NEC Chapter 9 Table 1 limits conduit fill percentages to prevent overheating and ensure conductors can be pulled without damage. For one conductor, 53% fill is allowed; two conductors allow 31%; three or more allow 40%. These percentages are based on the conduit's internal cross-sectional area from NEC Table 4, and individual conductor areas (including insulation) from Table 5. Proper fill calculations ensure compliance, prevent installation problems, and maintain long-term system reliability. Always account for equipment grounding conductors (EGC) when present, as they count toward total fill even though they don't carry continuous current.