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According to the book 'Kabel und Leitungen für Starkstrom' by L. Heinhold, 5th edition 1999 (in German).
The equation below is relevant for cyclic rating calculations based on Neher-McGrath, Heinhold, or Dorison methods. When using one of these methods, the result of the calculation of $I_c$ is actually the peak current. By using this equation one can derive an approximate and conservative average load current which is simlar but lower than the permissible continuous current rating. This can then be used to calculate the average losses over time and the average temperature over the cycle period.
Heinhold used this for calculation of cables in non-ventilated tunnels and channels. He explained that because the time constant of the air inside a tunnel is large compared to the time constant of a cable, the heating of the surroundings can therefore be determined either using the losses calculated from the root mean square current over 24 hours or using the average losses per loaded conductor, taking into account the daily load cycle. This principle can be applied to buried cables as well but depending on the shape of the load variation the error can become quite large. One can find that for quite extreme load shapes such as for photovoltaic, the total heat generated over 24 hours using the $I_{c_{LF}}$ is approx. 20 % higher than considering the actual loads.