Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

NFPA 13R and domestic water allowance. Am I doing this correctly?

NFPA 13R and domestic water allowance. Am I doing this correctly?

NFPA 13R and domestic water allowance. Am I doing this correctly?

I'm pretty sure I am correct in my approach but I would like any input.

If the 8" loop was part of the city watermain I would not have to add any domestic water allowance as the domestic water would be fed from the 8" city main and not from the main feeding the fire sprinkler riser.

However, as you can see the loop is all private and supplies both sprinklers and domestic water.

The way I read the standard is for the 48 Unit building I would have to add 185 gpm at the 4" sprinkler connection so if my sprinklers require 60 gpm I would carry 245 gpm all the way to the 12" city watermain.

Am I correct in thinking I don't have to add the domestic water use for all three apartment buildings? And no, even if I have to I would not have 485 gallons of domestic water allowance I would simply add the total units 612+612+816 for 2,040 domestic water units which would bring me to 325 gpm for all three buildings. Add to this the 60 gpm for sprinklers and I would have a total of 385 gpm sprinkler and domestic from the point of the 4" connection to the 8" loop to the city water main.

I will probably go with the 385 gpm just to play safe and avoid any possible argument.

So how would you handle this?

Travis? I know you have run into this a lot.

Construction just started and my drawings are about 50% completed.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

Not a designer, but you are allowed only one fire at a time.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?


I understand what you are saying and I agree and if the 8-in was all public water main I wouldn't need any domestic allowance at all.

But the issue is that is a private main and the way I interpret 13r it is that we need a domestic allowance if the fire sprinkler and the domestic is tied together anywhere on the private main.

It was several years ago that Travis ran into a very similar situation where he had three or four buildings very much like what I have and I'm just curious on to how that was resolved.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

Ok. This seems to be an issue of confusion for many, myself included. I'll try to break this down.

If this were an existing complex and you could do a flow test on the private main / loop, you would add the domestic demand on the 8" loop where the domestic and fire are supplied from. I hope that makes sense.

Here is where it gets tricky. If this is a new complex, your flow test doesn't account for the know water usage that will be present when the complex is fully functional. If this is a shared private main, I would add the domestic demand of each building at the point where the domestic supply taps the 8" new main. So, you may have a fire sprinkler demand of 80 gpm, but a total demand of 500 gpm when you add in all of the domestic use.

This is grey because 13R says to only consider the building you are calculating. If this were an existing complex with 1 new building, or a new hotel off a city road, that makes sense.

Our static pressures of flow testing are not really no flow, but rather "normal flow" for a given time. The residual is the additional strain of the water we flow from the hydrant at that time. So, if you have a flow test on the 12" city main, it is not going to account for all on this new development.

Feel free to call me if you want to go over it further, if I'm not explaining it well here.

Travis Mack, SET, RME-G,
Ferguson Fire & Fab, dba MFP Design

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

Well that sounds like a lucid thought.

From non engineering person,,, I always took it if you

Come of the main with a line to a building,,,

Than you pull both the domestic and fire service,,,

You calculate the domestic use.

What if you have one of these mega apartment complexes, with say 100 buildings, say four stories,,

Are you going to add all 100?

I understand the unknown of no mains on the ground yet.

Do another water flow after utilities are installed,,, will not show domestic use,,, but get you in the ballpark.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

On those mega complexes, they are usually 13 and not 13R so that wouldn't be an issue. But yes, we have done some where we have a domestic demand of 1200 gpm and a sprinkler demand of 80 gpm when they got them to be 13R.

Travis Mack, SET, RME-G,
Ferguson Fire & Fab, dba MFP Design

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

But it gets more interesting that 12-in line is not 12 inch it's 8 in and it is a dead end main.

Just to be on the safe side I'm going to go with calculating the entire domestic demand for the entire complex.

On a project like this I would rather err on the side of caution.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

I wish the standards would be consistent. I was always under the impression, in the OP's image, that the 8" loop was a private WATER main and that the FIRE MAIN would be the 4" to the building and that because it's dedicated to fire it would be exempt from domestic allowance. If the domestic tap came off the 4" fire line the domestic demand would be added at that point. That seems to be consistent with how it's done in NFPA 13. Digging in a little deeper to the definition of 'private fire main' in NFPA 24 it would seem that the OP's situation would mean that all of the private water lines would be common domestic/fire mains since they're feeding hydrants as well. It just doesn't seem consistent across the standards though and I'm not thoroughly convinced that the OP's situation would require any domestic demand. It just blows my mind that you could have the exact same situation as the OP's image but have all the buildings designed per '13 and not have to include any domestic demand at all. Or better yet, have his exact same piping infrastructure supply a medical plaza complete with a multi-story, x number of beds hospital, a separate dialysis center, lab buildings etc... and not have to factor in any domestic because his mains are all over 4" in diam. I tend to think my original assessment is correct even though strictly speaking it could be shown otherwise. I don't know. I saw mention of the 2016 13R handbook commentary explaining a little better than the previous editions. Can someone post the relevant portion of that?

