I need to run electricity to three garages. I will run a 100 amp service to a box in one of the buildings and then run 3 30 amp circuits, one for each building.

I know I will need an electrician to do the final hook up and check my wiring, but I don't mind doing the grunt work myself.

What gauge wire do I need to use for the 100 amp circuit? Do I need a load box or will a lug box work?

I plan to use 10 -2 with ground for the 30 amp circuits. Will that work for them?

Since the structures are metal, if I use plastic switch boxes, can I attach them directly to the metal supports in the buildings?

I think our electrical expert will need the distances. Post back with them, and he can give you a good answer to all your questions. 10-2 will limit you to only 120 volts.

Depends on the run also.

Find a voltage drop calculator online. It will size the line for you factoring in run.

In my 30 AMP circuits I stepped up to 8 gauge since my run was 80 feet.

Everyone is right, need the distances.

Since the electrician is making final connections, he will be responsible, best to check with him if the plastic boxes can be used in this type construction in your area.

The 10-2 can be used for 240 volt circuits that need no neutral. Just color the white wire at both ends, or where exposed in a junction box.

depends on the run also.

Find a voltage drop calculator online. It will size the line for you factoring in run.

In my 30 AMP circuits I stepped up to 8 gauge since my run was 80 feet.

Even though #8 will work fine, what Calculator told you 8AWG. You can get less than a 3% voltage drop using 10AWG for your distance. Unless you used Alum?

I looked in my computer but apparently I didn't save it.

I don't remember the name of it, bu I found it using Google.