The paper presents the results of a numerical investigation of a fast ionization wave in a non-preionized gas as the initial stage of a nanosecond capillary discharge. The wave was created in a 5 cm long narrow capillary filled with nitrogen at a pressure p=2 Torr by applying a nanosecond voltage pulse of negative polarity, which was supplied by an electric circuit consisting of a preliminary charged capacitor, a thyratron switch and a cable. Propagation of the wavefront along the capillary and formation of a conducting plasma channel were simulated using the fluid approach to description of processes in low-temperature plasma. Including electrical circuit into consideration allowed obtaining realistic voltage pulse shapes as well as current rise-rates in the system immediately after the ionization wave has reached the grounded electrode. The latter was used as a parameter indicating the efficiency of the consequent initiation of a capillary discharge. Obtained dynamics of wave propagation and structure of the wavefront are discussed. Influence of dielectric permittivity of the capillary material on the wave properties in general and on the capillary discharge initiation is analyzed.