The point dipole (PD) model is routinely used for estimating the dielectrophoretic (DEP) force acting on a particle placed in the nonuniform electric fields of dielectrophoresis devices, such as square cages. We show that if the particle size is much smaller than the dielectrophoretic cage size, the PD model accurately approximates the actual DEP force, computed numerically using the Maxwell stress tensor method. However, when the two sizes are comparable, the actual DEP force differs significantly in both magnitude and direction from that given by the PD model.