Sorry. Your on the right track with a two pole 20 or 30 amp off the bottom of your main then from the two pole breaker in your main run to your shed . Stick with an 8/3 for the welder and such . How far away is it from the main ? Every 100 ft you will have a voltage drop. You can get some db (direct burry) wire and add ya a hot tub breaker box on your shed as a disconnect at the shed and there pretty cheap at lows
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