your calculation if the line is 6" or larger.

Jonathan Sullivan

4/21/2020 10:54:52 am

I'm used to separate feeds for these services. When do you see combined used and how are these designed? NFPA 24 and local utility requirements from street into the building, and then tee off to domestic and fire services inside?

Pete D

4/21/2020 11:03:34 am

Don't forget to consider that you are designing for the current occupancy and all future occupancies, and material is not the largest contributor to cost.

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