How does ambient temperature reduce cable current capacity?
The current limit of a cable is set by how close it gets to the temperature its insulation can withstand. As the ambient gets hotter, the margin left for the cable's own heating shrinks, so a correction factor is applied to the table value: relative to a 30 °C reference, about 0.87 at 40 °C and about 0.71 at 50 °C. In addition, because the resistance of copper rises with temperature, the voltage drop grows somewhat as well.
The mechanism is simple: the insulation has an upper temperature limit. While the cable carries current, the heat it generates itself is added on top of the ambient temperature. The hotter the ambient, the less heat the cable can generate before reaching the limit, meaning the less current it can carry.
That is why the values in the tables are given for a reference ambient temperature and are multiplied by a factor for the real ambient. Common factors relative to a 30 °C reference are 0.87 at 40 °C and 0.71 at 50 °C.
Worked example: a cross-section shown as 44 A in the table, working at 50 °C in a closed, hot compartment, gives you a practical limit of 44 x 0.71 = about 31 A. That is nearly a third of the capacity lost.
The second effect is on voltage drop. The resistivity of copper rises with temperature; the value 0.0175 Ω·mm²/m is for 20 °C. On a hot run the real drop comes out somewhat larger than you calculated, which makes it necessary to leave a safety margin in the calculation.
In practice:
- Always apply the correction factor for the engine compartment, the vicinity of the exhaust, closed panels and metal surfaces exposed to the sun.
- Choosing a route away from the heat source is a cheaper solution than increasing the cross-section.
- Laying cables in a bundle combines with the temperature effect and reduces the capacity further.
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