In order to prevent voltage deviation and voltage fluctuation beyond its upper limit caused by the installation of large amount of distributed photovoltaic (PV) generators in distribution grids, the calculation models of PV generators are established to analyze the voltage deviation and voltage fluctuation. An index of the extreme capacity limitation of distributed PV generators interconnected into the idle feeder without causing the problem of voltage deviation and voltage fluctuation is defined. The extreme capacity limitations are deduced for six PV distribution patterns. The typical urban and rural distribution grids are used as examples to demonstrate the extreme capacity limitations in various cases. The results show that proposed index is rather conservative and neither problems of voltage deviation or voltage fluctuation could be avoided once the index was met.