Capacitance per unit length and related wave resistance refer to the main characteristics of a number of cable products, such as communication cables, control cables or radio-frequency cables. The capacitance per unit length value is regulated by the standards for appropriate types of cables. This study describes the technical implementation of the electrical method of measuring of the capacitance per unit length of the electric cable directly on the production line. It is shown that the change in specific electrical conductivity of water, which a capacitance transducer is dipped into, has a significant impact on the outcome of in-process control of cable capacitance per unit length. A method of offset from the impact of this measurement on the control results is suggested.