FDC Couplings – Threaded vs. Threadless

The threads for Fire Department Connections (FDCs) are mentioned by the International Building Code and the International Fire Code (Section 912.3 in both), NFPA 13: Standard for the Installation of Sprinkler Systems (Section 6.8.1), and NFPA 14: Standard for the Installation of Standpipe Systems (Section 4.8.2). All four of these documents require the Authority Having Jurisdiction (AHJ) to approve the thread type for an FDC, while the NFPA standards provide a bit more guidance to a designer or installer as to the thread type. What are the options, and why are there choices?

There are two primary types of couplings for fire hose in the United States – threaded and threadless. A threaded coupling is the traditional style of coupling which screws together, similar to that of a garden hose. The other style, which became popular for fire hose in the United States in the early 1990s, is a threadless coupling, also called a Storz coupling. This style of coupling has identical mating surfaces, with two lugs and two notches in each coupling. The lugs and notches engage, and a quarter-turn of the coupling makes the connection. Threadless couplings are generally found on hoses 4 inches and larger, and hoses smaller than 4 inches generally use threaded couplings. Threaded couplings can be found for hoses 4 inches or greater in diameter, but are very rare for hoses that would be utilized to supply an FDC.

While threaded couplings are the most common style, there are variants between them. The most commonly used fire hose coupling is the American National Fire Hose Connection Screw Thread, better known as NH thread. This thread style is described in great detail in NFPA 1963: Standard for Fire Hose Connections, and has been around since 1905 (after mismatched coupling styles complicated firefighting efforts at the Great Baltimore Fire of 1904). Even with the long history of this standard, there are other standards for threaded hose. Some departments use National Pipe Straight Thread, or NPST, also known as “iron pipe thread.” These threads are finer than NH threads, and they will not connect to an NH coupling. Additionally, some cities continue to use their own locally-designed threads.

Most fire departments have their own requirements for FDC coupling sizes – be it 2.5-inch threaded, 4-inch threadless, or 5-inch threadless, to name a few. The couplings usually reflect the specific operational procedures of the fire department. Proximity of fire hydrants, capacity of the water supply, fire department vehicle access, and fire department staffing all play a part in the decision.

Beyond the fire department’s coupling style preference, there are two additional key factors that can play into the decision as to what couplings are used.

The first is the number of inlets that are expected to be fed by a single fire engine. Most fire engines have at least three, and usually up to six 2.5-inch discharges. They also generally only have one large-diameter threadless discharge sized 4 inches or larger. If multiple FDCs are expected to be simultaneously supplied by a single fire engine, the design team and the AHJ should reevaluate what couplings are used to ensure adequate discharges are available on the responding fire apparatus.

The second factor that can change coupling styles is the required pressure for the sprinkler or standpipe system. NFPA 1961: Standard on Fire Hose, details the construction requirements and capabilities for fire hose. Hose that uses 2.5-inch couplings is classified as “attack hose” and has a maximum operating pressure of at least 275 psi, with some hoses exceeding 400 psi or more. Hose that is 4 inches and larger – which uses threadless fittings – is classified as “supply hose” and has a maximum operating pressure of 185 psi. If a sprinkler or standpipe system requires more than 185 psi at the FDC inlet, the fire department cannot use a supply hose to supplement the system at its required pressure, and should not be requiring threadless couplings 4 inches or larger.  This type of condition is commonly encountered in high rise or very large floor area buildings, where greater pressures at the FDC inlet may be required to serve the sprinkler and standpipe systems on upper floors or distant parts of the building.

The bottom line is that speaking with the responding fire department(s) early in the design process to ensure the proper FDC type is specified for a project is imperative – and a code requirement.