Add a breaker

I'd like to add a breaker for my garage. Lots of tools may be operating at the same time thus lots of power's needed.

I live in canada so I need 120V 60AC. I was thinking about a 30A breaker.

I need to know where to place the newcomer.

I have to breaker boxes, a big (the main one plugged to the line), a small plugged to the big.

Breakers from the big box are big and the clothes dryer's plugged into it. The other has smaller breaker. Does this mean the big box is under 240V and the small one is 120V?