Are Separate Fire & Domestic Lines Required?

6/17/2025

Do I need a separate 6" fire line to a building sprinkler system, and a second water line for all other uses?

This is for an NFPA 13 system. Thanks in advance.

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7 Comments

Chad

6/17/2025 07:26:25 am

Depends on the water company/department plus the sections below (2019 NFPA 13) As long as it calcs out, it could be shared. The domestic demand is required to be accounted for. Same for 13R and D, although D uses a fixed 5GPM domestic demand

For a full 13 system typically, yes its a dedicated main regardless of the calcs due to water department regs.

5.1.3.1
Except as provided in 5.1.3.2 or 5.1.3.3, no pipe smaller than 6 in. (150 mm) in diameter shall be installed as a private service main.

5.1.3.2
For mains that do not supply hydrants, sizes smaller than 6 in. (150 mm) shall be permitted to be used subject to the following restrictions:
(1)
The main supplies only automatic sprinkler systems, open sprinkler systems, water spray fixed systems, foam systems, or Class II standpipe systems.

(2)
Hydraulic calculations show that the main will supply the total demand at the appropriate pressure. Systems that are not hydraulically calculated shall have a main at least as large as the system riser.

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Private fire service mains must be at least 6 in. (150 mm) in diameter where they supply hydrants. In some cases, the use of a minimum 8 in. (200 mm) pipe for dead-end mains is advised. Where the fire service mains do not supply hydrants, they are permitted to be sized based on hydraulic calculations, or they are permitted to be as large as the riser under certain conditions. Fire mains need a water supply sufficient to meet the demands of the sprinkler systems, inside hose, outside hose, and other anticipated water demands.

5.1.3.3

Where a single main less than 4 in. (100 mm) in diameter serves both fire systems and other uses, the non-fire demand shall be added to the hydraulic calculations for the fire system at the point of connection unless provisions have been made to automatically isolate the non-fire demand during a fire event.

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Simultaneous flow of the non-fire water and automatic sprinkler systems must be considered when small diameter [i.e., less than 4 in. (100 mm)] combination lead-ins are used. The additional non-fire waterflow can create increased friction loss, resulting in a reduction in the water available for the sprinkler system.

The term domestic was replaced with non-fire in the 2013 edition. This change was made to emphasize that any water that is flowing needs to be accounted for whether it is being used for domestic use or other uses.

Anthony

6/17/2025 08:16:21 am

Depends on the municipality. In some areas its common to have separate lines so the water authority can shut off your domestic water for non payment and not have your building burn down.

Per NFPA 13 you can definitely use a combined 6'' incoming line and its very common to do so. I would suggest adding a 'master' OS&Y often called a city valve so the building water can be shut off from the main water entry point w/o having to mess about with a road box.

Jesse

6/17/2025 08:17:17 am

Yep you can, but there are some pitfalls. I recently had a project in Houston where there was an existing fire line and RPZ. Everything calc'd and was though fine until we were told that the domestic demand, or non-fire water was also coming off of that same fire line. So we had to add additional backflow preventers to the risers in each building which killed our calcs.

So while you can do it, it doesn't always work out for the best. Most of our Austin metro projects have separate domestic / fire taps.

Glenn Berger

6/17/2025 08:26:49 am

It depends on your locality and their regulations. Also check with the Owner's Parties having interest on the project as they may have additional requirements.

NFPA 13 does not require a dedicated supply pipe into the building.

Dan Wilder

6/17/2025 09:06:40 am

Not a requirement and others have given some good feedback above.

One of the biggest drawbacks I have found when this is proposed is the fact that cross contamination becomes a big deal. Differing pipe type requirements to comply with AWWA requirements (C.L.D.I.P, Copper), the addition of a RPZ, and non-fire GPM allowances creates a heavy toll on available flow and pressure.

Jack G

6/17/2025 09:35:31 am

It’s probably for most public utilities that separate dedicated lines are required so if you don’t pay your water bill ( and they shut you down) your building still has fire protection. This is prevalent for water companies in my jurisdiction.

Mark

6/17/2025 01:42:30 pm

I am an AHJ for a local municipality in Illinois, and the municipality requires a combination main into the fire protection/meter room which then is teed to the separate mains. The plumbing inspectors have explained to me that this is a State requirement in the Plumbing Code to avoid cross-contamination of the municipal water distribution system and dead-end mains.

The explanation they have given me is that a separate fire protection main does not have backflow protection until in the building before the fire pump or riser and since the fire protection water is not regularly flowed this will create a stagnant water situation and a large dead-end main the length from the municipal water main to the fire protection system. The Plumbing Code limits dead-end mains to 2' maximum, so this installation creates a hazardous situation with no cross-connect protection to the municipal water main.

This applies to all occupancy types, and we further require RPZ's, not DDC, on any fire sprinkler system with an FDC or other possibility of contamination. The sprinkler systems are calc'd to include both domestic and fire protection system demands.

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Are Separate Fire & Domestic Lines Required?

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