Large PV plants vary in construction details. The aim of this thesis is to show the different impacts of well-known influence factors especially relevant to large PV parks with more than 10 MWp installed capacity. This thesis is based on the results presented at PVSEC 2014 on the varying relevance of influence factors on performance and will give full particulars on the effects of different string cabling. A comparison is made of two large (20 MWp) PV plants with very similar technical structures and a distance of less than 10 km distance between the locations and monitoring values were access over a period of more than two years. The main methodology is the comparison of the parks and the correction of the power production caused by local weather differences. Any additional differences and irregularities of other impacts than those caused by cabling are corrected and an estimation of the impact is shown. An optimized park design reveals that design cabling in single row strings leads to ~ 460 000 kWh less losses during one season in the 31.5 MWp park.