I'm in the process of building a DC electric motor(a SRM). I haven't been able to find information on selecting a wire gage for a given amperage in a coil. All of the ampacity charts are for wire in 'free air'. I know that the only source of heat from a DC coil is from the resistance of the coil/wire. When you look at the windings in a motor or transformer, the gage of wire doesn't match the amp rating of the 'free air rating'. It is usually a smaller gage than the 'free air' would be. Can anyone point me to either literature or a chart that shows wire gages for coils?