The electrical double layer (EDL) profiles of electrical potential within charged slit-type pores is calculated in this work via a combination of grand canonical Monte Carlo (GCMG) simulations and electrodynamics concepts. The electrical potential distribution inside the pore is calculated with respect to field-free virtual bulk electrolyte solution implicit in the grand canonical ensemble. The GCMC simulations for slit-type pores performed in this work show that entropy effects lead to the exclusion of co-ions from the pore to allow densely packed counterions to efficiently neutralize surface charge. This phenomenon, in conjunction with the fact that electrical effects are long range with respect to the source charge, leads to a semioscillatory behavior of the potential profiles not predicted by classical theory. While bulk conditions in terms of ion concentrations are achieved in the center of the pore for most pore sizes studied in this work, electrical bulk conditions are not achieved except for the largest pores at the lowest surface charge density conditions. The electrical potential at the center of the pore is lower with respect to the absolute potential of the bulk solution in most cases.