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.