IEC 60287 Current Capacity of Cables - An Introduction 

By on

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.

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. 

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. 


I - conductor current, A
R' - d.c. resistance of the conductor per unit length, Ω/m

Θ - maximum conductor operating temperature, °C
Θa - ambient temperature, °C
ΔΘ - temperature difference (Θ-Θa), K

T - thermal resistance per unit length between conductor and surrounding, K.m/W

The losses (watts per unit length) generated by the conductor is given by:


The heat flow (watts per unit length) from the conductor is given by:


At thermal equilibrium these will be equal and can be rearranged to give the cable current carrying capacity (in Ampere):


As an example, consider finding the current carrying capacity of a 50 mm2 conductor, with XPLE insulation directly buried (with an insulation thermal resistance of 5.88 K.m/W and soil thermal thermal resistance of 2.5 K.m/W)  and at an ambient temperature  of 25 °C

by using the related resources links given at the end of the posts, we are able to find the following:

  • the dc resistance of the cable is 0.387 mΩ/m
  • the maximum allowable temperature for XLPE insulation is 90 °C

and a total thermal resistance of 5.88+2.5 = 8.38 (insulation, plus soil)

ΔΘ = 90-25 = 65 K, giving

I = √ [65/(0.000387*8.38)] = 142 A

The Standard in More Detail

Applying the IEC 60287 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:

  • 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)

Each of these areas is discussed in more detail in the following posts (which together form a comprehensive guide to the standard):

Applying 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 are 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.

Tip:  a cable run can move through different installation environments (for example it may start in a cable basement, more through ducts in a wall, be buried for some of the route, suspended under a bridge, buried again, through ducts and into the receiving building).  In this instance the current capacity should be evaluated for each type of installation condition and the worse case taken.

Other Related Resources

Cable Sizing Tool  - describes the procedure for the sizing of cable to BS 7671 and IEC 60364

Standard Cable & Wire Sizes  - list of standard IEC 60228  wire sized and AWG conversion table

8  Steps to Low Voltage Power Cable Selection and Sizing - general guide to selecting/using LV cables

Cable Insulation Properties - typical properties of various types of cable insulation


Within the note the IEC 60287 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 60287.  If you have any comments or something is not clear enough, please post these below.

Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. nightex's avatar nightex says:
    2/5/2013 2:47 PM

    Thanks for great info. But I have little problem with calculation. in given formula I=(deltaQ/RT)^1/2 thermal reistance T is given by K*m/W and temperature diference deltaQ in celsius (in given example). So how can this formula be true? I was trying to convert C to K (deltaQ) but then I got huge amperage.

    • Steven's avatar Steven says:
      2/6/2013 2:56 AM

      Thanks for the comment.

      If you are looking at ΔΘ, then it is a change in temperature, so if you use Celsius or Kelvin it's the same. I've added the units on the example calculation so it may help. Also if you do a units analysis, it shows than answer would be in Amperes.

      In the second post (I'll write soon), I intend to cover the actual current rating equations in the standard. By simplifying these equations for the given example, they also reduce to the the same formula.

  2. Notes's avatar Notes says:
    2/17/2013 7:31 AM

    Trackback from Notes

    In the previous note we looked at the approach taken by the standard to the sizing of cables and illustrated this with an example.  We then looked at one method of applying the standard and identified resources enabling the calculation of all the... ...

Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us

Latest Questions:

  1. how to select MCB
  2. 9.5 Conversion 3Phase to 1Phase
  3. Neutral CTs in HV/LV transformer
  4. current transformer
  5. MCCB Selection
  6. Electrtical Relay
  7. Arc flash calculator
  8. Current on each phase

most popular notesMost Popular Notes:

