A method of characterizing electrically heterogeneous electroceramics for a full three-dimensional collection of randomly shaped grains is presented. Finite element modeling, solving Maxwell's equations in space and time is used to simulate impedance spectroscopy (IS) data. This technique overcomes several deficiencies associated with previous methods used to simulate IS data and allows comprehensive treatment of a full three-dimensional granular representation of ceramic microstructure without the requirement for equivalent circuits based on the Brickwork layer model (BLM) or the introduction of constant phase elements to describe any nonideality of the IS response. This is applied to a full three-dimensional ceramic microstructure with varying grain size and electrical properties to generate IS plots that highlight limitations of the BLM in data analysis.