Harmonic emission limits of customer installations are based on a pre-defined share of the compatibility level between low, medium and high voltage network. The efficient utilisation of the whole network regarding harmonics depends significantly on certain assumptions including the distribution of customer installations to the different voltage levels. This paper studies the impact of different distributions of customer installations to low voltage (LV) and medium voltage (MV) network on the utilisation of the MV network regarding harmonics by a probabilistic simulation. Realistic distributions of customer installations and network configuration are obtained from a survey recently conducted among distribution network operators in Austria, Czech Republic, Germany and Switzerland, which is also described in this paper. The simulation shows that the ratio between customers directly connected to the MV grid and customers connected to the downstream LV grids have a strong influence on the harmonic utilisation level. The results suggest that a certain reallocation of the share of compatibility levels from MV network to LV network can improve the harmonic utilisation of networks. This can serve as input for the revision of related standards and rules for calculating harmonic emission limits for customer installations.