IEC-287 method evolved from the Neher-McGrath method using the same thermodynamic fundamental basis. Both methods employ the application of thermal equivalents of Ohm's and Kirchoff's Laws of thermal resistance circuit. Notice that conductivity is the inverse of resistivity.
The main differences in both methods are the units system used (English vs. SI/metric). Also the IEC method includes new developments of cable laying, configuration and drying soil effect due to heating by loaded power cable. Another standard, IEC 853-1 & 2 supplement IEC-287 covering cyclic load and emergency ratings.
See the enclose links for information regarding thermal resistance consideration in IEC 287.
