OKAPPI, a numerical model for calculating cathodic protection (CP) problems involving underground pipelines, was extended to situations involving stray currents. Aboveground structures such as rail bars, power stations, electric train or tram vehicles, and overhead lifelines were integrated in the existing model in a straightforward and transparent way. As a result, it was shown that the model can be used to deal with most of the CP problems encountered with underground pipes. Situations where the sources of stray current, such as other CP installations or earthing systems located in the soil, were modeled and compared with theory. Cases involving the four standard kinds of CP interference (anodic, cathodic, combined, and induced) were calculated and discussed. A few complex situations involving overhead systems such as trams or railways were investigated. The theoretically expected phenomena were confirmed by the calculations.