What is the Design Area for 600 sqft Dry System?

9/13/2024

I'm designing a dry system feeding an area of about 600 sqft. It's a Light Hazard area, so the design criteria is just 0.10 gpm/sqft over 1,500 sqft.

Should the design area include the entire area, or could I just calculate the 5 most demanding sprinklers?

Per NFPA 13 2016, Section 11.2.3.2.3.2 under the quick response sprinkler section states, "The number of sprinklers in the design area shall never be less than five."

The section above that does state that the area of operation can be reduced, but that only applies to a wet pipe system, amongst a few other points. Of course, this is a dry system, so I don't believe the reduction can be used for this situation.

How should this design area be calculated?

Thanks in advance.

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9 Comments

Pete H

9/13/2024 06:50:49 am

I'd do the whole area of the system unless it's just a 600 square foot corridor being fed, then I'd just do the five pursuant to 11.2.3.3.6 (and if you're using extended coverage, all sprinklers within 75 linear feet of corridor's worth).

But take it with a grain of salt, there might be exception you can use that I'm missing.

Pete H

9/13/2024 08:15:04 am

I also forgot, you need to add a phantom flow if you're not hitting the minimum flow for the required square footage and density if you're doing the Density/Area calc and not room design.

Mike Morey

9/13/2024 08:10:13 am

If you have less than the required area you calculate the entire area. That said, you may be subject to "phantom flow" (see 23.4.4.2.4 and 5) in that case. If you meet the requirements to call it "room design" however, phantom flow doesn't apply, so that is probably going to be the preferable route to go.

9/13/2024 08:10:46 am

I would add a "phantom flow" as per 23.4.4.1.1.5 NFPA 13 2013 to the design area if it is a room within a building that has other sprinkler systems around it. I would most likely add this flow to a base of riser or connection to the wet piping for the dry pipe.

Of course there are other design methods that may apply such as the corridor calculation mentioned above by Pete that may be considered.

Glenn Berger

9/13/2024 08:14:10 am

Need more information before advising on an answer -

What is surrounding the new dry-pipe sprinkler system area? Are there other hazards protected by separate sprinkler systems? Is the 600 square feet bounded by fire-rated construction?

Matthew Ostroff

9/13/2024 08:14:54 am

I believe you need to have a minimum of 195 gpm (.1gpm/sf x 1,950 sf) calculated for a dry system to meet the requirements of 27.2.4.2.4 of NFPA 13 (2019). You would need to add back in the flow at the furthest cross main.

27.2.4.2 Density/Area Method.

27.2.4.2.1*
Where the design is based on the density/area method, the design area shall be a rectangular area having a dimension parallel to the branch lines at least 1.2 times the square root of the area of sprinkler operation (A) used, which shall permit the inclusion of sprinklers on both sides of the cross main.

27.2.4.2.2
Any fractional sprinkler shall be carried to the next higher whole sprinkler.

27.2.4.2.3
In systems having branch lines with an insufficient number of sprinklers to fulfill the 1.2 requirement, the design area shall be extended to include sprinklers on adjacent branch lines supplied by the same cross main.

27.2.4.2.4*
Where the available floor area for a specific area/density design criteria, including any extension of area as required by 19.2.2 and Section 20.10, is less than the required minimum design area, the design area shall be permitted to only include those sprinklers within the available design area.

A.27.2.4.2.4
The following steps outline the procedure for calculation in accordance with 27.2.4.2.4:

(1) Calculate the hydraulic design discharge including those sprinklers within the available floor area.
(2) Calculate the minimum required discharge by multiplying the required design density times the required minimum design area.
(3) Subtract the discharge calculated in Step 1 from the discharge calculate in Step 2.
(4) Where the discharge calculated in Step 3 is greater than 0, the hydraulic design discharge is recalculated including an additional flow equal to that calculated in Step 3. The additional flow is added at the point of connection of the branch line to the cross main furthest from the source.
(5) Where the discharge calculated in Step 3 is less than or equal to 0, the hydraulic design discharge is as calculated in Step 1.

27.2.4.2.5
Where the total design discharge from these operating sprinklers is less than the minimum required discharge determined by multiplying the required design density times the required minimum design area, an additional flow shall be added at the point of connection of the branch line to the cross main furthest from the source to increase the overall demand, not including hose stream allowance, to the minimum required discharge.

Anthony P

9/13/2024 08:22:41 am

Keep in mind Dry Systems are required to be designed to 1950 Sq Ft.

New York is a little Different but usually I would design it to the actual size of the Dry System because if you have an Air Compressor you need to know the Volume of the Dry System (Inspectors test needs to be located at the most remote Branch)
Worst Case if there is an adjacent Wet System run a Separate Calculation on that. (Extra Price of course) Cause how I look at it either the Dry system is going to go off or the Wet System.

Usually, I would bounce this off the AHJ. Since they will sign off on it.

Jack G

9/13/2024 09:16:42 am

Calculate 1950 sq ft. Add sprinklers from adjacent areas if dry.
If adjacent areas are wet add a phantom flow at nearest x main..

Jesse

9/13/2024 10:05:40 am

Well, it wouldn't be 1500 sq. ft to begin with. Adding the 30% it becomes 0.10 / 1950.

But since its only a 600 sq. ft area protected you'd calc the whole thing. And you might need to add phantom flow to get to you minimal flow requirements

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What is the Design Area for 600 sqft Dry System?

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