Basis for No Hydraulic Calcs Under 20 Sprinklers?

4/2/2021

When 20 or fewer sprinklers are involved in a modification, codes and standards suggest new hydraulic calculations are not required.

What is the scientific or engineering basis for the "20-sprinkler" number?

Is there some study or analysis that found this to be a reasonable breakpoint?

I'm researching whether we should require full plans for less than 20 sprinklers. Thanks!

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14 Comments

FIREPE25

4/2/2021 10:08:05 am

I do not know of any code or standard reference that suggests that calculations are not required for less than 20 sprinklers.

Kelley Stalder

4/5/2021 11:58:05 am

You are right, however NFPA 13 Chapter 25 contains this statement: "Modification affecting 20 or fewer sprinklers shall not require testing in excess of system pressure" has been used as the basis for this statement in our rules:

"When an alteration consists of 20 sprinklers or less and all floor
areas were protected prior to the alteration, updated plans are not required."

I was really looking for a data-based reason for 20 sprinklers. Thanks!

Franck

4/2/2021 10:15:50 am

I don't think there is any scientific rule behind the 20. But maybe I'm wrong.
NFPA 13 (to my knowledge) does not speak about this.

But there might be some sense of judgement.
For a small extension of an existing installation, if the design for the small extension is based on the pipe schedule method and you have a light or ordinary hazard occupancy, I would not spend lot of money/effort to make a calculation, as long as you still respect the pipe schedule for the entire final system.
Even for a new small system, NFPA 13 allows you to use the pipe schedule method for light and ordinary hazard occupancies (up to 5000 sq ft or more dependning on the available residual pressure - as per 19.3.2.3 (2019 Edition).

On the other hand, if I add an extra hazard occupancy with 15 or 20 heads, and the remaining of my system is based on a light hazard occupancy, maybe an hydraulic calculation would make sense, as the main pipes for the LH design might be too small to carry the water demand to the EH area (friction loss).

The same for storage occupancies where the pipe schedule method does not apply.

I would also recommend an hydraulic calculation for an extension whenever there is a reasonable doubt that the demand might not be fulfilled because of additional friction losses (like adding sprinklers at the far end of the system).
Or for any specific system, such as adding 20 in-rack sprinklers as you need to balance the in-rack demand with the ceiling demand.

Kelley Stalder

4/5/2021 12:02:33 pm

Thanks

Jonathan Griffin

4/2/2021 10:46:04 am

I believe that you are confusing the allowance within the NFPA 72 standard which allows for a sprinkler system containing less than 20 sprinklers to forego Central Station monitoring.
NFPA 13 requires a minimum of 2 or 4 sprinklers within a 13D/13R system to be calculated, and no less than 5 sprinklers in a standard NFPA 13 system.

Casey Milhorn

4/2/2021 11:50:30 am

Franck is right on. I'm kind of split on putting a calc requirement based on QTY of sprinklers being added or remodeled. As Franck's examples show, there could be a case for the addition of one sprinkler requiring a calculation, or the addition of 1000 sprinklers not requiring a calc. For instance, if someone came in and built a 400 sqft paint application area in the middle of light or ordinary hazard building, you might only be adding 4 to 6 heads, but that demand could be more than the riser, main, or branch lines can handle, not to mention how do you know what size your new piping should be feeding the new heads? On the other hand, you could have an extra hazard multi-system building that was bought for office use, ACT ceilings were installed throughout with 1000 pendent sprinklers fed from 1" drop downs from the roof system. Common sense tells you that system is going to work extremely well and calculations are a waste of time. Some options would be just to trust the NICET (in most states) putting their signature on it and/or better training for the plan reviewer. I think NFPA stays away from this topic for these exact reasons and leaves it to the local/state AHJs to make their own rules on this.

MONTY

4/5/2021 07:35:59 am

I think he may be referencing 25.2.1.4.1 in NFPA 13-2016 which deals with hydrostatic testing and acceptance requirements.

