This paper addresses the topic of automatically computing cable layout designs of large scale wind farms. A network of cables in a wind farm's electrical collector system collects power generated by turbines and brings to the wind farm substation. Frequently, sections of the land area of a large wind farm are restricted for excavating and burying these cables, i.e. trenching. Such restrictions might arise from the landowners, presence of water bodies etc. It is important to take into consideration these real-life constraints in the process of automating designs of optimal wind farm electrical collector systems. This paper presents a graph-theory based methodology for addressing these trenching constraints in optimal collector system designs. The developed methodology has been tested on a real-life large wind farm.