This paper presents a tool to automatically generate distribution network models for the power engineering software OpenDSS. The tool was developed in a free and open source Geographical Information System (GIS) software that allows a direct translation of GIS to OpenDSS files. The paper shows the potential of the developed piece of software and how it can help power engineers to carry out detailed studies of medium and low voltage networks. It also explains data requirements in the GIS database and all the steps followed to create the OpenDSS files. In order to demonstrate the capabilities of the model builder, the paper presents the translation of a real distribution circuit GIS model to OpenDSS.