Current-Carrying Capacity (Ampacity)
It is the current a conductor can carry continuously without exceeding the temperature limit of its insulation. It is determined at the point where the heat produced by the cable's resistance balances the heat dissipated to the environment. Bundled runs, high ambient temperature and passing through conduit lower this value.
The resistance of copper rises with temperature: in environments reaching 90–105 °C, such as the motor area or under the bonnet, the resistance is 27–33 percent higher than its nominal value. That means both the loss and the voltage drop come out larger than you calculated.
In low-voltage DC systems there is an important distinction: the cross-section is most often determined not by current-carrying capacity but by the acceptable voltage drop. At 12 V a 3 percent drop is only 0.36 V, and this limit comes into play well before the heating limit.
- Over short distances the cross-section is determined by ampacity.
- On runs longer than a few metres the cross-section is determined by voltage drop, and the result usually comes out one or two sizes thicker.
- On cables run in bundles a derating factor is applied.
In practice the safest method is to do both calculations and choose the larger cross-section. Check that the terminal and the fuse holder also accept that cross-section.
Context where this term is used: DC Motor Driver Selection Guide