I am removing an existing overhead Three Wire system to a storage barn and putting the service underground. I found that the previous owner connected the overhead wiring to the Line Side of the Main house panel therefore no way to shut power off unless Meter is pulled. I would be installing a 60-Amp Two Pole Breaker to feed the Barn, the main panel in the House is a Three Wire service. The Barn is 200 feet from the House. I looked at some URD Cable, is this the best type to use for direct burial? Or do you have another recommendation? Can you recommend the correct wire gauge to use? Also it will be difficult to dig the Trench at the minimum 24" for direct burial due to numerous large tree roots. Please advise.

If you can trench, bury conduit. (PVC) It's a bit more work but well worth it to protect the wiring. #6 wire is rated for 60 amp, but at 200 feet you should use #4. And don't forget the ground.

If you can trench, bury conduit. (PVC) It's a bit more work but well worth it to protect the wiring. #6 wire is rated for 60 amp, but at 200 feet you should use #4. And don't forget the ground.

The OP stated the wire was also installed ahead of the main, I would suggest installing a sub-panel fed from the service and demolishing the original feed.