Absolute Roughness of Drawn Tubing and Plastic Pipe

Drawn copper, PVC, PE and glass are hydraulically smooth for most practical Reynolds numbers, with ε near 0.0015 mm. In the smooth regime the friction factor stops depending on roughness at all and follows the Prandtl or Blasius relation on Reynolds number alone, which is why the Moody chart's curves converge at the bottom left.

Do not over-claim the benefit. Because friction factor varies with the logarithm of ε/D, going from steel to copper at the same diameter typically saves only 10–20 % of straight-run friction loss, and in a real system fittings, valves and the coil or exchanger usually dominate anyway. The genuinely useful implication is different: smooth pipes hold their performance, while steel degrades with age. Also note that plastic pipe of the same nominal size often has a different inside diameter than steel (CPVC and PEX in particular have notably smaller bores than copper of the same nominal size) and that diameter change swamps the roughness advantage entirely.