I'm running 100' away from 150A main Service panel to 100A sub panel, my question is: what wire gage shoul I use? Thanks
First, don't be pennywise about the size of the panel. You want a lot of spaces. Spaces go a lot faster than you think, especially with 240V equipment. I like to finish an installation with more than 50% of the breaker spaces unused. I mention that because 100A panels are usually quite small.
#1 aluminum will suffice, 100' is just shy of the 115' where you start thinking about a size bump for distance. But if you're considering that...
One option is to go clear up to 150A, or 3/0 aluminum. Why would you do that? It makes the subpanel feed breaker in the main panel unnecessary. The existing 150A breaker protects the wire and the 150A+ rated subpanel. So you can just use thru-lugs or a subfeed lug kit (a breaker that's just lugs).
Generally commercial work requires conduit - if you use metal conduit, that serves as a ground wire, and you don't need one.