The more cables you're routing, the larger your conduit needs to be. Use manufacturer specs to determine the outer diameter of each cable. This is particularly useful when planning an installation. Proper conduit fill prevents three critical problems: Heat Buildup: Overcrowded conductors trap heat, accelerating insulation degradation and increasing fire risk. Can a conduit be too big? Yes, it is possible for a conduit to be too big, but it is. Conduit fill is the percent of area inside the conduit taken up by the cable (s). A great tool to help electricians, engineers and contractors answer the question, "What size conduit do I use per the NEC®?" The app. To find out how many cables you can run in a given conduit size, enter your Belden cable part number, or enter the diameter of your cable.