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Calculate with SxL or Room Area in Small Rooms?

1/14/2022

7 Comments

 
In Section 19.4.1.3(3) of NFPA 13-2019 Edition, discusses that rooms or compartments 800 sqft or less (small rooms) should be calculated based on delivering 0.10 gpm/sqft over the room or the compartment by using the area of the room divided by the number of sprinklers in the room.

I believe I've overcalculated remote areas in the past.

My project now is an apartment building designed according to NFPA 13. Using residential type sprinklers with non-sprinklered combustible concealed spaces. According to Section 19.4.1.2 I need to calculate a minimum of eight sprinklers.

In the past I've calculated the eight sprinklers using the actual spacing based on the S x L rules with a 0.10 gpm/sqft density.

One room example - room dimension of 24 x 12 is 288 sqft. Two sprinklers spaced in a compartment 8'-0" off one wall and 6'-0" from adjacent wall. Using the S x L rule: 16' x 12' = 192 sqft x 0.10 gpm/sqft = 19.2 gpm each.

Or, according to Section 19.4.1.3 (3), would I divide the room size 288 sqft by 2 sprinklers, which is 144 sqft per sprinkler, regardless of the actual sprinkler distance from the walls? This second scenario would result in a 14.4 gpm required flow.

Or, was the code option (3) assuming the sprinklers are evenly centered within the compartment.

Thanks in advance for your input and clarification.

​​​​​​​​​​​​​​​​​​​​​​​​Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
7 Comments
Dan Wilder
1/14/2022 07:56:13 am

Here is a great explanation that should help you out.

https://nfsa.org/2020/12/01/the-nfpa-13-small-room-rule-frequently-asked-questions/

Reply
Alex
1/14/2022 08:17:46 am

Hi,

You can use the room area divided by the number of sprinklers. Note, that the sprinkler coverage cannot exceed the maximum protection area found in NFPA 13 (2022) Table 10.2.4.2.1(a). For instance, maximum coverage area is 225 ft2. If you room is 245ft2, you will need (2) sprinklers.

The link that Dan provided is a great help! Wish I had found that earlier when I was designing.

Alex

Reply
Danefre
1/14/2022 08:55:45 am

Just wanted to add that the NFSA article Dan linked was written in 2020, so while it does say that "small room rule" does not apply to residential sprinklers per section 10.2 (2019), you still calculate it as area/# of sprinklers per 19.4.1.3 (2019).

in the past, 2007-2013 editions, Chapter 11 would redirect you back to Chapter 8. In 2016, the area/# sprinklers verbiage was included in Chapter 11. You can see the difference in how 11.3.1.3 is written between the 2013 and 2016 editions.
Credit to Travis M. for explaining this.

Reply
Dan Wilder
1/14/2022 09:55:02 am

Nice...I glossed over the "residential" part of that because I don't equate small room (exceeding distance from walls) & residential sprinklers (basically EC spacing)...and the area/# of sprinklers approach typically is less than the minimum required for the sprinkler so I don't get to utilize it as much (what little residential I do anyway)

and yes, Travis has provided some pivotal advice in the past for me as well, so double credit for sure!

Reply
Jack G
1/14/2022 12:15:17 pm

You can find a formal interp that preceded the paragraph being added. It’s been in the book for quite a while.
Small room rule only applies to light hazard occupancies.

Reply
Casey Milhorn
1/14/2022 12:54:04 pm

This is one of the most common things that I find designers don't do correctly. You basically do two calcs when using residential sprinklers in a 13 system.
#1 is at a .05 density using the SxL method (but rounded up at 16x16, 18x18, etc per cut sheet) over the four hydraulically most demanding adjacent sprinklers. (Use manufacturers end head pressures).
#2 is at a .10 density using each compartments total sqft and area of heads is total sqft divided by number of heads in compartment. (use 7 psi end head pressure).
99% of the time you will find that the .05 calc is more demanding because of the SxL method and rounding up to 18x18, 20x20, etc... and of course the end head pressure.

Reply
Coorey Power
1/17/2022 05:32:02 am

This! I've been questioned on several residential under 13 projects Where I show the 0.05 calc is more demanding. My experience is for the most part when the room is under 800 sqft the 0.05 calc is the most demanding. A lot of reviewers seem to miss the part where you divide the area of the room over # of sprinklers.

Reply



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  • Blog
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    • DOMESTIC DEMAND*
    • FIRE FLOW CALCULATOR*
    • FIRE PUMP ANALYZER*
    • FIRE PUMP DATABASE*
    • FRICTION LOSS CALCULATOR
    • HANGER SPACER*
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    • PLUMBING FIXTURE COUNTS
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