Fill % · Jam Ratio · Minimum Size
Calculate the fill percentage of a conduit given one or more cable sizes and quantities. A common rule of thumb is to keep total cable fill at or below 40% for three or more cables, allowing room for cable installation and heat dissipation. This calculator also checks for jam ratio risk — a condition where exactly three similarly-sized round cables can wedge inside a conduit during pulling.
Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.
Fill percentage is calculated as the total cross-sectional area of all cables divided by the internal cross-sectional area of the conduit. Jam ratio (conduit ID ÷ largest cable OD) is checked when exactly three cables are present — ratios between approximately 2.7 and 3.2 create a mechanical wedging risk during cable pulling, since three round objects of similar size can lock together inside a circular conduit.