[LazyPup]

No problem Fred,

Copper pipe is measured by its inside diameter while copper tubing is measured by its outside diameter.

1/2" copper pipe is 1/2" ID but it is 5/8" outside diameter.

Normally compression fittings are only used on copper tubing so most compression fittings are rated by tubing sizes (OD), therefore if you desire a compression fitting to fit on 1/2" copper pipe (1/2"ID) you must use a 5/8" compression fitting (5/8" is the OD of a 1/2" copper pipe)

An angle stop or line stop valve is actually a transition fitting which joins the copper pipe in the walls to the copper tubing supply lines to your faucett or water closet tank. Quite often we see stop valves with the input listed as a "nominal" size.

The word "nominal" is indicating that the actual measured size is different than its rated size, which in this case it is rated to fit on a 1/2" copper pipe which is 1/2ID, but since the compression fitting must fit over the outer wall of the pipe, its actual measured size would be 5/8".

You will also see nominal sizing when examining PVC Pipe. PVC pipe is made in numerous grades or wall thicknesses to meet differing standards for temperature or pressure. While pipe is normally always sized by its inside diameter the must keep the outside wall diameter constant on PVC to interchange the fittings. In this case, in order to create a thick wall to meet higher standards the actual inside diameter will vary slightly. I.E. a 1/2" Schedule 40 PVC pipe will measure a true 1/2"ID, but a 1/2" schedule 80 PVC pipe will measure about 7/16ID to allow the thicher wall. The 1/2" schedule 80 pipe is then said to be 1/2" nominal, which means it is actually slightly undersized but it meets the trade standards for the volume and flow rate of a 1/2" pipe.