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Iron Pipe Size Vs. Copper Tube Sizes: How They Differ

December 3, 2019 Viega
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If you trace the history of pipe dimensions, iron pipe size (IPS) is the first to pop up on the industry timeline. But with the rise of copper tubing over the years, another industry standard has emerged: copper tube sizes (CTS).

Commonly available in stainless steel, carbon steel and CPVC (commonly over 2"), IPS is prevalent in general piping and industrial applications. Meanwhile, CTS is commonly available in copper, CPVC (commonly under 2") and PEX and is specific to plumbing and potable water systems.

The dimensions of pipes typically made from certain materials are defined according to IPS, CTS and in some cases both.

Yet this only scratches the surface of what divides these two industry standards. Here’s a closer look at what makes each unique.

How Do IPS and CTS Differ?

The best place to start this conversation is with some history behind IPS. IPS used to be a standard for wrought iron pipe only. This was controlled by inner diameter due to the fact that back in the early 19th century, iron pipe was made by welding two halves of pipe together. Today’s methods include cold working the pipe and welding the rolled sheet, or the use of a mandrel to extrude seamless pipe.

The main difference between IPS and CTS is the actual outside diameter. It used to be that CTS was controlled by outside diameter (OD), and so the outside diameter is actually the same as the nominal diameter plus 1/8". The 1/8“ of inch came from the fact that it was the double standard thickness of copper tubing at the time (1/16” times 2), so a 2” pipe was actually always 2.125” in diameter. IPS was controlled by inside diameter (ID); however due to advances in technology and the production of piping, a new system had to be created to be able to match fittings and piping together. When nominal pipe size (NPS) essentially took over for IPS, it also became an OD-controlled value based on ASME B36.10., and thus the diameter differs from the nominal diameter (i.e., a 2” fitting actually has a 2.375” OD in IPS, and a ½” fitting has a 0.840” OD).

(We still use IPS rather than NPS to avoid potential confusion with National Pipe Straight, which is part of the ASME B1.20.)

For both CTS and IPS, there is another measurement that refers to the wall thickness of the pipe or tubing. In CTS, tube dimensions are specified by an exact OD and a tube wall thickness. (Again, due to advances in technology, we can now create differing thicknesses versus the standard ⅛” that used to be the only thickness available.) In IPS, pipe dimensions are defined by a nominal OD (which is different from the measured or actual OD) and a “schedule” that relates to pipe wall thickness.

Because CTS is OD-controlled, any change in the wall thickness of the tubing will cause a change to the internal diameter and alter the flow inside the pipe. In copper, the wall thickness of the tube is designated by the letters K (thickest), L (intermediate) and M (thinnest). For other types of pipe like PEX and CPVC, wall thickness is designated by the standard dimension ratio (SDR), which is defined as the ratio of the OD to wall thickness and has a typical range of 7.4 to 13.5.

With IPS, you have an extensive range of wall thickness options. This variance in wall size is specified as different schedules — a list that stretches from schedule 5 to 160. As the schedule number increases, the wall thickness increases and the ID decreases, thus enhancing the ability of the pipe to handle greater pressures. Regardless of the wall thickness, the nominal OD (and the measured OD) won’t change.

Whether you are working with IPS for industrial applications or CTS for potable water systems, Viega offers press fittings to meet your needs. In a matter of seconds, you’ll be able to make secure, consistent connections that ensure system success. To learn more about the products and services we offer, contact a Viega sales representative today.

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