Ampacity is commonly used in place of the formal term ampere capacity and is the maximum amount of current, in amps, that a particular device can carry continuously without exceeding the device’s temperature rating. This amount can also be called the current-carrying capacity or the thermal limit. For an electric conductor, the ampacity depends on its ability to dissipate heat without damage to the conductor or to any line insulation.

For electrical transmission and distribution lines, ampacity is a key factor in determining the current carrying capacity (also known as line rating) of the line. All electrical conductors have resistance to electrical flow. The resistance results in voltage drop and power dissipation, which heats conductors. As electrical conductors heat up, they expand, resulting in line sag. If an overhead line sags too far, distortion of the line becomes irreversible resulting in conductor damage. The line may also violate required clearances from vegetation resulting in fault risks. In a worst-case scenario, an overheated conductor can even melt off a pole. 

Because conductor temperature is the limiting factor, ampacity of overhead electric lines is an approximation based on assumptions about the weather. More heat is dissipated in cold or windy conditions, less heat in hot and still conditions. If a static rating is used, engineers must err on the side of safety and rate lines for hot weather with no wind. But in other conditions, this will reduce the ability of operators to utilize lines to their full capacity. The industry is now commonly using dynamic ratings, which means that ratings are adjusted based on current weather conditions. 

In the case of underground conductors, excess heating can result in damage to conductor materials resulting in failure. The heat limit for underground lines is largely determined by the conductor material but also by the soil’s ability to dissipate heat. Underground high-voltage lines are sometimes cooled by circulating oil or water.