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This paper studies the effects of natural convection on longitudinal heat transfer and on the air-gap thermal resistance of cables inside conduit installations. Oversimplification of the physical placement of cables inside unfilled conduits is the main shortcoming in currently available thermal models. The study closely investigates the share of each heat-transfer mechanism and the effect of the natural placement of trefoil cables inside the conduit. Measurements from various installation setups are investigated for their impact on heat transfer. The installation-dependent convection correlations adopted in this study have broader applications for the dynamic thermal rating of underground cables inside conduit, troughs, and tunnels. Laboratory measurements are compared with numerical solutions from the IEC 60287 standards, Electra 143 methods, and FEA simulations.