In accordance with ASME standard B16.11 - “Forged Fittings, Socket-Welding and Threaded” this table shows the schedule of pipe corresponding to each class of fitting for rating purposes.
ASME B16.11 provides that the maximum allowable pressure of a fitting be computed in accordance with the applicable piping code or regulation for straight seamless pipe or for material of equivalent composition and mechanical properties to the fitting. Any corrosion or mechanical allowances and any reduction in allowable stress due to temperature or other service conditions must be applied to the pipe and fitting alike.