Ampacity of a Conductor

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Ampacity is the current carrying capacity of a conductor. Ampacity calculation should take into account natural variables such as solar warming, wind and air density, viscosity, and thermal conductivity. Ampacity is a temperature rating. In order words, as temperature changes, the ampacity of a conductor changes. 

Increase in ambient/surrounding/medium temperature can significantly limit the current carrying capacities of cables. As cable temperature increases, its resistance increases thereby reducing the amount of current that can be carried. 
According to the National Electrical Code, article 310.15(C), the ampacities of conductors can be calculated by the following general formula:
TC    =  Conductor temperature in degree Celsius
TA    =  Ambient temperature in degree Celsius
ΔTD  = Dielectric loss temperature rise
RDC  = DC resistance of conductor at temperature TC
YC    =  Component AC resistance resulting from skin effect and proximity effect
RCA  = Effective thermal resistance between conductor and surrounding ambient.
The NEC specifies that the above formula can only be applied under engineering supervision. 

The table below gives the ampacities for portable power cables at 90 degree Celsius insulation and under an ambient temperature of 40 degrees Celsius. When the temperature changes as discussed above, correction factors are applied to determine the true ampacity of the cable at the new temperature. Correction factors normally applied are given here

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