Conductor current

The permissible current rating of a power cable can be derived from the expression for the temperature rise above ambient temperature.

The equation is different for cables in air and buried cables.

• Cables in air consider the temperature rise by solar radiation and the thermal resistance to ambient is for a continuous load.
• Buried cables consider the temperature rise by other buried cables and heat sources/sinks as well as the effect of drying-out of soil by applying the ratio of the thermal resistivities of dry and moist soils. The thermal resistance to ambient may have transient load variation and correction factors due to backfill material.
• For DC cables, the dielectric losses $W_d$ are zero and $\Delta\theta_d$ disappears. And since there is no induced current in the screen and sheath, the factors $\lambda_1$ and $\lambda_2$ will also be zero.

The current rating for a four-core low-voltage cable may be taken to be equal to the current rating of a three-core cable for the same voltage and conductor size having the same construction, provided that the cable is to be used in a three-phase system where the fourth conductor is either a neutral conductor or a protective conductor. When it is a neutral conductor, the current rating applies to a balanced load.

Where it is desired that moisture migration be avoided by limiting the temperature rise of the cable surface to not more than $\Delta\theta_x$, the corresponding rating shall be obtained from the equation below.However, depending on the value of $\Delta\theta_x$ this may result in a conductor temperature which exceeds the maximum permissible value. The current rating used shall be the lower of the two values obtained.

For transient calculations, $I_c$ is the constant steady-state current applied to cable prior to application of a step function, a cyclic load or prior to emergency loading.

*** These formulae are valid when the outer temperature $\theta_{omax}$ defines the load of a system.

Symbol
$I_c$
Unit
A
Formulae
 $\sqrt{\frac{\theta_c-\theta_a-\Delta \theta_d-\Delta \theta_{sun}}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} \left(T_{4i}+T_{4ii}+T_{4iii}\right)\right)+n_{cc} \lambda_4 \left(\frac{T_{4ii}}{2}+T_{4iii}\right)\right)}}$ Cables in air $\sqrt{\frac{\theta_c-\theta_a+\left(v_4-1\right) \Delta \theta_x-v_4 \Delta \theta_p-\Delta \theta_d}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} \left(T_{4i}+T_{4ii}+T_{4\mu} v_4\right)\right)+n_{cc} \lambda_4 \left(\frac{T_{4ii}}{2}+T_{4\mu} v_4\right)\right)}}$ Cables buried $\sqrt{\frac{\theta_c-\theta_{de}-\Delta \theta_d}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} \left(T_{4i}+T_{4ii}\right)\right)\right)}}$ cables in tunnel $\sqrt{\frac{\theta_c-\theta_a-\Delta \theta_d-\Delta \theta_{0t}}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+n_c \left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(T_3+T_{4t}\right)\right)}}$ Cables in tunnel (IEC 60287-2-3) $\sqrt{\frac{\theta_c-\theta_{at}-\Delta \theta_d}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} \left(T_{4i}+T_{4ii}+T_{4iii}\right)\right)\right)}}$ Cables in trough $\sqrt{\frac{\theta_c-\theta_a-\Delta \theta_d-\Delta \theta_p}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} T_{4iii}\right)\right)}}$ Cables subsea $\sqrt{\frac{\theta_c-\theta_e}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) n_c T_3\right)}}$ Cables in riser $I_c F_{red}$ With reduction (derating) factor $\sqrt{\frac{\theta_{omax}-\theta_a-W_d n_{cc} T_{4iii}-\Delta \theta_{sun}}{R_c n_{cc} T_{4iii} \left(1+\lambda_1+\lambda_2+\lambda_3+\lambda_4\right)}}$ *** Cables in air $\sqrt{\frac{\theta_{omax}-\theta_a+\left(v_4-1\right) \Delta \theta_x-v_4 \Delta \theta_p-W_d n_{cc} T_{4ss} v_4}{R_c n_{cc} T_{4\mu} v_4 \left(1+\lambda_1+\lambda_2+\lambda_3+\lambda_4\right)}}$ *** Cables buried $\sqrt{\frac{\theta_{omax}-\theta_a-W_d n_{cc} T_{4t}-\Delta \theta_{0t}}{R_c n_c \left(1+\lambda_1+\lambda_2+\lambda_3\right) T_{4t}}}$ *** Cables in tunnel (IEC 60287-2-3) $\sqrt{\frac{\theta_{omax}-\theta_{at}-W_d n_{cc} T_{4iii}}{R_c \left(1+\lambda_1+\lambda_2+\lambda_3\right) n_{cc} T_{4iii}}}$ *** Cables in trough $\sqrt{\frac{\theta_{omax}-\theta_a-\Delta \theta_d-\Delta \theta_p}{R_c n_{cc} \frac{1}{U_{OHTC} \pi D_{ext}} \left(1+\lambda_1+\lambda_2\right)}}$ *** Cables subsea $\sqrt{\frac{\theta_{omax}-\Delta \theta_d-\theta_{air}}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2\right) n_c T_3\right)}}$ *** Cables in riser $\sqrt{\frac{\theta_c-\theta_a+\left(v_4-1\right) \Delta \theta_x-v_4 \Delta \theta_p-\Delta \theta_d}{R_c \left(T_1+n_c \left(1+\lambda_1\right) T_2+\left(1+\lambda_1+\lambda_2+\lambda_3\right) \left(n_c T_3+n_{cc} T_{4\mu} v_4\right)+n_{cc} \lambda_4 \left(\frac{T_{4ii}}{2}+T_{4\mu} v_4\right)\right)}}$ Cables buried in ducts with bentonite filling and transient loading
Related
$D_{ext}$
$\Delta \theta_{0t}$
$\Delta \theta_d$
$\Delta \theta_p$
$\Delta \theta_{sun}$
$\Delta \theta_x$
$T_1$
$T_2$
$T_{4i}$
$T_{4ii}$
$T_{4iii}$
$T_{4\mu}$
$T_{4ss}$
$T_{4t}$
$\theta_a$
$\theta_{air}$
$\theta_{at}$
$\theta_c$
$\theta_{de}$
$\theta_e$
$\theta_{omax}$
$U_{OHTC}$
Used in
$\Delta W$
$I_{ar}$
$I_{c,LF}$
$I_{c,peak}$
$I_{c,sum}$
$I_{EMF}$
$I_s$
$I_{sp}$
$L_{crit}$
$S_G$
$W_c$
$W_{encl}$
$W_h$
$W_{sar}$