This study investigates a multiple input buck dc-dc converter. Different operational modes of the converter for two-, three- and four-input sources are studied then, for each item the relation of critical inductance between continuous conduction mode and discontinuous conduction mode is calculated. Finally, a generalised relationship is proposed for calculation of critical inductance of n-input topology, which is true for any number of input sources. Experimental and simulation results in power systems computer-aided design/EMTDC are presented to validate the theoretical concepts and proposed generalised relation of converter critical inductance.