Current Rating of a Cable

Current Rating of a Cable

Once all the thermal resistances are known then the current carrying capacity of a cable can be determined. When the cables carry an excessive current the cables get heated up. It is not advisable to operate the cable at excessively high temperature because of the following reasons :

1. Due to high temperature, oil in the oil filled cables get expanded and this may lead to bursting of the sheath.

2. High temperature can cause unequal expansion which leads to the formation of voids. Such voids may lead to ionisation and finally lead to insulation failure.

3. The dielectric losses increase with temperature which also can lead to breakdown of insulation.

Hence the cable must be operated at a current less than the maximum current carrying capacity of the cable.

The current rating of a cable is dependent on the following factors,

1. The maximum permissible temperature at which conductor insulation can be operated.

2. Heat dissipation arrangement through the cable.

3. The ambient conditions as well as conditions at the time of installation.

Let us see how to determine current carrying capacity of a cable. 

Let     n = Number of phases

R = Conductor resistance at 65 °C in Ω /m

I = R.M.S. value of current in each core

Hence the total core loss is given by n I²R.

The heat generated in the core of the cable passes through the dielectric medium to the sheath.

Let     θ_m = Maximum permissible temperature of the core

θ_S = Sheath temperature

Then we can write,

This is the heat flowing through bedding, serving and ground. While the total thermal resistance of bedding, serving and ground is S₄ + S₅ + G. And the corresponding temperature difference is difference between sheath temperature θ_S and ambient temperature θ_a. Hence we can write,

This is the required current carrying capacity of a cable.

The allowable temperature values for the various types of cables are given in the Table 6.12.1. 

Type of cable : Allowable temperature in °C

VIR and PVC insulated - 60

Impregnated paper insulated - 60

1. Oil filled and gas pressure - 85

2. 33 kV, solid, armoured       - 65

3. 22 kV, screened, armoured - 65

4. 11 kV, belted, armoured - 65

Practically the current carrying capacity also depends on the factors like grouping and proximity of other cables, load factor, load cycle, ambient soil temperatures in which the cable is to lay etc.

In practice S₄, S₅ and  λ can be neglected hence current carrying capacity can be expressed as,

Review Question

1. Write a note on thermal rating of cables.