This is the closest I got to finding the section that requires a sub panel in detached garages when there are more than two 20a circuits. (or is it one 20a circuit?) I've heard about this requirement but can't find it in the '99 code. Does anybody know what section this would be in or is it just a California thing?
I don't believe that the NEC comes right out & states that a sub-panel is required. I also don't believe that the circuits are limited to 20 amps. You are limited to one circuit and it can be a multiwire circuit. 225-30
A disconnecting means is required (225-31) and it must be suitable for use as service entrance equipment 225-36, but there is an exception (of course) that applies to garages & outbuildings on residential property.
By applying all of the above & 225-33(b), you eventually come to the conclusion that a single multiwire circuit (could be 3 hots, I'm sure there are some individual houses that are served with 3 phase) is all that is allowed without going to a panel that would comply with 225-36.
I'm sure I've left something out, but for the life of me, can't figure out what it is. I'm sure others will jump in & cover any of my deficiencies.
Few things are harder to put up with than the annoyance of a good example.