The factor for fictitious diameter by Neher $K_x$ depends on the cable diameter and the transient period, usually it lays in the range of 0.97 to 1.37 for cable diameters of up to 100 mm. For larger diameters (e.g. ducts and tunnels), the value of increases with the heating source diameter and the Neher method is not recommened.

Neher evaluated empirically a value of 1.02 for the factor $K_x$ under the assumption of a sinusoidal load variation with a length of the loss cycle $\tau$ of 24 hours and a soil diffusivity $\delta_{soil}$ of 0.5*10-6 m2/s.

$\frac{D_{\mathrm{x}}}{1000 \sqrt{\delta _{\mathrm{soil}} \tau}}$
$\delta _{\mathrm{soil}}$