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What Does Ampacity Mean?

Ampacity is a portmanteau of “ampere capacity”, and refers to the maximum current (measured in amperes, or amps) that a conductor can carry under normal usage conditions. Exceeding the ampacity of the conductor will cause the conductor or surrounding insulation materials to degrade due to the excessive heat generated by the current. Ampacity may also be referred to as “current carrying capacity”.

Because ampacity refers to the ability of a conductor to carry current without generating excessive heat, it can be considered to be 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 ampacity necessary to safely conduct the amount of current that system will generate.

Safeopedia Explains Ampacity

The term ampacity is generally used to specifically describe the carrying capacity of wires or cables. It is an important electrical safety concern as exceeding the ampacity of an electrical system may damage it, resulting in a dangerous electrical fault such as an arc flash or electrocution hazard. Exceeding a system’s ampacity can also pose a fire risk.

The amount of heat generated by a given current depends on the resistance of the conductor the current is flowing through. Because ampacity measures the ability of a wire or cable to dissipate heat, the ampacity of a wire will generally increase with the diameter of the wire—the larger the wire, the more effectively it can dissipate heat. A wire’s ampacity is limited by the ability of its jacket and insulation to withstand the heat the wire generates.

Electrical workers in the United States generally follow the ampacity guidelines laid out under the National Electrical Code, also known as NFPA 70. The Institute of Electrical and Electronics Engineers (IEEE) also provides a recognized and comprehensive list of ampacity ratings. These ratings are provided as tables that list the maximum ampacity of conductors based on the material of the conductor, its diameter, and the maximum temperature it needs to withstand.

Ampacity ratings are given for conductive material that is 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.


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