This paper is the first of two and presents a planning methodology for low-voltage distribution networks. Combined optimization of transformers and associated networks is performed, considering the street layout which connects the different consumers. In this first part, micro-optimization, the planning zone is divided into small zones, mini-zones, which are optimized independently. A repetitive procedure is used in order to locate transformers using clustering techniques. Optimum capacity, customers to be satisfied, the optimum network to be used and losses associated to this network are determined for each location. The methodology is applied over an area of 12.9 km² with nearly 20 215 consumers. In the second paper, two macro-optimization methodologies are discussed based on the planning results for each mini-zone, one based on the Voronoi polygons in order to improve load grouping into mini-zones and the other based on the combination of neighboring networks into a single transformer by means of a Tabu search. Finally, the methodology is applied to a zone with a surface of 2118 km² and approximately 1300 000 customers.