Hi, thanks for posting your question on JustAnswer...

Please let me know when you have joined the chat.

Great...

You'll need to run four wires total (hot, hot, neutral, ground). You'll need at least #10 conductors. With that, and a balanced 30A load (15A on each leg) you'll be within the recommended 5% Voltage drop range. But, if for example, one leg carries 18A, that leg will exceed the 5% voltage drop recommendation. So, on that basis, I'd recommend #8 conductors.

A couple of other notes you may not have considered:

1) Make sure your neutral and ground bars are separated in the sub-Panel (you usually have to purchase a ground bar kit as they don't typically come with sub-panels)
2) Make sure you drive two ground rods (at least 6' apart)outside the building and bond them to the ground bar in the panel with one continuous ground wire
3) Make sure your sub-panel is located nearest the point of entrance of the feeder cables. In other words, the feeder cables can't come into the building, travel through the building to the other side, then to the sub-panel.

(on that last note, the sub-panel needs to be be either a)outside the building or b)inside nearest the point of entrance of those feeder cables)

Personally, I always always use conduit. It's not much more $$, and it provides for a more flexible, reliable installation.

(conduit with THHN conductors, that is)

Not a problem. Glad to help. Happy New Year's, and take care!

Copper

Thank you!