Aiming at the HVDC cross-linked polymeric (XLPE) cables, the applicability of equations describing electrical conductivity of insulating materials under DC voltage to the XLPE material is discussed and studied. Based on two conductivity expressions, the critical temperature difference for field reversal, the temperature difference across insulation, the product value of local electric field, the electrical conductivity and the associated distance from the cable centre, and the field distribution across insulation at steady state are deduced and proposed. Simulation results from the finite elements analysis and the calculation results according to equations are compared, and these equations are proved reliable. The values calculated from the conductivities are compared, as well as the comparison in corresponding thermal and electrical characteristics, both forms of equations describing conductivity are proved exchangeable for the HVDC XLPE cable.