Ok, I’ve just built a detached workshop. I am going to have an electrician run a sub-panel to it since code will only allow me to run 1 circuit to a detached garage and the sub-panel is more than I want to deal with. I will do the internal wiring. I have three questions.
1. I plan to run two 120 volt, 20 amp circuits for two 75 watt double florescent lights and five outlets. Lights on one circuit and outlets on the other. I will run things like a fan, buffer, 12” drill press, and a band saw on the outlets. For this I plan to use #12/2 Romex.
2. I will run one 220 volt, 20 amp circuit for a metal grinder. The grinder has a 7.5 amp 220 volt single phase motor. For this I plan to use 10/3 Romex.
3. Third question. I am going to leave the inside un-finished (open studs). Is it ok to run Romex in the open studs?

 

question #1, good plan, question # 2, 12/3 is all you need. Question # 3 romex is ok to leave in an open wall.

 

#1. It's okay if you don't ever need to run two heavy tools simultaneously. Otherwise, you might want to split up the receptacles on two circuits. In fact, why not do it anyway. Once you have the subpanel, an extra circuit is only about $5. Don't skimp!

#2. 12/2 might be all you need. Check to see if your grinder needs a neutral. Probably not. If you run 12/3, you might be posting back here later asking what to do with the extra wire.