This paper presents a new approach for the determination of underground cable ampacity that considers natural convection of air inside a conduit and heterogeneous soil around the conduit, and compares calculated results with conduit-less considerations. It shows how ampacity of a buried cable is affected by thickness of the soil above it, and by the surrounding soil's heterogeneity and thermo-physical characteristic. Commercial ANSYS software for three-dimensional simulation of a 400kV polyethylene-paper cables with copper segments is used. The investigation yields numerical solutions for Fourier conducting heat transfer, continuity, energy, and momentum equations. Poor convection of air in conduit is shown to result in cable threshold temperature to be greater than that in conduit-less, other conditions being similar. Also results show that the longer the conduit, the lesser the acceptable ampacity. The proposed method gives criterion and flexibility to change strategy for situations which are not discussed in standard literatures on cable routing.