Cable Ampacity

Ampacity — the ampere capacity of a conductor — represents the thermal limit of a cable under specific reference conditions. Every cable has a maximum continuous operating temperature determined by its insulation material: 70°C for PVC, 90°C for XLPE and EPR, and 105°C or higher for silicone and mineral insulation. When the conductor current produces I²R heating that would raise the conductor temperature to this limit, the cable is at its ampacity. Exceeding ampacity accelerates insulation aging, reducing the cable's service life from its design value of 30+ years. Tabulated ampacity values assume reference conditions (30°C ambient air, 20°C ground temperature, single circuit in free air or standard burial depth with 2.5 K·m/W soil). Real installations require adjustment factors for higher ambient temperatures, grouped cables with mutual heating, and conduit fill restrictions. The NEC uses the term ampacity while IEC and BS standards use current-carrying capacity (CCC) — the concept is identical. Engineers should always use the insulation's rated temperature for ampacity lookup but may need to use the terminal temperature rating (often 60°C or 75°C) for termination sizing.