Ampacity

From the words Ampere and capacity. A rating of the capacity of an electrical conductor to carry current.

The main concern in rating a conductor's ampacity is heating. All conductors offer some resistance to the flow of electrons. This resistance causes some of the electrical energy to be converted to heat, warming up the conductor in the process. If a conductor's ampacity is too low for it's task, it could overheat, melting its insulation or possibly even the conductor itself. Undersizing a conductor also wastes that electrical energy that is being lost as heat..

Ampacity changes with the cross-sectional area of a conductor and the type of material the conductor is made of. The larger the cross section, the higher the ampacity.

Rated ampacity is also affected by the type of insulation a conductor has. The higher the temperature rating of the insulation, the higher the ampacity. Also affecting the rating is the location of the conductor. A conductor inside a conduit or raceway cannot be cooled as well as the same wire in free air so its ampacity must be derated (lowered).

The U.S. National Electric Code has tables that cover the ampacity of wires 14 AWG (American Wire Gauge) and larger and with insulation classes rated for 75 degrees C and 90 degrees C.

Ampacity chart for 60, 70 and 90 degree C rated copper wire in free air