Why can a cable not be loaded with the same current amperage when used in different ambient temperatures?
The current transfer raises the temperature of cables and wires on the basis of the current amperage or the selected conductor cross-section. If the current is too high, a cable installed in a room temperature of +20°C can easily reach a surface temperature of +80°C. If the ambient temperature were also to increase significantly, the maximum permissible conductor temperature of the cable would be greatly exceeded. This could result in damage to the core insulation material, the cable sheath and even the copper conductor, or cause the premature failure of these components.
Depending on the applicable standards, the various copper conductor cross-sections are all assigned maximum current ratings. The core insulation material plays little to no part here. What is important is how the cable is installed and whether it is a single core or multicore cable. In accordance with DIN VDE 0298, part 4, table 11 (see catalogue appendix table T12-1), the power rating values specified here apply to an ambient temperature of +30°C.
As per column B of table T12-1, the maximum continuous current that can be supplied to an ÖLFLEX® 450 P 3 G 1.5 cable for hand-held equipment at an ambient temperature of +30°C is 16 A per core (1.5 mm²).
If the ambient temperature rises to +50°C, for example, the so-called “correction resp. reduction factor" comes into play, the aim of which is to reduce the current load on the cable.
The reduction factor to be applied is derived from the prevailing ambient temperature and the maximum permissible conductor temperature of the cable. On the catalogue page for the ÖLFLEX® 450 P cable, the maximum permissible conductor temperature is specified as +70°C. Based on these two temperatures, the reduction factor 0.71 can be read from table T12-2 ("Correction factors") in the catalogue appendix; the maximum current rating is then multiplied by this factor.
If a customer wants to supply for his application a 16 A current to an ÖLFLEX® 450 P 3 G 1.5 mm² cable at an ambient temperature of +50°C, a conductor cross-section of 1.5 mm² will be insufficient!
ÖLFLEX® 450 P 3 G 1.5 mm²:
Max. load at +30°C as per table T12-1, column B: 16 A
Max. load at +50 ℃ as per table T12-2: 16 A x reduction factor 0.71 = 11.36 A
Result: To be able to conduct a 16 A current at an ambient temperature of +50°C, the conductor cross-section must be increased to a suitable size.
ÖLFLEX® 450 P 3 G 2.5 mm²:
Max. load at +30°C as per table T12-1, column B: 25 A
Max. load at +50 ℃ as per table T12-2: 25 A x reduction factor 0.71 = 17.75 A
Result: Increasing the conductor cross-section from 1.5 mm² to 2.5 mm² produces the required value of 16 A at an ambient temperature of +50°C.
Note that this calculation does not take account of other important factors for the correct determination of cable ampacity, e.g. the type of installation!
The amperage is measured in Ampere (A).