What Does Ampacity Mean?
Ampacity is short for “ampere capacity” and refers to the maximum current (measured in amperes, or amps) that a conductor can carry under normal usage conditions without exceeding its temperature rating. Exceeding the ampacity of the conductor will cause it or its surrounding insulation materials to degrade due to the excessive heat generated by the current.
Ampacity is also known as current carrying capacity.
Safeopedia Explains Ampacity
Ampacity is generally used to describe the current carrying capacity of electric wires or cables. It is an important electrical safety concern, as exceeding the ampacity of an electrical system can lead to dangerous electrical faults such as arc flashes, electrocution, and fires.
Electrical Wiring Standards
Ampacity is the ability of a conductor to carry current continuously under designed conditions without exceeding its temperature rating. In other words, it is a measurement of a conductor’s ability to dissipate heat without damaging itself or its insulation. In order to avoid arc faults, electrical workers must ensure that all wires used within a given electrical system have the required ampacity to conduct the amount of current that the system will generate.
The National Electrical Code (NEC) specifies electrical wiring for the United States. The NEC wire ampacity tables show maximum allowable currents for different sizes and applications of wire. The Institute of Electrical and Electronics Engineers (IEEE) also provides a recognized and comprehensive list of ampacity ratings. The IEEE tables list the maximum ampacity of conductors based on their materials, diameter, and the maximum temperature they can withstand.
Ampacity ratings are given for conductive material being used at ambient temperature without additional heat sources. When wiring is used at non-ambient temperatures, or when wiring is bundled together (limiting heat dissipation), it is necessary to apply an ampacity correction factor that accounts for how these differences will affect the ampacity of the system.
Calculating the Ampacity of a Conductor
The melting point of copper theoretically limits wire ampacity; however, the melting point of insulating materials used for conductors is much lower than that of copper. The practical ampacity rating is, therefore, based on the thermal limits of the insulating material. The ampacity of a wire will also generally increase with the diameter of the wire - the larger the wire, the more effectively it can dissipate heat.
When calculating ampacity, natural variables such as temperature, viscosity, wind and air density, and thermal conductivity should be taken into account.