The thickness of the tube is determined by the "Type". There are 3 "types" commonly used, Type M, L, and K. Type M is used for interior plumbing purposes, above ground sanitary lines and non-pressure applications. Type L is the most popular type for general plumbing purposes and for low pressure steam heating and condensate return lines. Type K tubing (heaviest wall) is most frequently employed for service lines but can also be used for general plumbing and heating.
Copper tube is minimum of 99.9 percent pure copper. The copper customarily used for tube supplied to these specifications is deoxidized with phosphorus and referred to as UNS C12200.