According to Houston Wire & Cable Company, National Electric Code Tables 310.15 B6 and 310.16 specify that 2/0 gauge copper wire can be used for service or feeder connections to a 200-amp panel. The source lists 4/0 gauge aluminum wire as another acceptable option.
It is important to check with local building code authorities, as there may be additional requirements in certain areas. Barring the service wire installed by a utility company, contractors are responsible for all wiring that extends from the service panel to the associated outlets or connections. Failure to conform to all relevant building codes can result in denial of occupancy, as well as dangerous conditions that could result in electrocution, fire or death.