yeah, i seem to remember the ampacity table 310-16 showing 30 amps for copper with cetain insulations if kept under 90 deg C, and in certain installations (e.g. open air, fewer then 3 current carrying cunductors bundled, etc.) anyway, we use the lowest 20 amp capability to be safe.
Since you are in Canada, is that drawing using Canadian codes. Most of the info you get as general info on this site will be based on the NEC. I, personally, have no idea about what is acceptable in Canada.
and as bricc stated, there are exception for certain uses that allow even a larger breaker than a 30 amp with #12 wire but those are special uses, not general uses.