The dc resistance of conductors 

By on

This is the first of two posts on the resistance of conductors.  In the next post I will look at the ac resistance, including skin effect and show how we can deal with this.  To get started, this post will look at the simpler case of dc resistance and how it can be calculated.

DC Resistance to IEC 60287

The international standard for conductors is IEC 60287.  The standard classifies conductors according to four classes:

- Class 1: solid conductors

- Class 2: stranded conductors

- Class 5: flexible conductors

- Class 6: flexible conductors (more flexible than class 5)

For each class of conductor, the standard defines the maximum allowable resistance at 20 oC:

Minimum Resistance of Conductors in mΩ/m
CSA mm² Copper (plain) Copper (tinned) Aluminium
class 1 & 2 class 5 & 6 class 5 & 6 class 1 & 2
0.5 36.0 39.0 40.1 -
0.75 24.5 26.0 26.7 -
1 18.1 19.5 20.0 -
1.5 12.1 13.3 13.7 -
2.5 7.41 7.98 8.21 -
4 4.61 4.95 5.09 -
6 3.08 3.30 3.39 -
10 1.83 1.91 1.95 3.08
16 1.15 1.21 1.24 1.91
25 0.272 0.78 0.795 1.20
35 0.524 0.554 0.565 0.868
50 0.387 0.386 0.393 0.641
70 0.268 0.272 0.277 0.443
95 0.193 0.206 0.210 0.320
120 0.153 0.161 0.164 0.253
150 0.124 0.129 0.132 0.206
185 0.0991 0.106 0.108 0.164
240 0.0754 0.0801 0.0817 0.125
300 0.0601 0.0641 0.0654 0.100
400 0.0470 0.0486 0.0495 0.0778
500 0.0366 0.0384 0.0391 0.0605
630 0.0283 0.0287 0.0292 0.0469
800 - - - 0.0367
1000 - - - 0.0291
1200 - - - 0.0247

DC Resistance - calculation

For solid conductors the resistance (theoretically) can also be calculated using the standard formula:

dbff7acff4ea0e173b19ee778b11c06b

If the length (l) is in metres, cross sectional area, a in m2 (mm2 x10-6) and resistivity ρ in Ω-m, then the Resistance will be in ohms. The resistivity in Ω-m (at 20 oC) for copper is 1.72x10-8 and for aluminium 2.82x10-8.

The above formulae does not take into account manufacturing tolerances, compactness of stranded conductors, etc.  These will result in the calculated resistance differing from any actual measured resistance.  For general use it is probably better to use the figures from the IEC 60287 table, rather than calculation using the above formula.

Temperature dependence

The above values of resistance are based on a temperature of 20 oC.  Resistivity of the conductor will varies with temperature, with the resistance increasing as temperature increases.  This variation can be simplified to a linear function for a reasonable temperature range as follows:

2b5815bcf14a91fa6f25795e6d6ee56c

  • R      = the resistance of the conductor at temperature T
  • R20   = conductor resistance at 20 oC
  • T      = operating temperature of the conductor
  • α      = temperature coefficient of resistivity

Actual values of α, depend on the composition of the material in addition to the temperature. For both copper and aluminium, α taken as 0.0039 will give sufficient accuracy for most conductor calculations.

The ac resistance of conductors ->



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. Notes's avatar Notes says:
    2/1/2013 10:30 AM

    Trackback from Notes

    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... ...


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



Microsoft OneNote

A couple of months ago I came Microsoft's OneNote and downloaded the 60 day free trail. Since then I have been using it regularly and now have a full license...

Understanding Motor Duty Rating

One of the comments on my Motor Starting Series was asking for something on duty cycles. Here it is. As a purchaser of a motor, you have responsibility...

Load Flow Study – how they work

A load flow study is the analysis of an electrical network carried out by an electrical engineer. The purpose is to understand how power flows around...

Lightning Protection and Earth Electrode Resistance

Most installations involve some form of lightning protection system which is connected to an earth electrode.  The function of the earth electrode is to...

IEC 60287 Current Capacity of Cables - Rated Current

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...

Medium Voltage Switchgear Room Design Guide

Many medium voltage (MV) indoor switchgear rooms  exist worldwide. The complexity of these rooms varies considerably depending on location, function and...

MIT OpenCourseWare

MIT OpenCourseWare, makes the materials used in teaching all MIT subjects available on the Web, free of charge, to any user in the world.

ANSI (IEEE) Protective Device Numbering

The widely used United Sates standard ANSI/IEEE C37.2 'Electrical Power System Device Function Numbers, Acronyms, and Contact Designations' deals with...

Fault Calculation - Symmetrical Components

For unbalance conditions the calculation of fault currents is more complex. One method of dealing with this is symmetrical components. Using symmetrical...

Laplace Transform

Laplace transforms and their inverse are a mathematical technique which allows us to solve differential equations, by primarily using algebraic methods...

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