Does the 'small room rule' apply to NFPA 13D sprinkler systems?

I could not find any mention of it in 13D. If 13D does not allow it, why not?

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Per NFPA 13D 2019 Edition Section 8.2.1.3, "Concealed Sprinklers shall be permitted to be installed in beams not greater than 4 in. (100mm) in depth."

Semi-recessed heads can be permitted to be installed in beams up to 14in.

Why do concealed sprinklers installed in beamed applications have a 4 inch limitation? 

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Where exactly does a hydraulic calculation need to end? 

When we run a hydrant flow test, there's a static/residual hydrant and a flow hydrant. I regularly run my hydraulic calculations from the building, through the service main, to a street main, and then up the tee'd branch that serves a hydrant so that my source point is exactly at the elevation of the static/residual hydrant. I do this so that I make sure to account for the proper elevation of the water supply.

Most other hydraulic calculations I see will end wherever the building's service main intersects with the street's supply. Is this correct? Wouldn't that place the source at an elevation lower than what the hydrant indicated is available?

Wondering what the proper way is that I should be precisely calculating systems. Thanks in advance.

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Why does a townhouse which is 4-story require an NFPA 13R system?

The IRC is specifically geared toward one-and two-family dwellings up to 3 stories, but the IBC allows 13D systems in R-3, R-4 condition 1 and townhouses (903.3.1.3).

Is there another prevailing code reference that overrules this allowance?

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I have an inspector who wants me to prove that calculating run or through tees is appropriate for 13 Residential systems.

I showed them that run tees are in 13D, he thinks they should be calc'd in 13? How do I prove something is not in a code?

Any help would be appreciated.

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Does the proximity to a residence and the number of windows make a difference in which system is used (NFPA 13 vs 13R vs 13D)?

​What are those requirements that make it change?

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Residential single-family dwelling is getting an NFPA 13D sprinkler system with a pump and water storage tank. 

Is the test line required to be routed to the exterior, or can it be run back to the water storage tank? I've seen it both ways but don't see an applicable code section.

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