This paper presents an experimental study aimed to determine the drying times of Kraft paper and pressboard during a transformer factory drying. Experimental measurements were performed over non-impregnated paper and pressboard samples by means of a thermo-gravimetric analyzer that allows characterizing drying curves of different materials at different temperatures. Samples of several thicknesses were considered in order to determine the differences of the drying times in the different parts of the transformer insulation.