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This paper presents the findings of an investigation concerning a calculation method for current-carrying capability (ampacity) of cables located in a shallow trough. The investigations proceeded in two parallel directions. On the one hand, an extensive search of published and unpublished records dealing with the subject was performed. The second path of investigation involved studies applying finite element analysis. Sensitivity analysis was also performed. In both cases, the recommendation is to use the same approach for determining the current rating of cables in a trough. They both use the cable rating in “free airv as the basis and add an extra thermal resistance to take into account the effect of the trough. Based on the investigation of the previous work and the finite element analysis, two new equations were developed for the external thermal resistance. The proper choice would depend on the field or laboratory tests that would confirm our findings or lead to the modifications of the rating formulae. In the absence of any additional information, it is recommended that the calculations be performed using one of three methods listed below. The methods are listed from the most to the least conservative. 1) method given by a new equation based on the finite element analysis; 2) method given in the IEC standard 60287; 3) method given by a new analytical equation presented in this paper. The comparison of the calculated results with the available test data shows that the method 3) above, the least conservative one, gives a good agreement with the measured values.