I assume 100A, so #8cu.

I would like to thank all of the responders for their valuable advice which I intend to take. To answer darrelectric's question, the run is about 165 ft.

I would also add some history to my problem. I originally had a 4 wire system using #4 AWG run in 2" conduit to this out building which serves as a workshop. This system was fed by a 50 amp breaker from a 100 amp sub panel. During our latest power outage I back fed the house using a 7 Kw generator thru the workshop panel. Some time after the power was restored,one phase failed. I can not pull the old lines out of the conduit so I intend to run new lines and conduit using heavier #3 wire so that I can run a 13 Kw generator in the future.
The workshop houses 3 phase machine tools run from Phaseamatics. I also have industrial strength tig and mig welders so I could use the extra amperage.