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Ampacity AmpacityThis definition of ampacity includes three main issues: continuous current-carrying capacity, specified conditions of use and not exceeding temperature rating of the conductor (insulation). is defined in NEC Article 100 as "the maximum current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."The temperature rating of a conductor is the temperature rating of its insulation. The melting or fusing temperature of copper is about 1080° C or about 1980° F. Aluminum melts at a much lower temperature, about 660° C or 1220° F. However, the most common types of conductor insulation are only 75 and 90° C (167° and 194° F), and the highest temperature rating of an insulation type recognized in Article 310 is 250° C (482° F), so obviously the insulation will be damaged by heat long before the conductor material itself is damaged.
Of course, the temperature rating is based on insulation type, so certain applications may need a very high temperature rating because the conductor will be installed in a high temperature location. Most commonly, the types of insulation chosen will be rated either 75° or 90° C.
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