Connect 4-in FDC Feed into 2-in Sprinkler Main?

5/5/2025

When connecting a FDC to a system at a point other than the riser, is it acceptable to connect a 4 x 2½ x 2½ (2-inlet) FDC using 4" pipe feed for the FDC and tying the 4" pipe to a 2" sprinkler main?

Thanks in advance.

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10 Comments

Glenn Berger

5/5/2025 08:12:36 am

Connecting a 4 inch FDC into a 2 inch pipe does not seem to meet its intent.

Chad

5/5/2025 08:16:37 am

We allow a single 2 1/2 for small systems…. i’d run it by the AHJ

Todd E Wyatt

5/5/2025 08:29:10 am

Per the referenced standard (e.g. NFPA 13 - 2022), the size of the FDC shall meet ONE of the (3)16.12.4 requirements :

NFPA 13 -2022
Chapter 16 Installation of Piping, Valves, and Appurtenances
16.12* Fire Department Connections.
16.12.4* Size.
The size of the pipe for the FIRE DEPARTMENT CONNECTION shall be in accordance with ONE of the following:
(1) Pipe size shall be a minimum of 4 in. (100 mm) for fire engine connections.
(2) Pipe size shall be a minimum of 6 in. (150 mm) for fire boat connections.
(3) For hydraulically calculated systems, the pipe size shall be permitted to be less than 4 in. (100 mm), but NOT LESS THAN THE LARGEST RISER BEING SERVED BY THAT CONNECTION.

A.16.12.4
The purpose of a fire department connection is to supplement the pressure to an automatic fire sprinkler system. It is not the intent to size the fire department connection piping based on system demand. For multiple system risers supplied by a manifold, the fire department connection need not be larger than that for an individual system.

J.H.

5/5/2025 08:42:37 am

Here also are some (NFPA 13, 2019 Ed.) restrictions on FDC connection to the system:

16.12.5.1.1 The fire department connection shall not be
attached to branch line piping.
16.12.5.2 For single systems, the fire department connection
shall be installed as follows:
(1) Wet system — on the system side of system control, check, and alarm valves (see Figure A.16.9.3)
(2) Dry system — between the system control valve and the
dry pipe valve
(3) Preaction system — between the preaction valve and the
check valve on the system side of the preaction valve
(4) Deluge system — on the system side of the deluge valve

16.12.5.3 The fire department connection shall be permitted to be connected to the main piping on the wet pipe or deluge system it serves.
16.12.5.4 For multiple systems, the fire department connection shall be connected between the supply control valves and the system control valves.

Casey Milhorn

5/5/2025 08:59:04 am

Nothing wrong with it, but worth asking the AHJ if they prefer a 2-1/2" single snoot or siamese connection in a case like this. I've ran into a couple AHJs that still want the siamese for redundancy. The cost difference is fairly negligible since the single snoots aren't that common. I personally like the idea of the siamese connection to keep things standardized and due to the negligible cost difference.

5/5/2025 09:47:40 am

It's not spelled out in NFPA 13, but we always make sure that there is a piping path from the FDC to the system riser that meets the minimum requirements of 16.12.4. I think 16.12.5.1.1 was added to make sure people weren't connecting 4" FDCs to smaller branchlines which implies the minimum pipe size from the FDC to the system riser should be maintained.

NFPA 13, 2025:
16.12.4* Size.
The size of the pipe for the fire department connection shall be in accordance with one of the following:
(1)Pipe size shall be a minimum of 4 in. (100 mm) for fire engine connections.
(2)Pipe size shall be a minimum of 6 in. (150 mm) for fire boat connections.
(3)For hydraulically calculated systems, the pipe size shall be permitted to be less than 4 in. (100 mm), but not less than the largest riser being served by that connection.

A.16.12.4 The purpose of a fire department connection is to supplement the pressure to an automatic fire sprinkler system. It is not the intent to size the fire department connection piping based on system demand. For multiple system risers supplied by a manifold, the fire department connection need not be larger than that for an individual system.

16.12.5* Arrangement. The fire department connection shall be arranged in accordance with Figure 16.12.1.

16.12.5.1* The fire department connection shall be on the system side of the water supply check valve.

A.16.12.5.1 The fire department connection should be connected to the system riser. For single systems, it is an acceptable arrangement to attach the fire department connection to any point in the system, provided the pipe size meets the requirements of 16.12.4.

16.12.5.1.1 The fire department connection shall not be attached to branch line piping.

5/5/2025 09:51:17 am

To be more specific for your question: "When connecting a FDC to a system at a point other than the riser, is it acceptable to connect a 4 x 2½ x 2½ (2-inlet) FDC using 4" pipe feed for the FDC and tying the 4" pipe to a 2" sprinkler main?"

Answer: If the largest riser being served by that connection is 2" then the answer is "yes", it is allowable per NFPA 13, 2015 16.12.4(3). If the largest riser being served by that connection is larger than 2" (for example a 4" riser) then answer is "no" per NFPA 13, 2015 16.12.4(1).

ANTHONY

5/5/2025 10:06:46 am

If the main, the whole way from the riser is 2'' then I find this to be acceptable. If there is any point on that main that is less than 2'' then you'll need to take 4'' feed back to that point. For instance, if you have a 3'' main for 40 ft with 3 branch lines off it then you'll need to tire back at the 3'' main.

Pete H

5/5/2025 10:11:19 am

Todd Wyatt nailed it:

FDC pipe less than 4" can't be smaller than the riser feeding the connection.

Jack G

5/5/2025 02:12:14 pm

So are we saying the size of the largest main is 2” ? Like a 2 x 2 x 1.25 grid ? And the fdc is on the back main ?
Is the Riser 2 inch with a 2 inch bulk ?
Would depend on system demand — 1– 2.5 outlet for each 250 gpm.
If riser and bulk are larger— and water demand is over 180 gpm ( 32 fps - high friction loss in 2 inch)
I would run 4 inch to the grid and just be done with it. Covers up to 500 gpm system demand.

Connect 4-in FDC Feed into 2-in Sprinkler Main?

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