Design Area for Very Small Extra Hazard Rooms?

3/5/2026

Working on the fire protection for a new industrial building. I only have two small areas to protect with sprinklers:
1. 400 ft² hydraulic room
2. 520 ft² transformer room

Both are considered Extra Hazard due to the presence of flammable liquids.

My understanding of NFPA 13 forces me to use 2500 ft² and 0.50 gpm/ft² as the minimum water supply. This will give me 1250 gpm of water to apply to very small rooms, which doesn't make much sense to me.

Am I understanding this right?

Is there a way around it, or a different way of calculating the water supply need?

Thanks in advance.

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10 Comments

Pete H

3/5/2026 06:18:56 am

From NFPA 13 (2025 edition)

As per 19.2.3.1.5, we're only required to go up to 3000 square feet if there are combustible concealed spaces.

As per table 19.2.3.1.1 - Extra Hazard Group 1 under the density/area method directs you to use 0.3 over 2500 square feet and Extra Hazard Group 2 directs to use 0.4 over 2500 square feet.

-

It is entirely possible/plausible that you are getting that 0.5 number from some flammable liquids code in the NFPAs or a separate part of NFPA 13, but my initial thought is that it could possibly be 0.4 gpm over 2500 sq ft.

Though the room design method may also be a goal for you, especially if you additionally calculate the other areas of the building to justify a design with smaller pipe for the areas with lesser hazards.

Quentin F

3/6/2026 01:27:53 am

Our insurance asked for a 0.5gpm/ft2 density.
Indeed, I think the proper number is 0.4
But that still gives me 1000gpm which seems very high

Dan Wilder

3/5/2026 07:12:42 am

Ya, there's a bit more to this for information needed.

On the surface, the two rooms, if smaller than the minimum remote area required, would need to be designed with either Room Design (rating of walls and doors for the hazard - likely 2 hour but would need more info) or the "Phantom Flow" requirements of 28.2.4.2.5 (13-25 Edition) would need to be applied due to the actual discharge vs minimum required. Or a possible 3rd case is a FPDBD that details out protection methods.

NFPA 850, 70, 13 and possibly others would also come into the design process review and if this is a water supply path, don't forget hose stream (likely 500 but again, need to confirm).

Glenn Berger

3/5/2026 08:09:54 am

You have some very good answers so far.

In a new industrial facility - Why is only 2 rooms being considered for sprinkler protection?

Need to review the code analysis and all applicable documents to determine the full extent of where fire protection systems needs to be installed.

Chad

3/5/2026 09:21:17 am

Glenn is on to something here, kinda odd.

Quentin F

3/6/2026 01:34:18 am

The rest of the site is protected mostly with fire hydrants.
Some electrical rooms will have inert gaz.

The main activity on site will involve liquid metal which does not mix well with water at all!
Thus the reason why we have limited amount of automated water based fire protection.

Anthony

3/5/2026 09:38:18 am

I agree with others that the rooms could be designed independent of other areas. Phantom flow can be descoped with an e-mail to the AHJ often.

I'm also concerned with the total project scope of work for a limited sprinkler system, not whole building system for a new build? Odd, makes the architectural compliance more complicated by reducing egress paths and increases wall ratings among other items.

Dustin T

3/5/2026 09:54:47 am

I too question why you have only two rooms in a new industrial building.

Your information suggests your situation is equivalent to an example in NFPA 13, small high hazard room separated from the surrounding hazard by full height partitions capable of delaying heat from one side fusing sprinklers on the other side. See the guidance in NFPA 13 A.11.1.2 (2016). If this is the case, the surrounding area hazard occupancy dictates the design area size. If the surrounding area is a lower hazard, your demand may be lower than the "phantom flow" option.

Ensure your design's hazard occupancies are correct. In my state, hazard classification and criteria are the role of a licensed engineer.

RayJ link

3/5/2026 10:27:09 am

Why would you use 2500 ft² when you are only protecting 420 ft² in one area and 520 ft² in the other?

Jack G

3/5/2026 04:21:33 pm

The way it’s stated in NFPA 13 , phantom flow is only for density area method calculations say an ordinary hazard calculation with a small room that may be extra hazard.
If “ Room design method” is used for the extra hazard room, and the walls and doors/windows of the room were rated appropriately, then the high density applies only to the room with “ no phantom flow “ added.
Density Area vs Room design methods.

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Design Area for Very Small Extra Hazard Rooms?

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