I was wanting to know it it would be safe to replace a 30 amp double pole breaker with a 50 amp double pole breaker. The 30 amp currently feed a well pump and a barn. The barn and the pump piggy back off the 30 amp breaker. I want to abandon the barn feed and run 300 ft of 6/3 to my RV service panel and piggy back the pump off of that 50 amp double pole breaker. Can I do this?
What @Harper said. The problem we have have with a question like yours is that we could say "yes you could update it" but we really don't know is what kind of loads you are applying at the end of a circuit. We are guessing that it is greater than 30A because you want to upgrade. Also you give us a distance of 300' which is a substantial distance so voltage drop will surely come into play.
For example if your load is 40A at 240V which would be 9.2KVA then at a 3% Voltage Drop. Then you could run 333' with a #4. A #6 would only be good for 210'. If you load were 45A or 10.35KVA the distance with a #6 would be no longer than 186', A #4 would get you 296' so technically you would need a #3. Buy the way this is all in copper.
So you need to get distances and loads for the pump either in HP or KVA and voltage requirements and a demand load for the rv either in Amps or KVA and operating voltage. Then there could be a more complete and informed answer.