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Current carrying capacity is defined as the amperage a conductor can carry before melting either the conductor or the insulation. Heat, caused by an electrical current flowing through a conductor, will determine the amount of current a wire will handle. Theoretically, the amount of current that can be passed through a single bare copper wire can be increased until the heat generated reaches the melting temperature of the copper. There are many factors which will limit the amount of current that can be passed through a wire. These major determining factors are: Conductor Size:
The larger the circular mil area, the greater the current capacity. Insulation:
The amount of heat generated should never exceed the maximum temperature rating of the insulation material. Ambient Temperature:
The higher the ambient temperature, the less heat required to reach the maximum temperature rating of the insulation. Conductor Number:
Heat dissipation is lessened as the number of individually insulated conductors, bundled together, is increased. Installation Conditions:
Restricting the heat dissipation by installing the conductors in conduit, duct, trays or raceways lessens the current carrying Capacity. This restriction can be alleviated somewhat by using proper ventilation methods, forced air cooling, etc. Taking into account all the variables involved, no simple table of current rating can be developed and used as the final word when designing a system where amperage ratings can become critical. The first table on the next page shows the current required to raise the temperatures of single insulated conductor in free air (30° C ambient) to the limits of various insulation types. The second table on the next page gives a derating factor to be used when the conductors are bundled. These tables should only be used as a guide when attempting to establish current ratings on conductor and cable. Amperes
Single Conductor in Free Air 30° C Ambient Temperature Derating Factors for Bundled Conductors
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