newsletter logo

Our Newsletter

Receive updates on new posts by email
down arrow

Solar Energy: The Physics and Engineering of Photovoltaic Conversion, Technologies and Systems
Solar Energy: The Physics and Engineering ...
Olindo Isabella, ...
Paperback - 488 pages
Solar Electricity Handbook - 2015 Edition: A simple, practical guide to solar energy - designing and installing solar PV systems.
Solar Electricity Handbook - 2015 Edition: ...
Michael Boxwell
Paperback - 204 pages
Electric Power Distribution Engineering, Third Edition
Electric Power Distribution Engineering, ...
Turan Gonen
Hardcover - 1061 pages
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 712 pages
Solar PV Engineering and Installation: Preparation for the NABCEP PV Installation Professional Certification
Solar PV Engineering and Installation: ...
Sean White
Paperback - 248 pages
Power System Monitoring and Control
Power System Monitoring and Control
Hassan Bevrani, ...
Hardcover - 288 pages
Energy Systems Engineering: Evaluation and Implementation, Second Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 672 pages
Handbook of Solar Energy: Theory, Analysis and Applications (Energy Systems in Electrical Engineering)
Handbook of Solar Energy: Theory, Analysis ...
G. N. Tiwari, Arvind ...
Hardcover - 824 pages
Independent Energy Guide: Electrical Power for Home, Boat, & RV
Independent Energy Guide: Electrical Power ...
Kevin Jeffrey
Paperback - 280 pages
Photovoltaic Design and Installation For Dummies
Photovoltaic Design and Installation For ...
Ryan Mayfield
Paperback - 384 pages
Do It Yourself 12 Volt Solar Power, 2nd Edition (Simple Living)
Do It Yourself 12 Volt Solar Power, 2nd ...
Michel Daniek
Paperback - 128 pages
Solar Engineering of Thermal Processes
Solar Engineering of Thermal Processes
John A. Duffie, ...
Hardcover - 936 pages
Wind Turbine Operation in Electric Power Systems: Advanced Modeling
Wind Turbine Operation in Electric Power ...
Zbigniew Lubosny
Paperback - 262 pages
Large-Scale Solar Power System Design (GreenSource Books): An Engineering Guide for Grid-Connected Solar Power Generation (McGraw-Hill's Greensource)
Large-Scale Solar Power System Design ...
Peter Gevorkian
Hardcover - 704 pages
Electric Energy: An Introduction, Third Edition (Power Electronics and Applications Series)
Electric Energy: An Introduction, Third ...
Mohamed A. El-Sharkawi
Hardcover - 606 pages
Smart Power Grids 2011 (Power Systems)
Smart Power Grids 2011 (Power Systems)
Hardcover - 696 pages
Power Quality: Problems and Mitigation Techniques
Power Quality: Problems and Mitigation ...
Bhim Singh, Ambrish ...
Hardcover - 596 pages
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Paperback - 400 pages
Design of Smart Power Grid Renewable Energy Systems
Design of Smart Power Grid Renewable Energy ...
Ali Keyhani
Hardcover - 592 pages
Electrical Power System Essentials
Electrical Power System Essentials
Pieter Schavemaker, ...
Hardcover - 340 pages
Bottled Lightning: Superbatteries, Electric Cars, and the New Lithium Economy
Bottled Lightning: Superbatteries, Electric ...
Seth Fletcher
Hardcover - 272 pages
Submarine Power Cables: Design, Installation, Repair, Environmental Aspects (Power Systems)
Submarine Power Cables: Design, ...
Thomas Worzyk
Paperback - 296 pages
Power System Relaying
Power System Relaying
Stanley H. Horowitz, ...
Hardcover - 398 pages
Power Generation and the Environment
Power Generation and the Environment
Anco S. Blazev
Hardcover - 1333 pages
Nonlinear Modeling of Solar-Radiation and Wind-Speed Time Series (SpringerBriefs in Energy)
Nonlinear Modeling of Solar-Radiation and ...
Luigi Fortuna, ...
Paperback - 98 pages
Fundamentals of Electrical Drives (Power Systems)
Fundamentals of Electrical Drives (Power ...
Andre Veltman, Duco ...
Hardcover - 341 pages
How to Solar Power Your Home: Everything You Need to Know Explained Simply (Back to Basics Conserving)
How to Solar Power Your Home: Everything ...
Martha Maeda
Paperback - 336 pages

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note