I suppose by strict interpretation the detached garage is not a dwelling unit but is very likely fed from a dwelling unit. so by extension if the circuits are fed from a dwelling unit and the circuits must comply with those rules regardless of the sub panel. OK that is 1 side of my thought process.
What about the garage requires a 20 amp lighting branch circuit? What about a barn fed from a dwelling unit?
Where are each of the outbuildings fed from? What activities are going on? A commercial garage on a residential/ commercial property.
I think it is likely that unless the garage has a special need for a 20 amp lighting branch and it is on a residential property fed from a dwelling unit I might not approve the use of 20 amp branch circuits. If the building was actually large enough to need 20 amp lighting circuits I would consider it but those buildings are almost non existent in this city so it might be moot here.
The other thing I am curious about is the rational for limiting the branch circuit capacity. In commercial applications there are lots of problems with the large number of luminaires that can be connected to a 347 volt lighting branch. The problem is revealed every time the electrician wants to turn off the power to change a ballast he must turn 20 to 30 fixtures off which usually leaves too many people in the dark including the sparky changing the ballast.
I agree that a detached garage is not a dwelling unit but some electrical code rules and building code rules do make a garage on a residential lot part of the wiring requirements for dwelling units like 26-714 requires an outlet and branch circuit for a garage that is part of a single dwelling unit. Attached or detached it is still considered as part of a dwelling unit. By extension then, a detached garage could be considered as part of a dwelling unit.
