The process of virtual cathode formation in a gap is critically examined via particle simulations. It is found that the limiting current obtained from the electrostatic approximation is valid only in the deeply non-relativistic regime. When the injection energy reaches 30 keV, the transients in the injected current may produce an inductive voltage that can significantly lower the limiting current from the classical, electrostatic value. The self magnetic field is unimportant, however. At mildly relativistic energy (250 keV, the self magnetic field becomes very important in the determination of the limiting current. The inclusion of a strong external magnetic field along the mean flow reveals two quasi-steady states, whose presence is unique to the high voltage regime. Implications of this study on simulation codes are explored.