Coverage Area for Standard Uprights & Pendents?

MeyerFire University | FX108.72H

By Joe Meyer, PE

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RESOURCES

FX173 SERIES

RESOURCES

NOTES & SUMMARY

• One-Page Summary [PDF]
• Notes Page [PDF]

CODE & STANDARD REFERENCES

• NFPA 13 – 2022: 9.5.2.1 Coverage Area Formula and Definition
• NFPA 13 – 2022: Table 10.2.4.2.1 Sprinkler Spacing Tables for Standard Spray Uprights & Pendents

FX173 SERIES

1. ​Sprinkler-to-Sprinkler rules for pendent & uprights?
2. Sprinkler-to-Wall rules for pendent & uprights?
3. Coverage Area rules for pendents & uprights?
4. Height of pendents & uprights: Unobstructed?
5. Height of pendents & uprights: Obstructed?
6. Exercise: Procedure Room #8
7. Exercise: Electrical Room #9
8. Heights of pendents & uprights: Peaked Roofs?
9. Exercise: Sprinkler Layout for Peaked Roof
10. Spacing rules for sidewalls?
11. What is the correct height for sidewalls?
12. Exercise: Hotel Room Layout #10
13. How does a "coverage box" work?

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TRANSCRIPT

Coverage Area: Standard Pendents & Uprights 

THIS SERIES

Coverage areas for Standard Spray, Uprights and Pendents. 

In our last segment, we looked at maximum and minimum distances from sprinklers to an adjacent wall. Prior to that, we covered sprinkler to sprinkler spacing. 

We looked at the requirements in NFPA 13 and looked at different caveats that those rules bring with them. 

In this segment, we're looking at the area that's allocated for each sprinkler. This, again, is specific to Standard Spray Pendent and Upright Sprinklers. 

As a quick reminder, there are several rules that come into play with sprinkler spacing. 

#1 - Distance Between Sprinklers 

#2 - Distance Between Sprinklers and Walls 

#3 - Coverage Area per Sprinkler. 

That's what we're gonna cover here. 

#4 - Distance from a Sprinkler Below a Ceiling or Roof 

#5 - Distance from Obstructions 

#6 - Distance from Heat Sources 

So again, we're gonna look at number 3 in this video. 

HOW TO DETERMINE COVERAGE AREA

How is a Coverage Area per sprinkler determined? 

NFPA 13 gives us a real complex formula for this, and that is A = S x L. The coverage area equals the sprinkler spacing times the line spacing. 

This all refers to the Coverage Area for each sprinkler; just one individual sprinkler. 

So, in this formula, what is the S-dimension for a sprinkler?  

Well, the (S) dimension is the larger of two measurements. And of those two measurements, the first is the distance from the sprinkler to the next sprinkler upstream. And the second is the distance from the sprinkler to the next sprinkler downstream. Or if the sprinkler is on the end of a line, it's twice the distance to the wall. 

So, if a sprinkler is in between two others, you just take the greater distance from the sprinkler upstream or the sprinkler downstream and use that as your S. Or if the sprinkler's on the end of the line and it's near a wall, you take the distance to the wall and double that. If that's greater than the distance to the prior sprinkler, then you use that as your S dimension. 

Let’s look at a couple examples. 

So, Sprinkler A here has both an upstream sprinkler and a downstream sprinkler on that same branch line. In this case, the dimension (S) will just be the greater of those two spacings. The first dimension is 14 ft (4.3 m) in this case. The second dimension is 12 ft (3.7 m). We take the greater of those two and that is our (S) Spacing. That's pretty simple for Sprinkler A. 

Sprinkler B is the last one on the line. The distance to the adjacent sprinkler is 12-ft (3.7 m), while the distance to the wall is 7’-3” (or 2.2 m). So, the Dimension (S) is gonna be the greater of the sprinkler-sprinkler spacing or double the distance to the wall. So, the double the distance to the wall, that's gonna be 7’-3” (or 2.2 m) times two, that comes out to 14.5-ft (or 4.4 m). That’s double the distance to the wall. 

