Why isn't PEX the same size as copper?

[gadolphus32]

Is there a reason why the IDs for PEX tubing are somewhat smaller than for copper? In other words, why is the ID of (for example) a 1/2-inch PEX tube smaller than for 1/2-inch copper?

Do they make PEX smaller on purpose? Is there an advantage to it being smaller? I have always thought of it as an inconvenience because it creates problems in certain situations (for example, when plumbing a tub spout, you can't use PEX because it's too narrow).

I've just always been curious if there is a reason for the PEX's smaller IDs (and, by extension, lower GPM rates).

 

[Reach4]

1. The OD of PEX is normally the same as copper, which is 1/8 more than the nominal size. For example, "1/2 inch" copper or PEX is 5/8 OD.
2. PEX needs thicker walls than copper.

 

[Sponsor]

 

[gadolphus32]

1. The OD of PEX is normally the same as copper, which is 1/8 more than the nominal size. For example, "1/2 inch" copper or PEX is 5/8 OD.
2. PEX needs thicker walls than copper.

Needing thicker walls makes sense. But in that case why not increase the OD of PEX in order to match the ID of copper? OD doesn't matter in most situations but ID does because it impacts waterflow. Right?

 

[Reach4]

I am not sure what all went into the thinking. One advantage is that the same Sharkbite connector that can connect to copper can connect to CPVC or PEX because the OD is the same. The same compression fitting can fit, although an insert and I think a different ferrule is needed.

You can upsize the PEX if needed.

There are 1/2 inch brass threaded fittings that attach to 3/4 PEX. This includes a drop ear elbow that connects to 3/4 PEX and accepts a 1/2 inch nipple.

 

[Imintrouble]

From what I know and have read, uprising is recommended in certain situations such as the main feed for an irrigation system specifically when referring to the different internal diameter of PEX vs copper.

 

[Kreemoweet]

Needing thicker walls makes sense. But in that case why not increase the OD of Pex in order to match the ID of copper? OD doesn't matter in most situations but ID does because it impacts waterflow. Right?

Wrong. There is no definite "ID of copper", as the ID varies with the grade of copper tubing. It is the OD that needs to be
standardized, to ensure interchangeability of tube/pipe/fittings from different manufacturers. The ID is just one of many
factors that affects flow characteristics, and is rarely a prime consideration, assuming it is "adequate".

 

[gadolphus32]

Wrong. There is no definite "ID of copper", as the ID varies with the grade of copper tubing. It is the OD that needs to be
standardized, to ensure interchangeability of tube/pipe/fittings from different manufacturers. The ID is just one of many
factors that affects flow characteristics, and is rarely a prime consideration, assuming it is "adequate".

The OD could still be standardized between manufacturers. Standard sizing between different manufacturers is a separate issue from matching the measurements of one type of pipe to those of another.

I do get the value of having a consistent OD between PEX and copper so that you can use the same compression fittings. But the usefulness there seems to be limited. Sharkbite-type fittings are the only kind that are truly interchangeable between PEX and copper. As noted above you would need different ferrules if you want to join PEX to copper with standard compression fittings, so those fittings are not completely interchangeable. And as far as I know there are no other types of pipe that have the same OD as PEX and copper. Galvanized, iron, PVC etc. are different.

Anyway, this was just an "I wonder why" question. It seems like there is no official answer.

 

[Jadnashua]

The same exact compression fitting will work on pex as copper IF you add the reinforcing insert sleeve to prevent the tubing from collapsing.

Unless you try to run pex like copper with fittings to change directions, the smooth, continuous pex and higher flow rates can approach that of copper. But, distance will always be a factor, reducing the maximum flow, regardless of what you choose.

While pex manufacturers typically call for their tubing to only be used with their fittings, in fact, (assuming the same type of pex), things are interchangeable, in the same manner as fitting on copper. The three common 'flavors' of copper used in residential and commercial applications are K, L, and M...the ID is different on all of them, but the OD stays the same so it will fit into any fitting, valve, etc. The bursting pressure of copper greatly exceeds that of pex, as does their temperature ratings. They each have advantages.

Like most things, actually calculating the required flow and acceptable pressure losses will make a successful installation. Both the Copper Institute and each of the PEX manufacturers have design manuals to aid those that actually want to do it 'right'. That might mean using larger PEX tubing than the equivalent copper, but in many of today's flow restricted valves, 1/2" copper's capacity exceeds what's needed, and 1/2" pex's will work just fine. That's not always the case. A common issue is with a tub/shower...pex may not have the capacity and performance unless it is upsized.

 

[hj]

REGARDLESS of what you think, it is too late. There is no way the world is going to convert to your idea, even if it made sense to do so.