Plumbing Size Fun

I was brain-dead on “Thursday”: My neighbour helpfully pointed out that I can replace the cracked copper DWV piping with ABS; they make ABS < -> copper fittings for this purpose. No plumber required; even _I_ can work with ABS :-)

So I need to run out to the store and get a bunch of ABS piping, and adapters. However, I need to know what size to get. Applying my superior mathematics skills, I measure the outside diameter of the pipe at slightly less that 10″. Divide by pi, and the pipe diameter is slightly less than 3.18″. Now, I _know_ that copper pipe is made in 3″ and 3.5″ sizes, not 3.18″ :-). This could be an inside diameter vs. outside diameter thing, but the number is still strange. Still, it’s probably safe to assume 3″, right?

Fortunately, there is google. The third hit for “copper pipe diameter”: is “Notes on Pipe”:, which explains that:

bq. Oddly, there is nothing about a ½” pipe that is ½”, be it copper, iron, or PVC

bq. Well, it seems that back in the beginning of time – copper pipe was introduced in the 1930’s – copper pipe was indeed standardized at the nominal diameter inside with a 1/16″ wall, making it 1/8″ bigger on the outside. As the metallurgy improved, allowing manufacturers to use thinner metal (and thus increase profits), they increased the inner diameter rather than decrease the outer diameter simply to allow the pipe fit existing fittings.

bq. As the manufacturers began to make bigger and bigger pipe, they found that 1/16″ wall thickness was insufficient. Thus, the inner diameter of larger pipes is smaller than the nominal size, while smaller pipes are larger.

And, usefully, a chart of nominal sizes with actual measurements, telling me that my assumption is correct; my 3.18″ pipe is, in fact, a 3″ copper pipe!

Next week, we learn why 2″x4″ lumber is neither 2″ nor 4″ :-)

posted at 12:20 pm on Sunday, July 04, 2004 in Personal | Comments Off on Plumbing Size Fun

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