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Because the air thermal resistance is high, the current-rating of underground cables in pipes are much lower than the current-rating of direct buried cables. In order to improve the cable current carrying capacities, thermal material is pumped into pipes to improve the situation of duct heat output. Based on the combinatorial coordinates, a complex computer program employing the finite difference method is developed to calculate the temperature field distributions and current-rating of underground cables in pipes. The calculating results are consistent with results obtained by simulation test and field test. Current-rating influenced by pumping thermal material into pipes from three single conductors to eighteen single conductors are studied. The results reveal that the current-rating of three single conductors can be improved about 5.6% or the temperature of cable cores can be reduced about 7°C for the same circulating current by pumping thermal material into pipes. When six or more cables are laid one another, their current carrying current-rating improved by pumping thermal material into pipes is obviously reduced by the mutual heating effect. The current-rating of the cable can be improved and operation temperature of cable cores can be reduced by pumping thermal material into pipes.