IEC 60287 "Calculation of the continuous current rating of cables (100% load factor)" is the International Standard which defines the procedures and equations to be used in determining the current carry capacity of cable.  The standard is applicable to all alternating current voltages and direct current cables up to 5kV.

This note will introduce the concepts adopted by the standard, provide some guidance on using the standard and direct the reader to further resources.

The methodology taken to the sizing of cables is that of treating the issue as a thermal problem. 

Losses within a cable will create heat.  Depending on the installation conditions this heat will be dissipated to the surrounding environment at a given rate.  As the cable heats up rate of heat dissipation will increase. 

At some temperature the rate at which heat is being dissipated to the environment will be the same as the rate at which it is generated (due to loses).  The cable is then in thermal equilibrium.

The losses (and heat generated) are dependent on the amount of current flowing within the cable.  As the current increases the losses increase and the thermal equilibrium temperature of the cable will increase. 

At some given current level, the cable temperature at thermal equilibrium will equal the maximum allowable temperature for the cable insulation.  This is the maximum current carrying capacity of the cable for the installation conditions depicted by the calculation. 

To illustrate the principle, we can consider a simplistic scenario of a d.c. cable (as shown in the illustration), surrounded with an insulating material and placed in a homogeneous thermal conducting material. 

The reality of any cable installation is more complex than described above.  Insulating materials have dielectric losses, alternating current introduces skin effect, sheath and eddy current losses, several cables are simultaneously producing heat and the surrounding materials are non-homogeneous and have boundary temperature conditions.

While the standard addresses each of these issues, the resulting equations are more complex do take some effort to solve.  Anyone attempting to apply this method should be working directly from a copy of the standard.  As an overview, the standard looks at the following situations:

• differences between alternating and direct current systems in calculating cable capacity
• critical temperatures of soil and possible requirements to avoid drying out the soil
• cables directly exposed to solar radiation
• calculation of the a.c. and d.c. resistance of conductors (including skin effect, proximity effect and operating temperature)
• insulation dielectric losses
• conductor I2R losses
• losses in sheaths and screens (including flat, trefoil and transposed formations)
• circulating current losses (including sheath, armour and pipes)
• thermal resistance (and it's calculation)

Guangdong Modern Zhujiang Cables Industrial Co., Limited