Despite fire hose and hydrant coupler standardization efforts that are at least 144 years old, there remain significant areas in Canada, the United States, and Mexico that use fire hose and hydrant threads and other couplings that are incompatible with those used by neighboring fire departments. This is notable because the first fire hydrant was invented by Manhattan fire fighter George Smith in 1817, making these devices 200 years old.
These incompatibilities have led to well-documented loss of life and buildings, including the Great Boston fire of 1872, the Great Baltimore Fire in 1904, and the Oakland firestorm of 1991. As of 2017, San Francisco still maintains fire hydrants with a size and thread that are incompatible with those used by most or all other nearby fire departments that would respond in mutual aid conditions, such as occurred during the 1989 Loma Prieta earthquake.
As a result of the 1872 Boston fire, the International Association of Fire Engineers designed and published a fire hydrant coupling standard. As a result of the 1904 Baltimore fire, the National Fire Protection Association formed a committee, and in 1905 published its first report on the subject, which would eventually become an official standard, NFPA 1963. This standard specified that each fire hydrant have one large diameter pumper (a.k.a. "steamer") port 4.5 inches in diameter with 4 threads per inch (meant for supplying water to a pumper truck or other high-capacity distribution device), and two medium-diameter ports, each 2.5 inches with 7.5 threads per inch, meant for supplying individual attack hoses directly.
During at least two periods, specialized thread-adjusting tool sets were developed to enable fire departments using diameters and threads similar to but incompatible with the NFPA standard to convert them to the national standard. The first of these was used around 1911, developed by the Greenfield Tap and Die Corporation, and documented as late as 1922, wherein it was claimed that the 70% of municipalities not already using the NFPA standard threads could convert their couplings to the new standard. Around 1950, San Diego Battalion Chief and Master Fire Mechanic Robert Ely developed a similar machine, now known as the "Ely Fire Hose Thread Standardizer" that could do the job in 90 seconds.
One of the reasons for the incompatibilities is that there are three U.S. national hose threaded hose coupling standards. NFPA 1963, which defines the vast majority of fire hose couplings in existence, and ANSI-ASME B1.20.7, which defines garden hose thread (sometimes used by wildland fire fighting crews) along with (non-tapered) iron pipe thread, and ANSI B26, FIRE-HOSE COUPLING SCREW THREAD FOR ALL CONNECTIONS HAVING NOMINAL INSIDE DIAMETERS OF 2 , 3, 3 , AND 4 INCHES".
Note: the straight iron pipe thread is a temporary connection and seals with a gasket, just like garden hose threads and fire hose threads, and is distinct from tapered iron pipe thread (NPT), which is a permanent connection sealed by the threads in conjunction with pipe dope or teflon tape wrapped around the threads. However, because the straight and tapered iron pipe threads differ only in their taper, it is possible for small NPSH/SIPT female hose couplings in sizes inches to 4 inches (inclusive) to be joined to NPT male pipe ends. The connection uses a gasket to seal, and is temporary.
Abbreviations
National Hose Thread is abbreviated NH or NHT
National Pipe Straight Hose is abbreviated NPSH (defined by ASME; same as SIPT, compatible with NPT threads when using a gasket) National Pipe Tapered Thread is abbreviated NPT or NPTF National Pipe Straight Mechanical is abbreviated NPSM Straight Iron Pipe Thread is abbreviated SIPT (same as NPSH) Garden Hose Thread is abbreviated GHT, but its official designation is NH (for National Hose, because it shares its origin with fire hose, but is no longer part of the fire hose coupler standard) General Iron Pipe Straight Thread is abbreviated IPS
NFPA 1963-Specified Metric Equivalents
-inch hose/coupling = 19 mm
1-inch hose/coupling = 25 mm
-inch hose/coupling = 38 mm
2-inch hose/coupling = 52 mm
-inch hose/coupling = 65 mm
3-inch hose/coupling = 75 mm
-inch hose/coupling = 90 mm
4-inch hose/coupling = 100 mm
-inch hose/coupling = 114 mm
5-inch hose/coupling = 125 mm
6-inch hose/coupling = 150 mm
8-inch hose/coupling = 200 mm
Garden Hose
-inch hose with -11.5NH ASME threads (-11.5NHR for rolled threads) -inch hose with -11.5NH ASME threads (-11.5NHR for rolled threads) -inch hose with -11.5NH ASME threads (-11.5NHR for rolled threads)