Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.
Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation.
For example, the United States National Electrical Code, Table 310.15(B)(16), specifies that up to three 8 AWG copper wires having a common insulating material (THWN) in a raceway, cable, or direct burial has an ampacity of 50 A when the ambient air is 30 °C, the conductor surface temperature allowed to be 75 °C.
Ampacity rating is normally for continuous current, and short periods of overcurrent occur without harm in most cabling systems.
Electrical code rules will give ratings for wiring where short-term loads are present, for example, in a hoisting motor.