IEC 60364-5-54 gives the following formula for the calculation of the factor k:
where Qc = volumetric heat capacity of conductor at 20°C, J.K-1.mm-3
ß = reciprocal of temperature coefficient of resistivity at 0 °C
ρ20 = electrical resistivity of conductor material at 20 °C, Ω.mm
ϴi = initial temperature of conductor, °C
ϴf = final temperature of conductor, °C
Note: strictly speaking ß is specfied at 0 °C. Within myCableEngineering calcuations, ß is determined by taking the reciprocal of the 20 °C temperature coefficient. This introduces a small, but negligible error.
Typical Application
For manual calculations, the following will give realistic results:
|
β [°C] |
Qc [J.K-1.mm-3] |
ρ20 [Ω.mm] |
Copper |
234.5 |
3.45 x 10-3 |
17.241 x 10-6 |
Aluminium |
228 |
2.5 x 10-3 |
28.267 x 10-6 |
Steel |
202 |
3.8 x 10-3 |
138 x 10-6 |
Substituting the above values and rearranging the IEC equation slightly, gives:
- copper conductors
- aluminium conductors
- steel