Your facility’s plumbing system isn’t just for moving water from Point A to Point B. It needs to maintain consistent pressure and prevent costly system failures. But using the wrong pipe connection can cause pressure fluctuations, equipment malfunctions, and compliance issues — leading to operational downtime and expensive emergency repairs.

For businesses across Tennessee, Alabama, Kentucky, and Georgia, Lee Company ensures that the right connections are used for the right applications, preventing plumbing failures before they happen. Below, we’ll break down the key differences between Iron Pipe Size (IPS) and National Pipe Thread (NPT) fittings, when to use each, and why it matters for maintaining pressure stability and system integrity.

What is IPS in Plumbing?

Iron Pipe Size (IPS) is a standard for measuring pipes’ outer diameter (OD) rather than the threading style or connection method. This system dates back to the early 19th century and remains a foundational commercial and industrial plumbing standard today.

Unlike National Pipe Thread (NPT), which focuses on how pipes are threaded together, IPS strictly refers to pipe size — whether the pipe is threaded, welded, or joined by other means.

How IPS is Measured

IPS pipe sizing is based on the OD of the pipe rather than the internal bore. For example:

• A 2-inch IPS pipe has an actual outer diameter of approximately 2.375 inches.
• A 4-inch IPS pipe measures around 4.5 inches in OD.

This consistent OD sizing allows IPS pipes to be used in various applications where precise diameter matters more than threading, such as structural piping and irrigation systems.

Where IPS Pipes Are Used

Since IPS pipes can be threaded or non-threaded, they serve a variety of applications, including:

• Industrial Piping: Common in chemical plants, manufacturing facilities, and refineries where welded or flanged connections are used instead of threads.
• Scaffolding & Structural Uses: Many non-plumbing applications, such as steel frameworks and handrails, use IPS pipes for their uniform diameter.
• Irrigation & Drainage Systems: Smooth IPS pipes are often used in underground water transport and drainage networks.

Choosing the correct pipe standard is critical for long-term infrastructure reliability. IPS ensures durability in large-scale systems where consistency and load-bearing capacity are key considerations.

What is NPT in Plumbing?

As mentioned in the prior section above, National Pipe Thread (NPT) is the standard for threaded pipe fittings in the United States. It’s widely used in applications requiring a secure, leak-proof seal. Unlike IPS, which focuses on pipe diameter, NPT is defined by its threading style — specifically, a tapered design that tightens as it’s screwed in, ensuring a tight fit that resists leaks under pressure.

This threading method has made NPT the go-to choice for plumbing systems that transport water, gas, and other pressurized fluids.

How NPT Tapered Threading Works

NPT pipes and fittings feature a gradual taper — typically at a 1.79° angle — so that when two NPT-threaded components are connected, the threads compress against one another, forming a tight seal. This tapering effect allows for a stronger connection than straight threads, reducing the risk of pressure drops, system inefficiencies, and connection separation due to vibration or thermal expansion.

However, unlike compression fittings or flanged connections, NPT threads alone are not sufficient to maintain consistent pressure in high-demand systems. Without proper sealing materials, minor pressure fluctuations can occur, leading to operational inefficiencies or even system disruptions over time.

Why NPT Requires Sealant

To virtually guarantee water pressure stability and long-term performance, NPT fittings require:

• Teflon Tape: Wrapped around male threads to fill microscopic gaps, reducing the chance of leaks.
• Pipe Dope (Thread Sealant Compound): A liquid or paste that hardens to form a tight seal, particularly useful for high-pressure systems.

Improperly sealed NPT connections can lead to pressure fluctuations and system inefficiencies. Inconsistent pressure can cause operational disruptions, increased energy costs, and potential safety hazards that strain your facility’s budget.

Where NPT is Used

Because of its ability to create a reliable, high-pressure seal, NPT is commonly found in:

• Gas Lines: These are used for natural gas and propane delivery systems, where even a minor leak poses a serious hazard.
• Water Distribution Systems: Ensures secure, leak-proof connections in commercial and industrial plumbing.
• Fire Sprinkler Systems: Relies on NPT fittings to maintain water pressure and prevent emergency failures.

For applications requiring a strong, pressure-resistant connection, NPT plumbing is a critical component. Selecting the correct pipe standard is key to ensuring long-term reliability, operational safety, and pressure stability.

Why Pressure and System Age Matter When Choosing IPS vs. NPT

Pressure regulation and system age are critical factors when choosing between IPS and NPT fittings.

• Pressure Issues (PRV Failure): A failed Pressure Relief Valve (PRV) can result in pressure surges or drops, leading to equipment damage, operational disruptions, and safety risks. If NPT fittings are over-tightened or IPS pipes aren’t rated for high-pressure applications, you risk catastrophic failure.
• System Age: Older plumbing systems may have outdated components prone to corrosion, scaling, or wear and tear. In these cases, NPT fittings provide a more secure seal for high-pressure applications, while IPS pipes offer better structural integrity for aging infrastructures.

Choosing the right pipe type isn’t just about diameter or threading — it’s about ensuring pressure stability and accommodating system age.

How IPS vs. NPT Impacts Commercial Plumbing Projects

Choosing between IPS and NPT fittings isn’t just a technical decision — it has direct financial and operational consequences for commercial facilities. If the wrong pipe type is used, facility managers may face installation delays, compliance risks, and long-term maintenance challenges.