Is 14.5 ft (and 4.4 m) greater than 12 ft (3.7 m)? It is. So that becomes our (S) dimension. Double the distance to the wall is gonna be larger here, so it's gonna govern that is our (S) dimension. 

What about the (L) Dimension? 

Well, as we talked about in the last couple segments, the (L) dimension is the larger of the distance from the sprinkler on the adjacent branch line in either direction or twice the distance to the wall, if it's up against a wall. 

So going to Sprinkler A, it has a branch line on either side. In this case, the dimension (L) is just gonna be the greater of the distances to each branch line. Well, on the branch line to the north, we're 11 feet (3.6 m), on the branch line to the south, we're 13 feet (4.0 m). So, we take the larger of those two dimensions. Our dimension (L) for Sprinkler A is gonna be 13 feet (4.0 m). 

For Sprinkler B, we have a wall to the north. So, we are comparing 11 feet (3.6 m), which is the distance to the line to the south, with double the distance to the wall. Double the distance is gonna be 6 feet (or 1.8 meters) times two, that's gonna be 12 feet (or 3.7 m). And of course, 1.8 meters times two is not 3.7, it’s 3.6. But when you go to 12 feet and then round the 12 feet, it's actually more accurately 3.7 meters. So, our (L) dimension there is the greater of 12 feet (3.7 m) versus 11 feet (3.6 m). 

So, sprinkler A has a branch line to the north and a branch line to the south. We are gonna take that greater of those two spacings. So, to the north we have 11 feet (3.4 m). And to the south we have a branch line that's 13 feet (4.0 m). We take the greater of those two, that's 13 feet (4.0 m), and that is gonna be our dimension (L) for Sprinkler A. That's pretty simple. 

Now, Sprinkler B, just like we calculated before the distance north to south, which is our L distance in this case, is the greater of the sprinkler spacing distance to the south or double the distance to the wall. 

So going to the south, we have 11 feet (3.4 m). To the north, we have 6 feet (1.8 m). If you double that, we end up with 12 feet (3.7 m). Now, I know 1.8 times two is not 3.7, it’s 3.6, but if you round 12 feet, it's more accurately 3.7 meters. So, I'm doing a bit of the conversion there. Anyways, that distance to the wall, double the distance to the wall is 12 feet (3.7 m). And that's gonna be greater than our sprinkler spacing distance to the south of 11 feet (3.4 m). 

So, for sprinkler B, our (L) dimension is gonna be 12 feet (3.7 m). So then to get the area, we just multiply the dimension (S) times the dimension (L). Let's multiply that out. 

In our first example, we have 14 feet x 13 feet, and that comes out to 182 square feet. That's 17.2 square meters. 

In the second example, we have 14.5 feet x 12 feet, or 174 square feet; or 4.4 m x 3.7 meters, and that comes out to 16.3 square meters. 

So that by code is how we determine the Coverage Area for each sprinkler. Note here that because of the way that we take the greater of the two dimensions, our Coverage Area by code is actually gonna be greater than the actual floor area that the sprinkler is technically protecting. If we were to simply draw lines between each sprinkler and measure that floor area, well that's gonna be smaller than what our Coverage Area that we're calculating is because we're taking the greater of those two dimensions. 

So, as a code reference, this all comes from NFPA 13 Section 9.5.2.1. That's gonna give us that formula for the area, and it's gonna define what those (S) and (L) dimensions are. 

AN EXCEPTION: SMALL ROOM RULE

Now, there is an exception to this, and of course we love exceptions in the Fire Protection world. And that's for Standard Spray Pendents and Uprights, which we have here. And that is our friendly Small Room Rule. In that case, we take the area of the room and just divide the area by the room by the number of sprinklers in the room. And that's the Coverage Area per sprinkler. Again, this is only for the Small Room Rule. 

