ument IEC the current carrying capabilities of power cables can be mathematically modelled. Current rating of power cables can hence be done without. BS IEC describes a method for calculating the continuous current rating factor for cables of all voltages installed in ventilated. BS IEC Electric cables. Calculation of the current rating. Thermal resistance. Calculation of thermal resistance. standard by.
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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.
Principle- simple wire in homogeneous material 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.
IEC 60287 Current Capacity of Cables – Introduction
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.
Ieec 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. I – conductor current, A R’ – d. At thermal equilibrium these will be equal and can be rearranged to give oec cable current carrying capacity in Ampere:.
> IEC Current Capacity of Cables – Introduction
As an example, consider finding the current carrying capacity of a 50 mm 2 conductor, with XPLE insulation directly buried vs an insulation thermal resistance of 5. Applying the IEC Standard click to enlarge 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:.
Each of these areas is discussed in more detail in the following posts which together form a comprehensive guide to the standard:. Within the standard, there are a lot of equations and it can be confusing to persons who are new to the method.
However, a step by step working through it approach will enable the current carrying capacity to be calculated. The flow chart shows one recommended path for working through a cable sizing exercise in line with the standard. Given the number of equations which need to be solved, it is tedious to calculate in accordance with the standard by using hand or manual methods.
More practically software applications oec used, which allow the sizing of cables to take place quickly.
A quick Google search will turn up several software programs capable of performing the calculation. In this instance the current capacity should be evaluated for each type of installation condition and the worse case taken.
Within the note the IEC have been introduces and the problem of finding the current capacity of a cable boiled down to that of a thermal calculation. The note has given an overview of the contents of the standard, ways to navigate and perform the calculation and provided links to more detailed posts. Hopefully the note has achieved the objective of providing an introduction to the current capacity sizing methods of IEC If you have any comments or something is not clear enough, please post these below.
One topic per page All topics one page Export Print Email. Thermal Problem Principle- simple wire in homogeneous material The methodology taken to the sizing of cables is that of treating the issue as a thermal problem. I 2 R ‘ The heat flow watts per unit length from the conductor is given by: As an overview, the standard looks at the following situations: Summary Within the note the IEC have been introduces and the problem of finding the current capacity of a cable boiled down to that of a thermal calculation.
Cable Sizing Input Data Checklist. Calculation of k Factor. BS Voltage Drop. Dielectric loss in cables. Economic Optimisation of Cables. Earth Fault Loop Impedance. IEC Fault Calculations. Cable Sheath and Armour Loss. Derating Factors – cables grouped in air. Support Tip – Browser Cache. API Help and Tips. Release Notes [Cable Calculator]. Google Sheets – Electrical Functions. Standard Wire and Cable Sizes.
Symbols – Cable Construction. Symbols – Cable Derating Factors. Symbols – Circuit Breakers. Symbols – Fault Calculations.
BS IEC 60287-3-1:2017
BS Voltage Drop Calculation. Calculating Cable Performance a. Calculating Cable Performance d. Newsletter Subscription Signup to our newsletter to receive site updates.