Cableizer
Log in

External diameter of conductor for transient calculations

For calculation of the cyclic and emergency current rating of cables acc. IEC 60853, the multi-core cable is replaced by an equivalent single-core construction dissipating the same total conductor losses. The equivalent single-core conductor has a diameter as calculated by the formula below where $D_i$ is the value of diameter over insulation and $T_1$ the thermal resistance for the multi-core cable.

The actual conductors are considered to be completely inside the diameter of the equivalent single-core conductor, the remainder of the equivalent conductor being occupied by insulation.

Symbol
$d_{ct}$
Unit
mm
Formulae
$d_c$Single-core cables
$\sqrt{2} d_c$Two-core cables, round conductors
$D_{it} e^{\frac{-2\pi T_1}{n_c \rho_i}}$Three-core cables, round conductors
$d_x$Multi-core cables, sector-shaped conductors
Related
$d_c$
External diameter of conductor [mm]
$D_{it}$
Diameter over insulation for transient calculations [mm]
$d_x$
Equivalent diameter of a conductor [mm]
$n_c$
Number of conductors in object
$\pi$
Archimedes' constant $\pi$
$\rho_i$
Thermal resistivity of insulation [K.m/W]
$T_1$
Thermal resistance between one conductor and sheath [K.m/W]
Used in
$A_{it}$
Cross-sectional area of insulation (IEC 60853) [mm$^2$]
$p_i$
Factor apportioning the insulation
$Q_{ct}$
Thermal capacitance of conductor (IEC 60853) [J/(m.K)]
$Q_{it1}$
Thermal capacitance of insulation, 1st portion (IEC 60853) [J/(m.K)]
$Q_{it2}$
Thermal capacitance of insulation, 2nd portion (IEC 60853) [J/(m.K)]
$t_{1t}$
Thickness of insulation to sheath, transient [mm]