Generally two factors will melt and damage the cable insulation:
1- Overheating due to excess current;
2- Insulation breakdown due to over voltage or voltage surges.
In the case of excitation system it should be noted that during field forcing the excitation current will exceed the nominal current, therefore you should review your system document to find the maximum current during field forcing or other current demand of the excitation system during the operation as for control of system apparent power and power factor control; It should be also noted that the excitation voltage may also increase beyond the nominal voltage in different mode of excitation system and more severe it may be subject to voltage surges, that is detrimental to the cable insulation; these conditions shall be also be verified in the excitation documents and hence the cable insulation shall be selected based on the maximum voltage possible in the system. The appropriate cable shall be selected based on these factors together with other design factors as ambient temperature, the condition of the cable configuration in the cable and so on.