Many distribution grid management functions rely upon power flow algorithms to analyse the behaviour of the grid under specific conditions. The knowledge of the grid parameters is usually the starting point for the definition of the models behind power flow algorithms. However, in the distribution system scenario, several factors can affect the level of confidence with which these parameters are known. This paper aims at investigating the main sources of uncertainty in the modelling of distribution lines, taking into account the specific characteristics of the distribution system, and it studies the impact of those uncertainties on the power flow results. The goal is to identify which factors can potentially bring severe degradations of the power flow results, so that the distribution grid model can be improved accordingly or their impact can be duly considered when evaluating the results.