Area Per Unit of A vs. Floor Area Per Extinguisher?

9/23/2024

I have never quite understood the difference between the maximum floor area per unit of A and the maximum floor area per extinguisher in NFPA 10 Table 6.2.1.1.

If you are only permitted 6,000 sqft for a 2A (3,000 x 2) how does the 11,250 sqft limitation come into play?

Regardless, with a 75-ft travel distance maximum, you rarely get anywhere near these limits, but I wonder if anyone knows more. Thanks in advance!

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7 Comments

Eric R

9/23/2024 08:19:28 am

The difference comes from how Unit of A interacts with travel distance.

The 11,250sqft comes from creating a square area within the 75-ft radius circle required for travel distance in a fully open room. This square works out to be 106' x 106' or 11,236-sqft which is rounded up to the 11,250.

Now for unit of A, all this means is that for a light hazard occupancy you can install a single 4-A extinguisher to cover that entire travel distance area (3,000 x 4 = 12,000). Or if the hazard is higher you would need to install an 8-A or even 12-A extinguisher to cover that amount of floor space.

Eric R

9/23/2024 08:22:26 am

For more detail on how this works check out NFPA 10-2022: Annex E.3 which has a nice graphic showing the 106' square.

Dan Wilder

9/23/2024 08:25:13 am

The 11,250 from an extinguisher location accessible in a rectangular area as measured from the Extinguisher to the corner of a 106'x106' rectangle (what fits within a 75' radius circle) 10-22'-Annex E.3.4

Unit of "A" is just a baseline effectiveness of the extinguisher (typically the equivalent of ~1.25 gallons of water on a 1 sq ft of fire area) - 10-22'-5.3.1

It's to avoid someone putting in a 6A extinguisher to cover up to 18,000 sq. ft. (essentially one extinguisher the middle of an open office), that "could" still also meet the 75' distance requirement....this is extreme but there are layouts that exceed the 11,250 and comply with the 75' distance.

It also is looking at the ability of someone to carry or wheel the extinguisher to the location of the fire thus the additional limitations.

Quick review anyway....

Jack G

9/23/2024 08:49:22 am

Don’t forget that the spacing will change if the occupancy has quick response sprinklers per the IBC.

Jay

9/23/2024 01:42:14 pm

Hi Jack,
What is the section number in the IBC that permits a different extinguisher spacing when quick response sprinklers are installed?

Glenn Berger

9/23/2024 09:02:21 am

Eric and Dan has covered this subject perfectly. The travel distance limitations always governs the quantity of extinguishers required.

Sergei Gluhushkin

9/25/2024 11:51:43 am

What about the extinguishers beside exits?
Would you place extinguishers beside exits first and then fill in the rest based on 75'-0 travel distance??

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Area Per Unit of A vs. Floor Area Per Extinguisher?

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