Current Carrying Capacities

The current carrying capacity of a bare wire or cable is limited by the maximum permissible temperature rise of the conductor under normal operating conditions. For insulated wire and cable, however, the maximum permissible temperature rise is limited by the amount of heat the insulation will withstand without deterioration.   In determining the losses of an insulated cable at high voltages, consideration must be given to the dielectric losses,  for temperature rises considerably greater then those based solely on I²R losses will occur.  For bare wire and cable the maximum operating temperature should not exceed 80°C for above this temperature the oxidation of copper is greatly accelerated. For this reason a safe working temperature rise of 30°C above a 40°C ambient has been adopted.

    In addition to I²R losses and dielectric losses, cables have other losses such as skin-affect and proximity-effect developed by magnetic induction.  For single conductor cables, however, where conductors are not operating close to each other, proximity effect is negligible.    Skin-effect loss is caused by the reversing magnetic field, about the cable, which tends to concentrate the current toward the periphery of the conductor.  This affect then reduces the effective carrying capacity of a conductor in its central portions.  Proximity-effect loss is caused by the opposing force of magnetic fields set up by neighboring conductors.  This displaces the points of maximum reactance to a maximum distance from each other, resulting in maximum current density at the nearest surfaces of the two conductors.

    The current carrying capacity of bare cable when used outdoors is greatly influenced by the general atmospheric conditions. The sun's radiation is absorbed by the conductor causing an increase in temperature, while air currents or wind have a large influence in conducting away the heat from the cable, thus lowering the temperature.  However, at the maximum operating temperature of 70°C the radiant energy of the sun has little if any effect on the temperature rise of the cable conductor.     

ALLOWABLE CURRENT CARRYING CAPACITIES IN AMPERES

(BASED ON ROOM TEMPERATURE OF 30°C,  86°F)

ULTIMATE INSULATION TEMPERATURE.  In no case shall conductors be associated together in such a way with respect to the kind of circuit, the wiring method employed, or the number of conductors, that the limiting temperature of the conductors will be exceeded.