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# NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing2

## NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

(OP)
For an NFPA 13 residential calculation the minimum water supply is 30 minutes. Scenario: a series of identical and adjacent small rooms each with a 1 hour wall/ceiling rating and 45 minute self closing door. One sprinkler is able to adequately protect each room.

Question: Would the residential calculation require 4 sprinklers to flow simultaneously from four adjacent rooms each separated by a 1 hour wall?

My opinion is that if a fire begins in a room with a rating greater than the water supply required, then it stands to reason only the sprinklers in that room are needed to flow in the calculation. There is tremendous benefit to compartmentalization. In theory, the fire would not escape that room before the required water supply has been completed. Practically speaking, the fire would be suppressed by the sprinkler system and/or first responders before it escaped the 1 hour rated room.

### RE: NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

If the fire compartment can be adequately protected with a single sprinkler and separation is in accordance with NFPA 13, the calculation only needs to be based on a single sprinkler flowing. If the room is connected to a corridor, a second calculation for the corridor is required. The logic is if a fire occurs inside the occupied space and the sprinklers are unable to control it, the corridor sprinklers will protect the egress path by preventing the spread of fire from the room of origin.

### RE: NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

Let me take a different spin on this.

You are talking about using Residential sprinklers from what I understand. As such, that falls into Section 11.3 Special Design Approaches. Section 11.3 does not reference you back to 11.2 where the room design method would apply.

In your case, in order to apply the room design method, you would need to use standard spray sprinklers. The calculation method for standard spray sprinklers offers the density/area method and/or the room design method. If you have light hazard, and protected openings (as you described) then your single sprinkler calc would be correct for standard spray sprinklers.

Let me finish by saying that I believe your rationale is correct in that you should be able to do a single sprinkler calc in the scenario you described. However, since the residential design approach never points back to an application of the room design method, you would have to be going for an equivalency and require professional engineering for that. As a NICET IV tech, I can only apply what the prescriptive standards allow.

Stookeye is one of the best FPE's that I have had the pleasure of dealing with. He has the letters behind his name, and more importantly, the knowledge base to promote the method he described above.

Travis Mack
MFP Design, LLC
www.mfpdesign.com

### RE: NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

(OP)
Gentlemen, what about the option of invoking 11.3.1.3, which leads to 8.6.2.1.2, and then 8.6.2.1.2.1. That gets us the smaller coverage area (when sprinkler not centered in room) since it circumvents the S x L calculation. Then we can flow four congruent residential sprinklers satisfying the 4 sprinkler calculation requirement with less demand. Thoughts?

### RE: NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

Yes. Typically, your room is a small room. As such, you need to meet 2 requirements, the listing of the sprinkler and the area of the sprinkler x 0.10. In a small room, the area per sprinkler is the room area divided by the number of sprinklers in the room.

If your room is 12'x12', the minimum flow is 14.4 gallons for a single sprinkler in that room. If you are 9' off the wall and using a 4.9k residential sprinkler, you need to flow 17 gpm. If you are at 10' off the wall, then you need to flow 20 gpm per sprinkler listing. If you are 8' or less off the walls, then you flow 14.4 gpm.

I hope that makes sense.

Travis Mack
MFP Design, LLC
www.mfpdesign.com

### RE: NFPA 13 Residential Calculation - Effect of Room Rating on Number of Sprinklers Flowing

(OP)
Perfect, thanks for the support.

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