Ductile Iron push-on and mechanical joints are covered in ANSI/AWWA C111/A21.11 "Rubber-Gasket Joints for Ductile-Iron Pressure Pipe and Fittings." Section 4.2.2 of that standard states: "The mechanical and push-on joints shall have the same pressure rating as the pipe or fitting of which they are a part." In other words, if the pipe is rated for 150 psi working pressure plus 100 psi surge (250 psi), so is the joint. If the pipe is rated for 350 psi working pressure plus 100 psi surge (450 psi), so is the joint.
This is not to say that Ductile Iron pipe and push-on and mechanical joints cannot be rated above 350 psi working pressure plus 100 psi surge (450 psi). Footnotes under Table 7 in ANSI/AWWA C151/A21.51 "Ductile-Iron Pipe, Centrifugally Cast, for Water" state: "Ductile Iron pipe for working pressures higher than 350 psi is available." There are numerous Ductile Iron pipelines operating at working pressures well in excess of 350 psi throughout the United States. Additionally, Ductile Iron’s push-on joints have been proven effective in actual tests and/or service with at least 1,000 psi internal pressure, 430 psi external pressure, and 14 psi negative air pressure with no leakage or infiltration.