I would love for 13R to get in line w/ 13 and make it less murky. Make it based on main diameter and be done with it.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

I realize this is "cloudy," but if you think about it, you can make some sense of it. Most projects that we do are single buildings. When we run into these campus style arrangements (new business park, new apartment, new warehouse park, etc), your flow test may be good for the time you take it, but it won't account for "known" upcoming changes. So, with this complex if you take the flow test on the main street, you won't be accounting for the anticipated full domestic loads that will be present. If this was an existing complex and they are just adding one building, then you take the flow test on the site fire loop and not the main in the street. That flow test would basically account for all of the domestic use - provided it was done at peak time as required. Afterall, the static pressure we record on a flow test is not at zero flow, it is at current flows at the time. We then flow additional water and record the flow and pressure. This is our residual pressure and flow. This is why you have to include the domestic allowance for all of the buildings as you go around the private loop. If you have an industrial complex, it would be prudent to include the anticipated domestic demand, in addition to the hose allowance. I don't know many, if anyone, that does it this way, but it is the most complete.

I hope this makes some sense. So, there are good reasons for doing it that way, but it is pretty "cloudy" to say the least in the NFPA standards.

Travis Mack, SET, RME-G,
Ferguson Fire & Fab, dba MFP Design

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

I did what I felt was the right thing to do.

I added the load for the entire complex and came up with a total domestic load of 2,040 which comes out to 330 gpm for domestic.

I need 390 gpm total sprinkler and domestic demand.

What I find interesting is in all three buildings there's a total of 120 one and two bedroom apartments all having two bathrooms. If we divide the 330 gpm by the total number of apartment units we come out to 2.75 gpm for each apartment unit which seems a little high to me but I am going to stick with that number anyway.

What makes this project even more special is the 8" line is a dead end line and between where it comes off the city grid we have 1,620' of 8" which supplies a total of one small animal pet clinic and that is it. Between the grid connection to the site there's empty fields on both sides of the road and what happens in ten years when this gets built up?

As an added safety factor Georgia requires a minimum of a 10 psi "safety factor" and it looks like I will end up with around a 13 psi "safety factor".

I learned a long time ago that it just isn't smart to shave some corners to save a buck or two. Approaching it like I am I could probably save maybe $1,000 on material but on a job selling for $225,000 it just isn't smart to get worked up over a lousey $1,000. Our work begins 1'-0" AFF.

I've been doing sprinkler design work for nearly 50 years now (I ain't young) and what I am proudest about is I have never had to go back on any project to increase the pipe size because what I did was design with pipe to small to do the job. I like a job with an 18 psi "safety factor". Over design the stupid thing and sleep better at night.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

Was one idea ,,, moons ago,

Make 13R and especially 13d more affordable?

1. Allowing one line to supply both domestic and fire.

2. Trying to get around cities that charge outrage amounts for either two taps,

Or two meters, one which may record very little usage ??

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

Travis, yeah I understand what you're saying but it seems to cross the line into civil engineering. The good thing is, if you're only adding the domestic to the building you're calculating it's usually going to be a non-issue. Adding a few hundred gallons at the point of connection to an 8" main on a 13R system - Yawn! The trouble really comes in when they tap off the actual fireline (from the water main to the building) - that can hurt.

SprinklerDesigner2 - I do the same regarding safety factors. I really don't like to squeeze the system for every last psi unless it means the difference between being on city water or the cost of a fire pump/tank. Then I'll do what I have to but make sure everyone's aware that if even a few add'l 90's get added in it could spell death. Sometimes I get questioned about the amount of 'cushion' and I'm like 'If the AHJ requires asbuilts you'll thank me for it.'

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

CDAFD, yeah, tell me about it. I have that conversation at least once a year when you get these inspectors asking "what if" for a 13D system. I tell them if the house burns down to the ground but the occupants get out alive the sprinkler system is considered a success! Think about that! WHAT IF the fire originates in the attic? All is for naught. The more stipulations you put on these systems the more expensive they're getting making people dislike them even more. We want more sprinklered buldings, not less.

RE: NFPA 13R and domestic water allowance. Am I doing this correctly?

The IFC 903.3.5.2 just states a requirement for the domestic demand to be added to the sprinkler demand.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close