Ampacity for conductors should be obtained from the manufacturer - they'll typically site a particular NEC article/table.
Btw, welding cables are not listed for anything other than welding or temporary installations. They are certainly not listed for running in conduit. My understanding is that heat dissipation for these types of cables are different than typical permanent wiring conductors.
I'm dealing with this situation at the moment where the EC installed welding cables to connect UPS battery cabinets to separate UPS inverter cabinets and ran four current-carrying conductors in conduit. When I started asking Southwire how do you derate welding cables in conduit, they said you can't, since it's not meant for conduit applications. Let's see what the AHJ says.