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+Jason Hugo

#112 - A Guide to Fire Department Connection (FDC) Sizes and Threading

Posted by Jason Hugo on 6/12/2018 to Fire Department Connection
FDC Sizes and Threads

The wrong fire department connection can render life-safety systems inoperable

Fire department connections allow firefighters to pump water to certain sprinkler and standpipe systems – critical tools in the event of a fire. While they’re found on buildings all over the country, each fire department connection is designed to connect to a hose of a particular kind and size, potentially leading to confusion for building owners and facilities managers looking to repair or replace them.

Understanding the different kinds of fire department connections and the particular swivels they use to connect to fire hoses is essential when ordering components.

Do you already know the compatibility of parts for your FDC? If you’re simply looking to buy components, feel free to skip directly to our Fire Department Connection product category.

The basics of fire department connections

A fire department connection (FDC) has three main components: the swivel, body, and inlets. Threaded rings, or swivels, secure fire hoses to the FDC’s inlets, which direct and control water flow, and the FDC’s body, which bridges these components to a building’s standpipe or sprinkler system.

FDCs supply water to standpipe systems and in some cases, sprinklers, allowing firefighters to connect to an external water supply (usually a fire truck) via conveniently-placed locations on the exterior of a building. For many automatic standpipe systems, the water provided is merely supplemental, keeping the system pressurized and providing a back-up source should problems with an onsite supply arise. In manual systems, however, no pressurized water is available unless it’s provided through the FDC.

Fire department connections are generally one of three main types: exposed, freestanding, or flush. Exposed and flush FDCs are both found on a building’s wall, but the body of an exposed connection is visible, whereas flush connections conceal the FDC’s body behind a plate. Freestanding fire department connections stand apart from a building and supply water to a standpipe or sprinkler system through buried pipe. 

Flush Freestanding and Exposed FDCs
(Left to right) A flush FDC, a freestanding FDC, and an exposed FDC. Click here for an in-depth look at the role and components of fire department connections.

A mismatched fire department connection can be a fatal error

Working fire department connections are a crucial investment in life safety and property protection. Individual parts can be replaced affordably – swivels cost less than $40, some caps less than $5, and plugs less than $20 – but entire FDCs can cost anywhere between a couple of hundred dollars to nearly $4,000 before installation. And because they’re designed with specific fire hoses in mind – those used by the fire authority in a given jurisdiction – the wrong FDC can put firefighters in a difficult position.  

Without functioning standpipes, firefighters in tall buildings must run long lengths of hose throughout a building, leading to delay that risks property damage and loss of life. Mismatched threading was a major contributor to at least two historic fires: the Great Baltimore Fire of 1904, which destroyed more than 1,500 buildings in Baltimore, MD, and the 1991 Oakland Hills Fire, where incompatible hose and fire hydrant couplings delayed a fight against an Oakland, CA blaze that killed 25 people. 

How do I figure out what kind of thread and size I need?

Each fire department connection swivel connects only to hoses with couplings of the right size, threading, and gender. A few threading standards prevail, but some cities – like Phoenix, AZ, San Francisco, CA, and New York, NY – have their own specific standards for hose couplings and fire department connections.

National Hose/National Standard Thread (NH/NST) and National Pipe Straight Hose (NPSH) thread are two of the oldest and most widely-adopted thread types for fire department connections and hoses. Each connects the male end of a hose – where threads are located on the outside of the coupling – with a female FDC coupling, which has a threaded interior.

With this overview in mind, the following questions are designed to help you figure out your FDC’s size and thread type:

Is it threaded?

Most fire department connections have threads visible inside the coupling, as pictured below.

FDC Swivel and Gasket
The inside of this coupling has NST threads. 

Storz couplings, on the other hand, have no threads and no designated male or female end. Hose couplings are pressed against the fire department connection and rotated until the couplings latch together.  

4-Inch Storz Connection FDC
A Storz coupling. Note the lack of threads. 

If you have a Storz FDC, you’ll just need to take a proper measurement of the inside waterway – scroll down to the section on measuring the inside waterway for more detail.

Where do I live?

Some cities have their own distinct thread standards. Standard FDC threads may not be compatible with local fire equipment. Rather than simply ordering an FDC that looks like it fits, take the measurements described below and be sure to mention where you’re ordering from to your supplier.

Those cities include:

  • Chicago, IL
  • New York, NY
  • Cleveland, OH
  • Cincinnati, OH
  • Toledo, OH
  • Pittsburgh, PA
  • Detroit, MI
  • Denver, CO
  • Salt Lake City, UT
  • Richmond, VA
  • Raleigh, NC
  • Pittsburgh, PA
  • Louisville, KY
  • Phoenix, AZ

What size is the inner waterway?

Often, the answer is 2.5 inches. That’s because most Class I and Class III standpipe systems feature 2.5-inch hose connections. New York, NY and San Francisco, CA, which commonly use 3-inch FDCs, are exceptions to this rule.

For an in-depth review of fire protection standpipe systems, click here.

You can get an accurate measurement of your FDC’s size one of two ways. The first and best way is to measure the inside diameter of the inlet with no swivel attached. Another alternative is to measure the inside diameter of the back of the swivel.

FDC Missing a Swivel
An FDC with one swivel missing. Measuring the inside diameter on the inlet without a swivel provides a reliable indication of the FDC’s size.

FDC Measurement
This FDC is about 2.5 inches, as measured from inside the swivel. 

Does the thread taper?

NST and NPSH threads are straight-threaded. This means that the width of the threads remains the same along the entire coupling. Tapered threads, on the other hand, get progressively narrower. While these can be hard to spot (see below), tapering threads are a sure sign that your FDC isn’t NST or NPSH-threaded.

Tapered Thread
A tapered pipe-plug held by a caliper. Note how the thread walls move further away from the caliper’s jaw when viewed from right to left. Image credit: BenFrantzDale at English Wikipedia, CC BY-SA 3.0.

What if I’m still not sure?

Firefighters, other life safety professionals, and quality suppliers can help you determine which fire department connection you need. If you are certain of the size and coupling of your FDC, check out our selection of Fire Department Connection products.

If you still have questions about which parts are compatible, call us at 888.392.3362 or fill out our contact form and we’ll guide you through the process.

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