Ampacity is a shorter term for current carrying capacity first conceived by William Del Mar in the early 1950s. The term was first published in 1962. The term is defined as the maximum amount of current a cable can carry under prevailing operation conditions without sustaining immediate or progressive deterioration. These conditions include environmental and time considerations.

Cables are a source of heat whether they are only energized or carrying load current. This heat causes a temperature rise in the cable which must be kept within allowable limits. These limits have been established through years of experience. The various components can endure some maximum temperature on a sustained basis

There are several heat sources in a cable such as dielectric loss in the insulation, losses due to current flow in the conductor, current in the sheets, shielding, and armour. Sources external to the cable include adjacent cables, induced current in a surrounding conduit, etc. The sources of heat result in a temperature rise in the cable that must flow outward through the layers, that have varying resistances to the flow of heat. These resistances include cable insulation, shields, sheets, jackets, air, conduits, concrete, surrounding soil,and finally ambient air.

To avoid damage to the cables, the temperature rise must not exceed the maximum temperatures the cables can endure. It is the careful balancing of temperature rise to 27 acceptable levels and the ability to dissipate that heat that determines the ampacity of cables.

There are two current levels that must be considered when designing a cable system. The maximum current under normal operation and the maximum fault current. The maximum fault current is much higher but only needs to be endured for a shorter period of time.