re: Wiring a shed for a 220v 170 amp welder

The shed should have had its own sub panel from the beginning if it has power currently. It is a separate structure and has a few requirements as such from the NEC (National Electric Code). One of which is you have to have a circuit disconnect at that building. If you already have a circuit run out there that say runs only a light, the light switch counts as a disconnect at the building. It is when you go adding other circuits that you fall into a little different area as far as code requirements on a separate structure even with all the residential exceptions. Seeing as you are wanting to add 220V out there, I don't see how you get around adding a sub panel to the shed.