A method for minimal losses in primary distribution systems by changing topology is presented. The loss minimization in distribution systems is based on Simulated Annealing (SA) and Radial Power Flow. SA's major advantage over other methods is an ability to avoid becoming trapped at local minima. The SA algorithm does not require or deduce derivative information, it merely needs to be supplied with an objective function for each trial solution it generates. Thus, the evaluation of the problem functions is essentially a `black box' operation as far as the optimization algorithm is concerned, which is the most convenient with large real systems as electrical networks. The model is useful for planning and operating in primary distribution systems. Through different test network applications, the method was validated and compared to other proposed methods.