In the microcrystalline regime, the behavior of grain boundary-controlled electroceramics is well described by the “brick layer model” (BLM). In the nanocrystalline regime, however, grain boundary layers can represent a significant volume fraction of the overall microstructure and simple layer models are no longer valid. This work describes the development of a pixel-based finite-difference approach to treat a “nested cube model” (NCM), which more accurately calculates the current distribution in polycrystalline ceramics when grain core and grain boundary dimensions become comparable. Furthermore, the NCM approaches layer model behavior as the volume fraction of grain cores approaches unity (thin boundary layers) and it matches standard effective medium treatments as the volume fraction of grain cores approaches zero. Therefore, the NCM can model electroceramic behavior at all grain sizes, from nanoscale to microscale. It can also be modified to handle multi-layer grain boundaries and property gradient effects (e.g., due to space charge regions).