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



Photovoltaic (PV) - Utility Power Grid Interface

Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird. During periods when the PV system generates energy...

Wiki Depreciation

We have had the Wiki with us for a long time now, but at last I have decided to say bye bye – more details on why below.

New Mail Chimp

We've been sending out Newsletters on a regular basis for a few weeks now. To do this we have been using Google's Feedburner service. While Feedburner...

Battery Cars A to Z

Battery powered cars are a hot topic and widely debated. The pros, cons, issues and time frames can be talked about endlessly. An article by the Telegraph...

IEC 61439 - The Switchgear Standard

The new standard IEC 61439 replaces the old 60439. Compared to the old standard, the new 61439 is a more clearly defined and takes into account the assembly...

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

Difference Between Live and Dead Tank Circuit Breakers

A quick post in connection with an email question: Live Tank - the circuit breaker the switching unit is located in an insulator bushing which is live...

Maximum Demand for Buildings

Estimating maximum demand is a topic frequently discussed. Working out how much power to allow for a building can be very subjective . Allowing too much...

Introduction to Lighting

When looking at the design of a lighting scheme it is useful to have an understanding on the nature of light itself and some of the basic theory associated...

Multimeter

Multimeters are undoubtedly the most common item of electrical test equipment in use.  Often it is the first piece of equipment people will turn to when...

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