Only if you have more than 6 breakers.
I'm telling you, here they do not make you put one. I have run a #2 SER all the way across a crawl space to the other side of the house to then go underground another 60' to the garage and then terminate at a 100A ML panel.
I spliced the SER with mechanical lugs in a "J" box and then ran just 3 - #2 URD alum conductors UG to the garage and then grounded the service there.
This is not a seperately derived system and according to this particular county inspector, a main was not needed. Afterall, it is already protected for short and overload at its MB. So, if you have less than 6 breakers in that garage panel you are good (around these parts anyway).
:~)
