By solving the two-dimensional Poisson equation for fibers with circular internal electrodes, we show that the parallelplate approximation incurs significant error in the determination of the electric field applied to the fiber core. This also affects the value of χ(3) measured in electro-optical fibers. The exact solution of Poisson's equation is used to solve the field applied to the core of a photonic crystal fiber, where the many holes screen the field. A universal curve is derived to allow using the simplifying parallel plate approximation and correcting for the error in symmetric fibers with round holes.