Depends on what that unit has for load calculation.A single family dwelling with a 350 amp service has an total load of 183 amps, 43880 VA. A in law apartment is being added to the existing structure. My question is what size sub panel will be excepted by the NEC?
Zoning laws and whether such unit is allowed is one thing, NEC definition of a dwelling unit is another.If not a separate legal "dwelling," which would have its own service (?), I don't think there is a required minimum. Just do the load calculation and provide what you need/want.
Assuming you want the panel to be accepted, and not excepted...![]()