Required Size of FDC Pipe for Larger Risers?

4/5/2021

NFPA 13 states that for hydraulically calculated sprinkler systems the fire department connection (FDC) pipe can be less than 4-inch diameter, but not less than the system riser.

What size should the fire department connection feed pipe be for a 6-inch riser, or an 8-inch riser?

Can the FDC pipe still be 4-inch for these larger risers?

I have some clients believe that 4-inch is the maximum no matter what, and some say if the riser is 6-inch or 8-inch that the FDC pipe needs to match the larger riser size.

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8 Comments

Eric

4/5/2021 08:11:40 am

The code in NFPA 13 states that the size for the FDC is in accordance with ONE of the following. 4" minimum for fire engine connections is the most common and somewhat serves as a "rule of thumb" in the industry. In a hydraulically calculated system that is supplied by a smaller that 4 inch riser, the code referencing the smaller size comes into play. Then it is limited to the size of the riser servicing the system.

8.17.2.3* 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.8.17.2.3 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.

Dan Wilder

4/5/2021 08:28:43 am

It depends.....I know right?

Which version and year are you referencing? I understand you referenced 13 but 14 has some specific language as to sizing & number of outlets required that will override 13. The 13-2007 Edition called out for a 2½" outlet per 250GPM of system demand (matches 14), later editions removed this requirement (in 13, 14 still has this requirement in 19'-7.12.3). Most of my AHJ's have adopted some variant of the sizing with most asking for a Siamese connection AND a Storz connection (4"-5" but they vary as well) for any system with a demand over 500GPM. At that point, we provide 6" piping regardless. I cannot say I've provided an 8" FDC supply unless a pump was present on some of our larger demand systems (those typically get into multiple Storz connections as well).

Officially (per 13-19'), sizing is based on 16.12.4 and can be less than 4" but not less than the largest riser being served. Any pumper connections shall be a minimum of 4". So with nothing more than NFPA to fall back to, match your riser size for the FDC supply piping. Bigger question is how many 2½" outlets are you providing for the larger systems?

It's a little outdated but a good start to all your FDC questions:

https://www.meyerfire.com/blog/a-fire-department-connection-cheatsheet

Here is a Research paper on the matter showing that a single 2½" inlet can provide over 500 GPM, a Siamese can get over 1500 GPM but anything more than 2 inlets was tested with a 6" pipe:

https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Suppression/RFFireDepartmentConnectionInletFlowAssessment.ashx

Monday Morning

4/5/2021 09:07:43 am

NFPA 13, 2016 8.17.2.3 says "The size of the pipe for the fire department connection shall be in accordance with one of the following:" (which means any of the options are acceptable). 8.17.2.3(1) states "Pipe size shall be a minimum of 4" for the fire engine connections." 8.17.2.3(3) states "For hydraulically calculated system, the pipe size shall be permitted to be less than 4 in., but not less than the largest rise being served by that connection." To the best of my knowledge, the intent of this section is to require a 4" FDC for any riser size but to allow you to use a smaller FDC for smaller risers. This means8.17.2.3(3) is giving the option of making an FDC smaller when possible but not intended to require larger FDC piping. A.8.17.2.3 explains that the intent is supplemental so assuming you have a reliable water supply, there is no reason to size an FDC based on system demand.

Many AHJs have their own FDC requirements which should be followed, but outside of AHJ modifications the intent of 8.17.2.3 is to require a 4" FDC with two 2.5" outlets (minimum) for fire sprinkler systems. Designing and installing 6" pipe definitely doesn't hurt anything, but NFPA 13 provides minimum standards which in this case is a 4" FDC.

NFPA 13, 2019

4/5/2021 09:22:20 am

It isn't as clear in earlier editions of NFPA 13, but the 2019 edition provides more guidance about the 4" minimum pipe size and two 2½" outlets.

NFPA 13, 2019 A.16.12.3.1 "The purpose of the fire department connection is to supplement the water supply but not necessarily provide the entire system demand. Fire department connections are not intended to deliver a specific volume of water."

NFPA 13,2019 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."

NFPA 13, 2019 16.12.3.1 "Unless the requirements of 16.12.3.1.1, 16.12.3.1.2, or 16.12.3.1.3 are met, the fire department connection(s) shall consist of two 2½ in. connections using NH internal threaded swivel fitting(s) with "2.5-7.5 NH standard thread," as specified in NFPA 1963."

Casey Milhorn

4/5/2021 09:53:48 am

Everyone covered it pretty well. The only thing I would add is to use some common sense as well. We just had a job with one of those "surprise" fire pumps (I think we've all had one or more of these before). Well the fire pump is going to be installed at the far end of the building from the current lead-in, which meant that the FDC has to feed all the way from the discharge side of the pump back to the front wall. This is a storage type occupancy with some larger GPM requirements. Per the standard, we could have technically provided a 4" main several hundred feet back to the front, but we know the friction loss would be extreme with even just 500 gpm flowing through it. We used a little common sense and upped the size to 6". Of course many times us contractors don't have the luxury of making things "better" but fortunately this was a change order situation and we had the freedom to make a good fire protection decision with the fire fighters in mind.

Dan Wilder

4/5/2021 11:22:38 pm

No close mains you could have tied into from the front of the building? We've tied into the primary (or secondary/dead main) if it matched the size of the riser up to that point (6" riser feeding a 4" primary = 6" FDC feed tieing into a 4" primary/secondary/dead main). We still provide checks to each system, just not at the riser room.

Casey Milhorn

4/6/2021 08:42:17 am

Unfortunately it's a multi-system building. We would have had to tie into 3 separate systems with checks on each one and the distances would have been more than the one bulk main. Great idea though and I've used that one before too.

Deborah H

4/6/2021 05:53:19 pm

Croker and Potter - FDC Siamese only come in 4" 5" & 6" (back, bottom or top feed) these products are all UL FM & used everywhere.

NFPA 13, 2016 8.17.2.3
Minimum is 4"

Required Size of FDC Pipe for Larger Risers?

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