# Characteristic diameter

This is the diameter around a source at which the effect of the loss factor commences. Inside the circle of diameter $D_x$, the temperature changes according to the peak value of the losses. Outside this circle, it changes with the average losses. The characteristic diameter is a function of the diffusivity of the medium and the length of the loss cycle. In the majority of cases, the soil diffusivity $\delta_{soil}$ will not be known. In these cases, a value of 0.5*10-6 m$^2$/s can be used. This value is based on a soil thermal resistivity of 1.0 K.m/W and a soil moisture content of about 7% of dry weight.

The calculation of the characteristic (or fictitious) diameter for sinusoidal load is based on the IEEE paper 'Ampacity calculation for deeply installed cables' by E. Dorison et al, dated 2010. Three different methods can be chosen:

1. Neher McGrath: Neher empirically evaluated constants to best fit the temperature rises calculated over a range of cable sizes. Based on the evaluations by Neher, the characteristic diameter becomes 212 mm for a transient load period of 24 hours (fixed value in Cableizer) and a soil diffusivity of 0.5*10-6 m$^2$/s.
2. Heinhold: The method by Heinhold is from his book 'Kabel und Leitungen für Starkstrom' by L. Heinhold, 5th edition 1999 (in German). Heinhold defined three equations to calculate the characteristic diameter depending on the type of load curve (sinusoidal, rectilinear, or median).
3. Dorison: The method by Dorison uses the modified Bessel function K of order 0: k0() and the modified Bessel function of first order: k1(). The IEC standard 60853 uses an approximation with an exponential integral valid for an infinite thin wire whereas the formulas presented in the paper the cable diameter influences the diameter of the area affected by load variations. This is particularily important for cables in tunnels, where the tunnel diameter replaces the diameter of the cable.

Symbol
$D_x$
Unit
mm
Formulae
 $60000K_x \sqrt{\frac{\tau}{3600} \delta_{soil}}$ Method by Neher McGrath $\frac{k_H}{\sqrt{n_{cycle}} {\rho_4}^{0.4}}$ Method by Heinhold $D_e e^{\frac{1}{\frac{q_x D_e}{2000}} |\frac{\operatorname{k0}\left(\frac{q_x D_e}{2000}\right)}{\operatorname{k1}\left(\frac{q_x D_e}{2000}\right)}|}$ Method by Dorison
Related
$D_e$
$\delta_{soil}$
$\tau$
Used in
$T_{4d}$