Value of S is the allowable design strength for continuous long-term service of the tube, as determined by the ASME B31 Boiler and Pressure Vessel Code, Section 1-Materials. Allowable stress value depends on the service temperature and on the temper of the tube, hard drawn or annealed.

Allowable stresses for annealed and drawn temper copper tube are only a small fraction of copper’s ultimate tensile. In system design, joint ratings must also be considered, as the lower of the two ratings (tube or joint) will govern the installation. The rated joint strength in soldered tube systems often governs design. However, annealed ratings must be used in brazed systems since the brazing operation may anneal the tube near the joints.