Incorrect values of cable harmonic impedance can lead to wrong conclusions when assessing impact of current distortion on the power network. A theoretical derivation of resistance and reactance for harmonic frequencies is not sufficient as it neglects operating conditions of cables as well as their configuration and actual installation pattern. In this paper, a non-invasive procedure for estimating parameters of the low-voltage cable is proposed. On contrary to invasive methods, proposed technique does not require any external sources of harmonic excitation and consequently it does not introduce additional disturbances to the system. A disconnection of the cable is not required as the procedure meant to be performed online. The methodology employs natural variations of harmonic currents and voltages, however, the impedance angle is preserved due to the utilization of synchronized measurement samples. The developed signal processing algorithm provides noise reduction capabilities and smoothing of the output in presence of fluctuating harmonics. Additionally, an attention is given to the metrological characterization of the measurement system and evaluation of uncertainty of impedance values. The results of the laboratory experiment indicate that for selective harmonics the variations of impedance estimates are no more than 10 mΩ which is in the range of calculated uncertainty values. The practical applicability of the method is discussed together with its limitations.