What is the minimum size of the hot and cold water supply lines to a bathtub?

What is the minimum size of the hot and cold water supply lines to a bathtub?

• A. 3/4 inch ID
• B. 1/2 inch OD
• C. 3/4 inch OD
• D. 1/2 inch ID (5/8" OD)

Answer: (D)

The size tested is about fixture supply sizing and how copper tubing is measured. For a bathtub, the minimum is 1/2 inch nominal tubing. In copper tubing, the number refers to the nominal inside diameter, not the exact bore or the outer dimension. A 1/2" nominal tube actually has an outer diameter of 5/8", so the correct description is 1/2" ID (5/8" OD). This size provides enough flow at typical residential pressures while matching standard fittings; smaller lines would restrict flow and larger ones aren’t the minimum required.

The size tested is about fixture supply sizing and how copper tubing is measured. For a bathtub, the minimum is 1/2 inch nominal tubing. In copper tubing, the number refers to the nominal inside diameter, not the exact bore or the outer dimension. A 1/2" nominal tube actually has an outer diameter of 5/8", so the correct description is 1/2" ID (5/8" OD). This size provides enough flow at typical residential pressures while matching standard fittings; smaller lines would restrict flow and larger ones aren’t the minimum required.