When Does the 2,000 sqft Minimum Area Apply?

10/17/2023

Good morning all.

So I'm looking at an Extra Hazard Group 1 (EH1) dry system under NFPA 13 (2016 edition).

I want to apply multiple adjustments per 11.2.3.2.7 because I want to use high temperature sprinklers to get me a 25% reduction under 11.2.3.2.6.

This can result in the following math: 2,500 * 1.3 for dry system = 3,250 sqft. 3,250 * 0.75 (for 25% reduction) = 2,437.5 sqft.

And this always stays above the 2,000 sqft requirement of 11.2.3.2.6 and everyone is happy.

But it can also result in the following math happening: 2,500 * 0.75 = 1,875 sqft. 1,875 gets forced into 2,000 sqft because I'm not ever supposed to go below a 2,000 sqft design area under 11.2.3.2.6. So then 2,000 * 1.3 (for dry system) = 2,600 sqft.

So would the system have a 2,437.5 sqft design area or a 2,600 sqft design area?

Yeah, it's only two sprinklers, but at these densities that can change a system. I can see an argument for either conclusion. Thanks in advance.

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6 Comments

danefre

10/17/2023 07:06:46 am

Your first calculation is the correct one. Step 1: make all the area adjustments. Step 2. Check if you are below 2000 sqft.

And as you know, multiplication order doesn't matter.

Anthony

10/17/2023 07:33:19 am

I don't think either is wrong and this falls under good (or bad, depending on your point of view) "code lawyering." I'd take the smaller area if it was convent and would be compliant and happy with that.

Note NFPA does specifically states there is no order of operations to area reduction or increase:

13-16 annex:
A.11.2.3.2.7

Example 2. A wet pipe sprinkler system (light hazard) in a building with a 16 ft 8 in. (5.1 m) ceiling and a slope exceed-ng 2 in 12 in. (50 in 300 mm) (16.6 percent slope) employs quick-response sprinklers qualifying for a 30 percent reduction as permitted by 11.2.3.2.3. The initial area must be increased 30 percent for the ceiling slope and the resulting area decreased 30 percent for quick-response sprinklers. IT DOES NOT MATTER WHETHER THE REDUCTION IS APPLIED FIRST. If a discharge density of 0.1 gpm/ft2 (4.1 mm/min) over 1500 ft2(139 m2) is chosen from Figure 11.2.3.1.1, the 1500 ft2(139 m2) is increased 450 ft2 (42 m2), resulting in 1950 ft2(181 m2), which is then decreased 585 ft2 (54 m2). The final design is 0.1 gpm/ft2 (4.1 mm/min) over 1365 ft2 (127 m2).

Dan Wilder

10/17/2023 10:55:43 am

As it looks like you are referencing 2016, my approach would be:

11.2.3.1.1(1) - .3 / 2500
11.2.3.2.5 - Increase 30% = .3 / 3250
11.2.3.2.6 - Reduce 25% = .3 / 2437.5
11.2.3.2.7.1 - This indicates the adjustments shall be compounded based on the area of operation "originally" selected from Figure 11.2.3.1.1, not to apply any other criteria during the process. Any minimums would be checked at the end of the process (see next statement for why).

Also, section 11.2.3.2.7.2 (unsprinklered combustible concealed spaces for 3000 ft²) says that the minimum 3000 ft² is to be applied AFTER all other modifications, not starting with or during the process...so if there is a provision in another area for a minimum square footage, that should also follow the established intent from this section.

Now, looking into the 2019/2022 edition, you can also take an additional 25% off for 11.2K sprinklers. (19.3.3.2.7) but no less than 2,000 ft², which applying the order above, would come 3rd in the list.

I think there is a good argument for the lesser remote area size.

Jack G

10/17/2023 12:28:45 pm

I would do all required increases to the dry system (bigger number) Then I would apply the discounts.
Do the requirements for the system building first- better fire protection.
Opinion.

Pete H

10/17/2023 12:31:26 pm

Unfortunately the code doesn't spell it out... but I think the correct math is:

2500 * (All Modifications or Reductions)
2500 * (1.3 * 0.75)
2500 * (0.975)
2437.5

This way the modifiers are applied concurrently.

But as stated, the code does not spell it out plainly enough in my opinion.

To take it further, if you were to use the 11.2k reduction with high temp heads that Dan Wilder was kind enough to point out in 2019-2022 editions:

2500 * (1.3 * 0.75 * 0.75)
2500 * (0.73125)
1828.125, and now that is immediately forced to 2000 square feet.

Which does allow for a 2000 square foot design area for an extra hazard occupancy, which feels kind of dangerous, but it is my interpretation/opinion of the current writing of the code. I could be wrong.

10/17/2023 12:47:41 pm

A dry extra hazard occupancy. I forgot to point out dry in that last paragraph.

When Does the 2,000 sqft Minimum Area Apply?

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