In this paper, a state-estimation-based method is proposed to determine harmonic current contributions of iron and steel (I&S) plants supplied from a point of common coupling (PCC). Due to highly nonlinear nature of the I&S plant operation, accurate estimation of harmonic contributions of the individual plants is essential. In the proposed method, the nonlinear characteristics of the plants and the utility are modeled as harmonic current sources and the harmonic current contributions of loads are decoupled from the upstream effects on the basis of the sample-by-sample time-synchronized field measurements of load currents and voltage signals. Being a field-measurement-based method, it is necessary to mitigate the measurement errors of conventional voltage transformers (CVTs) at harmonic frequencies. Gross error of voltage transformers is reduced through calibration, while weighted least squares (WLS) state estimation is used to reduce the Gaussian error of the current transformer and calibrated voltage transformer measurements. Using the estimated harmonic voltages and currents, harmonic contributions of all plants are determined by employing the Norton equivalent circuits of the plants together with the superposition theorem. 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 supplied from the PCC. The proposed method eliminates the need for the usage of resistive-capacitive voltage transformers (RCVTs) for accurate harmonic voltage measurements, which are expensive and cumbersome, while reducing the Gaussian measurement error of conventional voltage and current transformers at the same time; hence, it provides accurate harmonic contributions of the electric arc furnace (EAF) plants.