In practice, sometimes long grounding wires are applied to decrease the grounding resistance but the lightning performance of the grounding system is still poor because the grounding wires have an effective length. This paper presents an analysis of effective length for horizontal/vertical grounding wires using the minimize per-unit length impulse impedance method. The grounding response due to lightning current was simulated by solving Telegrapher's equations with the finite-difference time-domain (FDTD) technique. During the simulation, the voltage distribution was low enough to limit the electric field intensity not exceeding the critical value for soil ionization, E_c. The results of the simulation were proved with scaling experiments. The simulation analysis showed that the effective length of the horizontal/vertical grounding wires decreases with front time of impulse current. For example, in soil resistivity of 50Ω.m, the effective lengths of the vertical grounding wire are 29m and 5.3m for impulse currents of 10/350μs and 0.25/100μs, respectively.