Ampacity (Current-Carrying Capacity)
Ampacity is the current a conductor can carry continuously without reaching a temperature that would destroy its insulation. For copper in a 30 °C ambient with free-air routing it is typically around 1.5 mm² ≈ 18 A, 2.5 mm² ≈ 26 A, 10 mm² ≈ 61 A. The value is not fixed; it changes markedly with the routing conditions.
A conductor carrying current produces heat because of its resistance. As long as the heat produced can be given off to the surroundings, the temperature settles at an equilibrium; if it cannot, the insulation softens, darkens and eventually punctures. Ampacity is exactly the current corresponding to that equilibrium point.
- Ambient temperature: tables are generally for 30 °C. A factor of about 0.87 is applied at 40 °C and about 0.71 at 50 °C.
- Bundling: if there are several loaded strands in the same conduit, each heats the others and the capacity falls.
- Routing: a cable suspended freely in air carries more current than one inside a tube or buried under insulation.
At hot spots such as the engine bay, near the gearbox or inside a closed cabinet, do not use the table figure as it stands. The practical approach is to choose one step above the cross-section you calculated and to keep the continuous current at about 80% of the ampacity; that way the insulation stays in a safe range even when the starting current peaks.
Context in which this term is used: Installation, redundancy and field continuity