Modeling of spurious coupling between a circuit and its metallic enclosure in a circuit simulator is an important feature. Especially simulating circuits with nonlinear components will benefit greatly from this, because these components can typically not be simulated in the field simulator directly, but require artificial ports inside the calculation domain. It will be shown in this paper, that these ports introduce erroneous coupling results, if misplaced or misorientated. This is validated by separating the coupling effects from the normal circuit response by introducing an impedance coupling network into the circuit simulator. This makes modeling of electromagnetic coupling inside the circuit simulator possible and provides fast and accurate results, as will be shown by example of a cavity embedded oscillator.