Loss factor of magnetic steel ducts

The loss in duct made of ferromagnetic steel is calculated in the same fashion as the loss in steel pipes of pipe-type cables.

${10}^{-5}\frac{0.0115s_c-0.001485Di_d}{R_c}$three single-core cables in trefoil in magnetic steel pipe
${10}^{-5}\frac{0.00438s_c+0.00226Di_d}{R_c}$three single-core cables in cradled formation in magnetic steel pipe
$\lambda_4 \left(\frac{f}{60}\right)^2 \sqrt{\frac{60}{f}}$other frequencies than 60 Hz
$0$steel pipe is non-magnetic or a pipe-type cable with flat-wire armour over all three cores after they are laid up is used