Dealing with realistic networks, the solution of lightning electromagnetic field coupling to transmission lines is generally obtained through a Finite-Difference Time-Domain (FDTD) algorithm interfaced with an electromagnetic simulator. As well known, the stability of the FDTD scheme is determined by the Courant stability condition. In this contribution, it is shown that a second stability condition that limits the ratio between the space step and the line length appears when spatial extrapolation is used to obtain the values of the currents at the line extremities to be interfaced with a transient electromagnetic simulator. Finally, this work aims at solving the aforementioned problem by using a different approach to obtain the currents at the line extremities.