225.33(A) General. The disconnecting means for each supply permitted by 225.30 shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be no more than six disconnects per supply grouped in any one location.
#93777 - 06/15/0512:29 AMRe: main breaker for subpanel
For a separate building, such as a detached garage, you are required to have a disconnecting means on the building. In this area, I typically have a 'safety switch' mounted on the outside, feeding the panel directly behind it (on the inside). A 'main breaker' in the panel is another way to do it- but is usually more expensive- and our AHJ likes the disconnect on the outside anyway.
There is one exception- no such disconnect is necessary if you're running only one circuit- and "circuit" is vague enough to include a single multi-wire branch circuit (or what we normally think of as two circuits). Assuming that you are using both full-size breaker spaces for a pair of full size breakers, you might be able to argue that you have but a single multi-wire circuit, with the breakers simpling being an additional, not required, disconnecting means.
#93778 - 06/15/0502:35 PMRe: main breaker for subpanel
IMO (and that of the Handbook) 225 section II effectively says to treat each separate building like it has it's own service entrance. Except when only one circuit (specifically allows multi-wire) is in the separate building.
225.30 only one source can feed the separate building (230.2, 230.40) 225.31 all ungrounded conductors must be disconnected (230.70) 225.32 disconnect where power enters building (230.71(A)) 225.33 number of mains (230.71) 225.34 group the mains (230.72) 225.35 accessible to occupants (230.70(A)(1), 230.72(C)) 225.36 suitable for service entrance(230.70(C))
#93779 - 06/15/0503:39 PMRe: main breaker for subpanel