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Distribution system circuit topologies, connections, and equipment pose interesting challenges to system analysts. An approach based upon fundamental circuit analysis principles to calculating, fault currents of various types is presented. The basic approach consists of obtaining the prefault phase voltages and a phase Thevenin matrix at the fault location, and using the phase Thevenin matrix to solve the equations imposed by the boundary conditions of a given fault. The approach also obtains sensitivity matrices to be used for calculating post-fault voltages and currents at locations different from the fault location. The approach requests services from a multiphase power flow software component and a circuit model software component. The approach works for both radial and looped distribution systems. The analysis can evaluate fault currents at a node regardless of whether it is three-phase or nonthree-phase and grounded or ungrounded. Experience obtained from solving circuits with more than 5000 nodes will be reviewed.