This is the first of a two-paper series, in which a field-data-based analytical method is proposed to determine the harmonic current contributions of each Iron and Steel (I&S) Plant supplied from the Point of Common Coupling (PCC) and the upstream loads at the utility side. The non-linear characteristics of the plants and the utility are modeled as harmonic current sources, whereas the linear parts of the proposed model consist of passive circuit elements. In the proposed method, the harmonic current contributions of loads are decoupled from the upstream effects and they are determined through the harmonic Norton's equivalent circuit model of the system constructed on the basis of the sample by sample, time-synchronized field measurements of load current and PCC voltage signals. This method can be applied to circuit schemes with any number of plants supplied from a given PCC. The results have shown that the proposed method can be used as a successful estimation tool to determine the harmonic current contributions of all I&S Plants connected to the PCC.