Which copper tubing type has the thickest wall and is rated for the highest pressures and underground applications, and is green?

• A. Type M
• B. Type L
• C. Soft & Rigid
• D. Type K

Answer: (D)

The main idea is that wall thickness controls how much internal pressure the tube can safely carry and how well it stands up to external forces. Among the standard copper tube types, the thickest wall—and thus the highest pressure rating—is Type K. Its extra thickness makes it stronger and more resistant to deformation and damage, which is why it’s chosen for underground and other demanding installations. The green color is a code used to identify Type K tubing. Type L has a thinner wall and is common for indoor use, while Type M has the thinnest wall and lower pressure ratings. Soft & rigid describes the tube’s form (annealed versus drawn) rather than its thickness or pressure rating, so it’s not the designation for underground/high-pressure service.