Temperature Derating Factor
Ampacity tables are generally for an ambient temperature of 30 °C; in a hotter environment the table value is multiplied by a factor and reduced. A factor of about 0.87 is used for 40 °C and about 0.71 for 50 °C. In a hot environment the same cross-section can carry less current.
The logic is simple: the current a conductor can carry is set by the difference between the insulation's temperature limit and the ambient temperature. As the environment heats up, that difference narrows and the temperature rise allowed for the cable shrinks.
- At 40 °C: the typical 26 A value for 2.5 mm² falls to about 22.6 A.
- At 50 °C: the same cross-section drops to about 18.5 A.
- Together with the bundling factor: the two factors are applied by multiplication; with six cores in a hot conduit the real capacity can approach half the table value.
This correction is the step most often ignored in the field. A tractor engine bay, the area near a construction machine's hydraulic unit, conduit on top of a machine sitting in the sun and the inside of a closed panel easily exceed 50 °C in summer. Move the cable away from the hot surface if you can; if you cannot, increase the cross-section by one step. Using cable with heat-resistant insulation also recovers capacity, but for that the cable's temperature class has to be verified from the label.
Context in which this term is used: Installation, redundancy and field continuity