The most frequently used fire hoses today come with NH / NST or NPSH threads.

The most common thread type in use by fire departments today is NST or National Standard Thread. It is often referred to as “fire hose thread”. NST also goes by the initials NH or National Hose. It can even be referred to as NHT or National Hose Thread but NST or NH is more common these days. NH = NST.

Fire hoses with National Pipe Straight Hose Thread (NPSH) have the same threads per inch as National Pipe Thread (NPT), but the threads do not taper. This allows a female coupling to thread onto a NPT fitting without being a permanent connection.

Both NH / NST and NPSH are “straight” threads which means the threads are consistent from tip to collar and the seal is made via the gasket inside the female coupling mating against the square face of the male coupling. The threads do not make the seal, they merely hold the two fittings tightly together.

The female and male side of a fire hose coupling with NH / NST threads will only connect to fittings, nozzles or hoses with NH / NST threads. For example, a fire hose with NH / NST threaded couplings will require a nozzle with NH / NST threads.

The female side of a fire hose coupling with NPSH threads will connect to fittings or hoses with NPSH or NPT threads. For example, a female fire hose coupling with NPSH threads will connect to the male side of a PVC pipe, metal pipe or couplings with NPT or NPSH threads.

The male side of a fire hose coupling with NPSH threads will only connect to fittings, nozzles or hoses with NPSH threads. It will NOT thread into a female NPT pipe. For example, a male fire hose coupling with NPSH threads will require a nozzle with NPSH threads.

Some fire departments and industrial locations in the United States have special threaded fire hoses.

How to determine which threads you might have on your current fire hoses.

1. Check the female coupling for letters like NH, NST or NPSH.
2. Measure the outside diameter of the male coupling. Do not measure the female coupling.
3. Measure the circumference of the male coupling, and check the thread charts found below
4. If the couplings are stamped something like 3.03 x 8 or 3.25 x 7.5, you likely have a “local” thread and will need to order a custom adapter (which usually has a lead time)

How to determine which size hose you might have.

1. Check the female coupling for numbers like 1.5 or 2.5
2. Measure the inside diameter of the male coupling. Do not measure the female coupling.
3. Check for numbers stenciled on the hoses.
4. Do not measure the hose laying flat.

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How To Measure the Circumference of an Adapter or Fire Hose Fitting

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• IDM - Inside Diameter Male
• ODM - Outside Diameter Male
• TPI - Threads Per Inch