Electromagnetic Models (EM) for analysis of grounding systems are based on least possible approximations in the solution of Maxwell's equations. The complexity of the EM calculations motivated the creation of more engineer-oriented and less time consuming methods for analysis of grounding systems. One of the most commonly used methods is the transmission line (TL) model. In this paper a new, improved formula for the inductivity parameter of the TL model is proposed. The new formula is implemented to calculate the grounding impedance of vertical and horizontal grounding conductors, as well as the transient potential at feed point when a subsequent lightning stroke is injected. Comparison of the obtained results to the classical TL model shows a significant improvement in precision with reference to rigorous EM calculations.