25.2.1.4.1
"Where modification is made to an existing system affecting more than 20 sprinklers, the new portion shall be isolated and tested at not less than 200 psi (13.8 bar) for 2 hours."

In this case, a modification to a system would be tested under system pressure (25.2.1.4).

However, the standard does not specifically state that any modification under 20 sprinklers does not require calculations. But I can see where the argument can be made.

Kelley Stalder

4/5/2021 12:05:03 pm

You see my question precisely. All I was hoping for was that someone would point me to a paper or analysis that justified 20 heads or fewer.

Or maybe it was just some number that was agreed to in a standards meeting...

Dorgan

4/5/2021 11:47:14 am

This is a common practice in ALL jurisdictions. Why would an AHJ require expensive calcs for a 3 head relocate? Its plain silly.

Denver Fire, and nearly all AHJ's, have amendments for their specific requirements for quick "walk-through" permits. I do them all the time. A simple call will see their requirements.

Denver requires no calcs if under 20 sprinklers are touched, after 20 its a PE stamp, and up to 50. After 50 a calc is required.

Another jurisdiction its 75 and I have done up to 300 is my most relocates without a calc. It was a OH office being returned to its core and shell for an open ceiling LH office area.

The only code off the top of my head, and one Ive been called out on for a no calc job, is if more than 1 sprinkler is pulled from one 1" outlet calcs can be required. Denver Fire I believe allows 2 from ea 1" outlet and calcs if more than 2.

Also AHJ's will require calcs if going up in hazard or any alteration of the sprinkler system

GL!
DT

Kelley Stalder

4/5/2021 12:02:01 pm

Thanks. I understand that various jurisdictions differ. I was looking for a scientific, data-based analysis for 20 heads. Or any number of heads.

I realize that NFPA 13 has some specific requirements as you mentioned.

Kim

4/6/2021 03:32:01 pm

I've fought with my bosses about this for years (VA/NC/SC). Around here, 19-20 heads in a light hazard upfit didn't require a calc or submittal.

Where I began to disagree with my bosses was when we started to use a lot of 48" AH1's (regardless of xyz dims to center of tile)

I said the 1" loss per the spec sheet was much more than a 1" sch40 hard pipe armover, and since we buy the 48" length in bulk, undoubtedly, our installers are tying up more than one "90", making it worse. Will it spray? Yes. To the extent in which it should? I don't believe so.

Am I looking at this wrong? Sorry to semi-Hijack the question.

Casey Milhorn

4/6/2021 06:08:00 pm

Yeah, I agree. There are times that adding flex on a non-flex job could require a calculation. I'm kind of split on this one too. I use a little common sense on this. You will gain some pressure due to elevation, typically the build out isn't in the remote part or floor of the building, and usually reduced area calculations aren't very common around here for shell systems and can be utilized for the buildout. Also be on the lookout from VIC, I think they said they are (or have) added other friction loss options for 1, 2, or 3 90 degree bends so you don't always have to use the worst case scenario. Remember that NFPA 13 already has tons of safety factors inherently in the original calc. Oversized area, elongated area, remote area is located at the worse case area, C-factor is based on future pipe conditions and not new and a lot of fire marshals throw some type of safety margin on top of that. Then you factor in that most light hazard fires are controlled or extinguished by just one or two heads. AND sometimes you pick up a little efficiency (back pressure) by restricting the non-driving heads with extra footage of pipe and fittings that you don't get with pure uprights. I just remember a lot of buildouts back in my early days coming out of 1/2" outlets. I worry more about that than I do flex. BUT all that being said, if I run into a build out on the top floor or I get a feeling of the system being marginal, I do a calc. You can't go wrong with doing a calculation when you suspect it's going to be tight.

Kim

4/9/2021 12:21:21 pm

Thanks Casey.

Matt Morelli

8/1/2023 09:24:46 am

Does anyone know if i have a floor zoned off by separate east & west FCA's and the total of heads modified off each FCA is less than 20, but the total floors head amount is 22- can the testing be omitted? Thanks

Basis for No Hydraulic Calcs Under 20 Sprinklers?

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