Taking into account both cable performance and manufacturing costs, aluminum and copper cores are the most commonly used conductor materials for cables. Long-term practices in cable design and combustion performance testing have shown that under the same external insulation formulations and structural conditions, cables with aluminum cores are more prone to vertical upward flame spread compared to those with copper cores. This paper utilizes the Fire Dynamics Simulator (FDS) software to construct a numerical model of ignition and combustion for a standard testing chamber of vertically burning bundled cables. Based on the GB/T 18380 standard, the injection flow field of a propane/air premixed flame was achieved with a fire source power of approximately 20 kW, consistent with experimental standards. Subsequently, for six typical bundled cables with a diameter of 29 mm each, the ignition and upward flame spread for both copper and aluminum core cables were calculated in detail. Results such as the rate of heat release, the rate of combustible gas production, and the internal conductor temperature distribution indicate that aluminum core cables burn more intensely and are more prone to upward flame spread. Therefore, for the design of flame-retardant cables with aluminum cores, the formulation of the external sheath layer should be appropriately enhanced in terms of flame retardancy to achieve the same level of fire protection as copper core cables.