Abstract: The temperature-rise, which electrical machines may safely withstand is determined by the limiting temperature of the insulating materials used in them. It is therefore, a vital requirement to qualify electrical insulating materials thermally by determining their insulation class. In this study, 25 sample varieties of Nigerian cloth fabrics were experimented with to determine their insulation classification. The samples were cut into definite dimensions and weighed. Each type of cloth fabric was made into two samples; one sample left in its ordinary state, while the other sample was impregnated with insulating varnish. Both samples were subjected to a heat-run in a sealed industrial oven, while measuring the insulation resistance of the given sample at regular temperature intervals, until the sample burns out. The measured values of weight, insulation resistance and temperature are shown in tables. Curves were plotted to show the variation of insulation resistance with temperature. From the experiments, only two of the unimpregnated samples can be used for class Y insulation whose limiting temperature is 90°C, while 24 impregnated fabrics can be used for class Y insulation. Only one cloth fabric is unsuitable for class Y insulation even with impregnation.