Abstract Impulse coefficient is calculated, for the first time, by considering the response of the current injection electrode connected to ground grids for lightning subsequent return strokes. It is observed that the impulse coefficient can be substantially amplified specially for low resistivity soils (e.g. forest grounds) and the enlargement is a function of the length of the injection electrode. Several rigorous finite-difference time-domain (FDTD)-based simulations have been performed and a new semi-empirical formula for impulse coefficient, which takes into account the injection electrode's length, is provided. The proposed expression is verified experimentally.