Conductor current–carrying capacity, or ampacity, is determined by the maximum safe operating temperature of the insulation used on the conductor. Heat generated as a result of current flow is dissipated into the environment.
Thus, for a given installation context (open-air, buried in earth, or enclosed), ampacity increases with increasing conductor size and with maximum permissible insulation temperature.
If more than three conductors are placed in a conduit, the resultant increase in temperature requires that the conductors be derated to maintain safe operating conditions.
Because heat dissipation from a conductor in free air is much greater than that from the same conductor enclosed in conduit or directly buried, its corresponding allowable ampacity is also greater.
Conversely, if the ambient temperature around a conductor is higher than 30ºC (86ºF), the temperature upon which all standard ampacity tables are based, the permissible ampacity must be reduced.
Ampacity tables for conductors in free air, for cable types not shown in Table below, and derating factors for high ambient temperatures are all found in the NEC.
Physical Properties of Bare Copper Conductors
Source: Except for millimeter dimensions, this table was extracted from NFPA 70-1999, the National Electrical Code. © 1999, National Fire Protection Association, Quincy, MA 02269.
Note: This extracted material is not the complete and official position of the National Fire Protection Association on the referenced subject, which is represented only by the standard in its entirety.