ampacity

Ampacity is the maximum amount of electric current a conductor or conductor assembly can carry continuously under specified conditions without exceeding its temperature rating. It is a property that depends on how much heat the conductor can safely generate and dissipate while carrying current.

Ampacity depends on several factors, including the conductor material (typically copper or aluminum), cross-sectional area, insulation type and temperature rating, ambient temperature, and how the conductor is installed (in free air, in conduit, or in cable trays). It also depends on installation details such as the number of current-carrying conductors in a raceway or bundle, which can require derating, as well as altitude and ventilation.

Standards and practical use

In many jurisdictions, ampacity values are specified by electrical codes and manufacturer data. In the United States, the National Electrical Code (NEC) provides ampacity tables that relate conductor size, insulation temperature rating, and ambient conditions. International standards, such as IEC, provide similar tables. Adjustment factors are applied for higher ambient temperatures and for derating when multiple conductors are present.

Application and purpose

Ampacity informs the sizing of conductors and protective devices (fuses or circuit breakers) to ensure continuous operation remains within safe temperature limits. It is distinct from short-circuit or fault-current ratings, which relate to protection devices and equipment under abnormal conditions. Ampacity considerations are fundamental to safe electrical design and vary with material, insulation, installation, and operating environment.