Technically you can weld 10" thick material but it will need multiple passes and preheat as required on most welds on thicker materials.

I work around a lot of pipe welders and they do not need 200A to weld a 10"+ Schedule 160 pipe because the the weld will take multiple passes. Much of this is done with a GTAW root pass and hot passes with 1/8 inch 7018 stick requiring about 120A.

How thick of material do you need to weld?