So, let's say this is our room where we have the Small Room Rule for our Coverage Area for each sprinkler, that's just gonna be the total area of the room divided by 2. And we get our coverage area per sprinkler. So, if that is light hazard, non-combustible, hydraulically calculated unobstructed, and we get 225 sq ft per sprinkler, then as long as that floor area divided by the number of sprinklers is less than 225 sq ft, and then we're good. 

WHAT IS THE MAXIMUM ALLOWABLE COVERAGE AREA?

So, we know how to calculate the Coverage Area for each sprinkler, but what is the maximum that’s allowed in NFPA 13?  

Well, in all cases, Chapter 9 is gonna tell us that no sprinkler of any type or orientation can cover more than 400 sq ft (or 37 sqm). It'll also point us to the individual sprinkler chapters for the maximums, for each sprinkler type. 

Chapter 10 will actually bring this 400 sqft (or 37 sqm) limit down to 225 sqft (20 sqm) for Standard Spray Pendent and Uprights and it'll then restrict things down further from there based on our hazard classification and a number of other things. So, for Standard Spray Uprights and Pendents, we first go to chapter 10, then to section 10.2.4, we can see here that exception that we just talked about for this Small Room Rule. 

What about the maximum limits? 

Well, they depend on the hazard class, whether we're obstructed or unobstructed, whether we're combustible or non-combustible, and whether our system is pipe schedule or hydraulically calculated. 

Based on this video series that we're doing, and any sprinkler spacing work you've done before, you're probably familiar with these spacing tables by now, along with the sprinkler-to-sprinkler maximums, they give us the maximum coverage area per sprinkler, and that's what our maximums are here. 

So as an example, let's go to the Ordinary Hazard Table. If we're using standard spray upright sprinklers, Table 10.2.4.2.1(b) is gonna tell us that we have a maximum coverage area of 130 sqft (12 sqm) per sprinkler. 

That's gonna be where we find our limits is in one of these tables. 

So in our examples, going back to our examples, would these sprinklers be okay if it were an open Ordinary Hazard space? 

No. Neither of these scenarios would be okay for Ordinary Hazard. 

Why is that? Because our Coverage Area per sprinkler that we calculated is more than 130 sq ft (12 sqm)? 

What about if this space was actually light hazard and it's non-combustible unobstructed like say we have a flat, smooth ceiling and we're not combustible ceiling hydraulically calculated? Well sure, in that situation, or even if we had a pipe schedule system, the coverage area is less than the maximum that's allowed by code and that's 200 sqft (18 sqm) per sprinkler. We're under that so the sprinkler spacing here would be okay if we are light hazard, not combustible and unobstructed. 

WHY WAS HAZARD CLASSIFICATION IMPORTANT?

So, we spent some time talking about hazard classification in an earlier series. Why was that so important? Well, we're starting to see here, and especially in the series, how our sprinkler-to-sprinkler distances, our sprinkler-to-wall distances in our coverage areas are all affected by that hazard classification. 

The determination that we made earlier on what that hazard classification is, these are some of the downstream effects that we're finally starting to see. 

So the lower the spacing and coverage areas that we're allowed, the more sprinklers will need on a given floor area. The more sprinklers we would need on a job, the closer those sprinklers are gonna be to a fire. Again, there's a lot of downstream effects that we're seeing now that are important from that hazard classification that we determined a ways back. 

SUMMARY

In this segment, we've introduced how to calculate our Coverage Area, and that is by first determining the dimension (S) and the dimension (L) for each individual sprinkler and multiplying those together.  

NFPA 13 uses Coverage Areas, or sometimes they refer to it as Protection Area, and that gives us our maximum areas that we allowed to attribute to a single sprinkler. 

So, this has all been under the pretext that we have standard spray, pendent and upright sprinklers. 

In our next segment, we're gonna look at sidewall sprinklers and how those rules work. 

I’m Joe Meyer, this is MeyerFire University.