How to Identify Fire Hydrant Threads: NST vs. NPSH Explained

Identifying fire hydrant threads is important when connecting hoses, adapters, or other equipment. But how do you know what type of thread your hydrant has?

Using the wrong thread type can prevent a proper connection, cause delays, and create unnecessary frustration. Understanding the difference between NST (National Standard Thread) and NPSH (National Pipe Straight Hose) threads can help you choose the correct fittings and avoid compatibility issues.

What Are NST (NH) Fire Hydrant Threads?

NST (National Standard Thread), also known as NH (National Hose), is a common thread standard used for fire hose and fire hydrant connections in the United States. 

NST threads are commonly used for fire service applications, including:

• Fire hoses
• Fire department connections (FDCs)
• Fire hose nozzles
• Fire hydrants
• Fire apparatus

While many hydrants use NST threads, not every hydrant does. Thread standards can vary by city, county, municipality, and water system, so always verify the thread specification before purchasing a hose, fitting, or adapter. 

What Are NPSH Threads?

NPSH (National Pipe Straight Hose) threads are straight threads commonly used for industrial, commercial, pump, and water-transfer hose applications.

You'll often find NPSH threads on:

• Pumps
• Industrial hose assemblies
• Water transfer equipment
• Agricultural equipment
• General-purpose water systems

Although NST and NPSH threads may appear very similar, they are not designed to mate together reliably. Even if the thread size looks correct, differences in thread dimensions can prevent a proper seal or secure connection.

What's the Difference Between NST and NPSH Threads?

NST (NH)	NPSH
·         Designed for fire protection applications	·         Designed for industrial and general water applications
·         Common on hydrants and fire equipment	·         Common on pumps and industrial equipment
·         Standardized for fire service use	·         Standardized for straight pipe hose connections
·         Not intended to connect with NPSH threads	·         Not intended to connect with NST threads

Because the differences are subtle, it's easy to mistake one for the other. Never assume two fittings are compatible simply because they thread together partway.

How to Identify Fire Hydrant Threads?

There are several ways to identify your hydrant's thread type:

1. Check with Your Local Fire Department or Municipality

The easiest and most accurate method is to contact the organization responsible for the hydrant. Many municipalities maintain records showing the exact thread standard used throughout their water system.

2. Measure the Threads

If records aren't available, you can measure the hydrant outlet using:

• A caliper to determine the outside diameter
• A thread pitch gauge to measure threads per inch

These measurements can then be compared with standard thread specifications to identify the thread type.

3. Use a Thread Identification Gauge

Thread gauges are designed specifically to identify fire hydrant and hose threads. They provide a fast and reliable way to determine compatibility before ordering fittings or adapters.

4. Look for Manufacturer Information

Some hydrants include identification plates or markings that list the manufacturer and model. With that information, the manufacturer can often confirm the original thread specifications.

What if my Equipment Has a Different Thread?

If your equipment uses a different thread standard than your hydrant, you'll typically need a thread adapter designed to connect the two thread types. 

Selecting the correct adapter requires knowing both:

• The hydrant thread specification
• The thread specification of your hose or equipment

When in doubt, consult your equipment supplier before ordering.

Why Thread Identification Matters

Using the correct thread type helps ensure:

• Secure, leak-free connections
• Faster equipment setup
• Improved safety during operation
• Reduced equipment damage
• Compatibility between hydrants, hoses, and accessories

Taking a few minutes to verify your hydrant's threads can save significant time and prevent costly mistakes in the field.

Need Help Finding the Right Fire Hydrant Adapter?

If you're unsure which thread type your hydrant uses, the JME team can help identify the correct fittings and adapters for your application. Give us a call at 1 (800) 333-3331 or contact us, one of our experts would be happy to help!

Check out our fire equipment at JME for products and solutions!