We have performed magnetic and transport measurements on several series of La0.7Ca0.3MnO3 polycrystalline pellets and thin films. The aim is to discriminate the effects of structural and chemical disorder on the electronic transport properties of polycrystalline manganites. We propose a macroscopic model that takes into account that electric transport is dominated by the evolution with temperature and external magnetic field of different kinds of parallel connected conduction channels. The relative weight of these channels is determining the overall measured resistance, insulator-to-metal transition temperature, and magnetoresistance. We demonstrate that transport properties of samples with the same magnetic behavior are dominated by the connectivity between grains, but not to grain size effects, at least down to 12 nm grain diameter. The model reproduces very well the magnetoresistance at low temperature for all measured samples with different grain sizes and grain connectivity. [S0163-1829(99)03934-X].