Here are the practical implications of these differences and why they matter for facility operations:

1. Compatibility and Installation Risks

Plumbing systems are complex, and even minor compatibility issues can cause major delays. When facility managers approve system upgrades or emergency repairs, they need confidence that the installed materials will fit without modification or workarounds.

One of the most frequent causes of installation delays is mismatched pipe specifications. If a contractor mistakenly installs an IPS pipe in a system designed for NPT fittings, the issue may not be noticed until the final connection is made. At that point, the only options are to:

• Order new materials, which could take days or weeks depending on supply chain availability
• Find an adapter, which may not always meet regulatory or performance requirements
• Attempt a workaround, which increases the risk of leaks or compliance failures

For facilities where plumbing disruptions can halt operations — like manufacturing plants, healthcare buildings, or high-rise commercial properties — using the correct standard upfront avoids costly rework and downtime.

2. Maintenance and Retrofitting Challenges

Beyond installation, the long-term maintenance of a plumbing system depends on selecting the correct pipe type. Many existing commercial and industrial buildings have legacy systems that rely on NPT threading. If a replacement component is incorrectly specified as IPS, facility maintenance teams may experience:

• Misalignment issues, making routine replacements more difficult
• Leaks from improperly sealed joints, increasing repair frequency
• Incompatibility with older plumbing infrastructure, requiring more extensive retrofits

Making the right choice in commercial plumbing networks prevents unexpected maintenance costs, emergency repairs, and long-term system inefficiencies.

Mechanical rooms often have complex piping configurations with both IPS and NPT connections. Mechanical rooms house critical plumbing components, including PRVs, pressure tanks, and backflow preventers. If these components are paired with the wrong type of pipe or fitting, pressure imbalances, corrosion,  increased wear on fittings, and system failures can occur.

Regular inspections of your mechanical room and entire plumbing system can identify potential failures before they disrupt operations, ensuring that the right piping standards are maintained throughout the system.

3. Hidden Corrosion Risk in Mixed-Metal Systems

Electrolysis occurs when two different metals, like galvanized steel and brass, come into contact with each other in the presence of moisture, creating an electric current that accelerates corrosion. This chemical reaction is especially problematic in plumbing systems where metal components are constantly exposed to water, humidity, or condensation. Over time, electrolysis weakens the metal, leading to leaks, reduced water pressure, and even catastrophic pipe failures.

This issue is particularly relevant when mixing IPS and NPT fittings, as these two systems are often made from different metals.

For example, IPS pipes are frequently galvanized steel, while NPT fittings are commonly brass or copper. When connected without proper insulation, the dissimilar metals create a galvanic cell, triggering electrolysis.

In high-pressure systems, even minor corrosion can compromise the entire system’s integrity, causing pressure drops, pipe bursts, or equipment failures.

To prevent electrolysis, your plumbing system needs dielectric unions to separate dissimilar metals. These connectors are specifically designed to break the electrical continuity between different metals, eliminating the conditions that cause electrolysis.

The Financial Impact of Incorrect Pipe Selection

Plumbing failures aren’t just a contractor’s problem — they become a facility management crisis when they lead to unplanned expenses, tenant complaints, or compliance violations.

• Leaks contribute to 14% of treated water loss in municipal systems annually — a direct cost to building owners and municipalities.
• Improperly sealed connections in high-pressure systems (like HVAC water loops or gas lines) can cause gradual pressure loss, increasing operational costs.
• Unexpected retrofits due to incompatible fittings lead to higher labor costs and extended project timelines.

By selecting the proper pipe type, facility managers can prevent financial waste, extend the lifespan of plumbing systems, and avoid costly emergency repairs.

Facility managers don’t need to memorize IPS and NPT specifications, but they do need to recognize the operational risks of incorrect pipe selection. Whether overseeing system upgrades, approving maintenance work, or handling emergency repairs, verifying compatibility upfront helps prevent costly mistakes.

Can IPS and NPT Fittings Be Used Together?

Because IPS and NPT have fundamentally different threading styles, they are not directly interchangeable. As previously discussed, NPT fittings use tapered threads to create a secure seal. In contrast, when threaded, IPS fittings typically have straight threads that do not self-seal. This key difference means that attempting to connect them without an adapter can result in leaks, pressure drops, and long-term system failures.

This distinction is critical for facility managers overseeing plumbing upgrades or emergency repairs. If an IPS pipe is mistakenly installed where an NPT connection is required, the issue may not be apparent until the final assembly stage — leading to costly delays. At that point, the only solutions are to replace materials, attempt a risky workaround, or use an adapter.

When an IPS-to-NPT transition is necessary, properly rated adapters provide a secure and code-compliant connection. However, not all adapters play nice with high-pressure systems or regulated applications like gas lines. Selecting the right transition fitting for the job ensures performance reliability and prevents compliance issues.

Protect Your Facility from Costly Plumbing Failures with Lee Company

Water damage is one of the most expensive risks for commercial properties, with the average insurance claim costing around $24,000. Even a tiny miscalculation in pipe selection can lead to unexpected expenses, downtime, and long-term maintenance headaches.

Lee Company has been a trusted name in commercial plumbing since 1944, providing expert installations, preventive maintenance, and 24/7 emergency services across Tennessee, Alabama, Kentucky, and Georgia. Our licensed and insured technicians ensure that your facility’s plumbing system is built to last, minimizing downtime and unexpected costs.

Don’t leave your plumbing infrastructure to chance. Contact Lee Company today for expert guidance on IPS and NPT fittings, custom solutions, and reliable service tailored to your facility’s needs.