The Steady-State Cable Temperature calculation determines the temperature of all the cable conductors involved in the raceway system under a specified loading condition. The calculation is based on the IEC 60287 standard or the NEC accepted Neher-McGrath approach, which employs a thermal circuit model to represent heat flow situations. It is assumed that the cables have been carrying the specified load long enough that the heat flow has reached its steady-state and no more changes of temperature will occur throughout the raceway system. The cable temperature calculated is dependent on raceway system configuration, cable loading, and the location of each particular cable.
The most important differences between the IEC 60287 and Neher-McGrath approaches are listed below.
- The Neher-McGrath approach for cable thermal calculation approach uses a user-defined load factor, whereas the IEC 60287 approach assumes a unity load factor.
- IEC 60287 for cable thermal analysis gives analytical expressions for the computation of the geometric factor of three-core cable insulation, whereas the Neher-McGrath approach makes a reference to the paper by Simmons (1932).
- The Neher-McGrath approach uses the thermal resistivity, power/loss factor and dielectrical constants as defined in the file insullib.mdb, located in the Table directory under the ETAP installation directory. The relevant values used in IEC 60287 are as defined in the standard. When a material is not given in IEC Table, a conservative value of 6.0 is used for IEC cable derating.
- Calculation of losses in magnetic armor is treated only qualitatively in the Neher-McGrath approach with references to the literature for complex computational methods. Relevant approximations are proposed in IEC 60287.
- The insulation resistance calculation for three-conductor cables is different between the Neher-McGrath approach and IEC 60287 standard, which may result in a significant difference in cable thermal resistance value.
Due the differences between the Neher-McGrath and IEC 60287 methods as mentioned above, it is expected that for the same underground system, the two methods may produce different results.