CTS and IPS are two outside-diameter sizing systems. CTS polyethylene tubing matches copper tube's outside diameter; IPS pipe matches iron pipe's. A 3/4-inch CTS tube measures 0.875 inch across and a 3/4-inch IPS pipe 1.050, so their fittings don't swap. Match the fitting to the system, not to the nominal size.
What do CTS and IPS actually mean?
The AWWA C901 standard defines both. Its scope reads: "Polyethylene pipe ranges in nominal size from 3/4 in. (19 mm) through 3 in. (76 mm) and conforms to the outside-diameter (OD) dimensions of iron pipe size (IPS) or to the inside-diameter (ID) dimensions of IPS pipe. Polyethylene tubing ranges in size from 3/4 in. (19 mm) through 2 in. (51 mm) and conforms to the OD dimensions of copper tube size (CTS)."
So in this standard the IPS product is called pipe and the CTS product is called tubing. JM Eagle's data sheet sorts them the same way: IPS pipe is made to ASTM D3035, and CTS pipe to ASTM D2737. Both lines also list AWWA C901.
The Plastics Pipe Institute adds a third family. Its municipal water manual says polyethylene can be made to controlled outside diameters or to controlled inside diameters. The inside-diameter product, called SIDR and made to ASTM D2239, goes with barbed insert fittings and clamps. The manual says the outside-diameter systems "are for use with fusion or with fittings that require a predictable outside diameter." IPS and CTS are two of those systems.
How different are the sizes?
The nominal size doesn't tell you the outside diameter. These numbers come from JM Eagle's data sheet for SDR 9 pipe:
| Nominal size | CTS outside diameter | IPS outside diameter |
|---|---|---|
| 3/4 inch | 0.875 in. | 1.050 in. |
| 1 inch | 1.125 in. | 1.315 in. |
| 1-1/4 inch | 1.375 in. | 1.660 in. |
| 1-1/2 inch | 1.625 in. | 1.900 in. |
| 2 inch | 2.125 in. | 2.375 in. |
IPS pipe is wider every time, by 0.175 to 0.285 inch in this table. The bore differs too. At 3/4 inch, JM Eagle lists an average inside diameter of 0.669 inch for CTS and 0.802 inch for IPS. IPS runs to 3 inches in that sheet, while CTS stops at 2.
For pressure, the same sheet rates both lines at 250 psi at SDR 9. The PPI manual says polyethylene water service lines carry rated pressures of 80 to 250 psi, depending on the pipe's dimension ratio. Check the rating printed for your exact product rather than assuming one.
Why don't the fittings swap?
Compression and stab fittings clamp the outside of the pipe, so they are made for one outside diameter. The table shows why a mix-up fails. A 3/4-inch IPS pipe is 0.175 inch wider than 3/4-inch CTS tubing. It won't go into a CTS coupling, and a CTS tube would sit loose in an IPS one. (That is arithmetic from the table, not a line from a manual, so read the fitting's own label.)
The PPI manual sorts the joining methods by sizing system. Heat fusion, electrofusion, outside-diameter compression fittings and stab fittings are listed for both IPS and CTS pipe. Insert barb fittings and clamps are listed for the SIDR pipe. The manual also says to make sure mechanical fittings are designed for use with polyethylene pipe, because PE pipe can contribute to leaking joints when it is used with fittings designed for other materials. On 1/2- to 2-inch pipe, it recommends internal stiffeners in stab and mechanical fittings.
What does the Phoenix code say?
Phoenix uses the 2024 International Plumbing Code, adopted by Ordinance G-7397 effective August 1, 2025. The city's amendment list doesn't include Section 605.3 or 605.4, so the model text applies.
Table 605.3 lists polyethylene pipe (ASTM D2239, ASTM D3035, AWWA C901) and polyethylene tubing (ASTM D2737, AWWA C901) as water service pipe. In other words, both the IPS pipe and the CTS tubing are on the list. Section 605.3 adds that service pipe "shall conform to NSF 61," and that pipe or tubing installed underground and outside the structure "shall have a working pressure rating of not less than 160 psi (1100 kPa) at 73.4°F (23°C)."
Table 605.4, the one for water distribution pipe inside a building, lists PEX, PE-AL-PE and PE-RT, but not plain polyethylene pipe or tubing. Section 605.3 says service materials not third-party certified for water distribution "shall terminate at or before the full-open valve located at the entrance to the structure."
On the city's side of the meter, Phoenix's Design Standards Manual says "Service lines for meters less than 3 inches shall be Type K copper tubing." That covers the line from the main to the meter. Your side is the line from the meter to the house. I did not find a Phoenix rule on PE for irrigation, and the code table above covers potable water service only. One Phoenix amendment matters for polyethylene: Ordinance G-7397 adds Section 605.25, which says non-metallic potable water pipe, fittings and valves "shall not be exposed in exterior outdoor locations." So PE service pipe stays buried, and any riser or stub-up that comes out of the ground has to be metal. If a pipe's job is irrigation, ask the city or your irrigation supplier which product applies.
What should you do before buying fittings?
- 1Find the sizing system. Check the maker's data sheet or the paperwork that came with the pipe for "CTS," "IPS" or "SIDR."
- 2Measure the outside diameter. Compare it to the table. 0.875 inch is 3/4-inch CTS. 1.050 is 3/4-inch IPS.
- 3Buy fittings built for that system. Look for CTS or IPS on the fitting, plus a stiffener sized for the pipe.
- 4Keep repair parts matched. Mixing systems at a coupling is where leaks start.
- 5Check what's underground. If you aren't sure what a yard line is, how to find where water pipes run and what to do about a broken yard water line are good places to start.
Related reading: how Arizona water systems inventory service line materials, copper tube types K, L and M (the tube CTS is sized to), PEX versus CPVC and push-connect fittings.
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