Ampacity, or current carrying capacity, is defined as the current a conductor can carry before its temperature rise exceeds a permissible value. The correct conductor size ensures that the heat generated by the flow of current through the conductor and the heat of the ambient temperature does not exceed the thermal rating of the insulation.
There are many factors which will limit the amount of current that can be passed through a wire:
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.
CONDUCTOR NUMBER: Heat dissipation is lessened as the number of individually insulated conductors that are bundled together are increased.
INSTALLATION CONDITIONS: Restricting heat dissipation by installing the conductors in conduit, duct, trays or raceways lessens the current carrying capacity. This restriction can be alleviated by using proper ventilation methods, forced air cooling, etc.