The transient temperature calculation yields cable temperature variations as a function of time in accordance to load changes. While the steady-state temperature calculation can be used to check the cable temperature under constant loading, the transient temperature calculation provides a tool to verify operation conditions of the raceway systems against the cable short-time or emergency temperature limits. In most cases, the short-time maximum allowable temperature of a cable is considerably higher than its steady-state temperature limit. For loads that have high peak values for only a short period of time, the transient temperature calculation can be used to determine the cable peak temperature and its duration, as well as compare it against its short time maximum allowable temperature, resulting in a more economical design of your raceway systems.
The transient temperature calculation is based on a dynamic thermal model of the raceway system, constructed mainly from thermal resistance, thermal capacitance, and heat sources. The thermal resistance is used to represent different thermal layers from the cable conductor to ambient soil. The thermal capacitance is used to represent the capability of each layer to absorb the heat. When you change the cable loading, the heat generated by the loss in the conductor will change accordingly, resulting in a variation of the heat flow dissipated from the cable conductor to the ambient soil. As a result, the cable conductor temperature will vary to follow the load change pattern, at a rate of temperature change that depends on the resistance and capacitance values of the circuit.