Cable sizing

Dear Sir,

 How will we calculate the cable size in sqmm if the current in ampere is given without using the charts mentioning the ratings and size..

Any thumb rule is available.

Please reply..

Thanks in advance

Syam


myElectrical

asked 3/11/2013
SYAM9
SYAM

6 Answers

This answer has been selected by the question author as the best answer.

Hi,

Steven is bang on!!! However I would like to add my two bit- over and above points mentioned by Steven, the laying method, whether in cable tray, buried underground or run in cable trays overhead along with parallel running of other cables, soil resistivity and temperature of soil all will contribute to  selection  of cable sizes. Also rating will differ on type of insulation used or whether is used like in ACSR conductors as free field conditions.

Pit is better to use manufacturer chart after including all derating factors applicable to select the cables.

Thanks

S.S. Pattabhiraman

answered 3/13/2013 Pattabhiraman 74
Pattabhiraman


  • I asked the question because some one asked the same question for an interview i attended i replied it is based on the cable ampacity table available in IEC standard 60364-5-52. He asked me what you will do if the table is not available...I have no answer for that...SInce i am entry level i thought t
    hat there are other methods which i don't know..So that is why i asked the question here..

    Thank you for your reply.. - SYAM 3/14/2013

The following post is a more formal approach to sizing cable current capacity (without using charts).

IEC 60287 Current Capacity of Cables - An Introduction

As for rules of thumb, I think ultimately they don’ work well.  Typically you end up oversizing the cable with the associated cost and installation issues or worse, under sizing cables with safety implications.   With a rule of thumb you also don’t have a standard against which to judge/substantiate your calculation.

It’s relatively easy (particularly with software) to formally calculate a cable size and my advice would be to approach the sizing from that view.

Also we need to bear in mind that cable sizing is not just current capacity, but is also voltage drop and fault  ratings.

answered 3/11/2013 Steven McFadyen 246
Steven McFadyen
  • I asked the question because some one asked the same question for an interview i attended i replied it is based on the cable ampacity table available in IEC standard 60364-5-52. He asked me what you will do if the table is not available...I have no answer for that...SInce i am entry level i thought that there are other methods which i don't know..So that is why i asked the question here...
    Thank you for your reply..
    - SYAM 3/14/2013

Choosing the cross-section of cables is quite complicated. This procedure is based on IEC 60909. It depends on the characteristics of cables and surrounding environment as well as cable laying and so on. To observe more details on cable selection, please check out cable selection.
answered 10/20/2014 pht 5
pht
Dear Sham,
To find out the cross section area of cable the thumb rule is S= Ixsqroot t / k where t= fault duration, I = short circuit current in kA, factor k=0.094 for aluminium conductor of XLPE
for eg:-
t= 0.25 sec
I= 26.24 kA
Now cross section of cable=26.24x sqroot (0.25)/0.094 = 139.6sqmm
so next standard size of cable is 185 sqmm
hope this thumb rule will help you
thanking you
Sathya Narayana JE E/M
answered 9/23/2014 Sathya Narayana JE E/M 46
Sathya Narayana JE E/M

No thumb rules, absolutely!

You must follow a methodical approach while sizing cables, duly considering continuous current carrying capacity under site conditions, the voltage drop & the short circuit withstand capacity.

answered 4/4/2013 EE65 19
EE65

Short Ciruit calculations should also  be included in the cable sizing calculation

 

Mr. Steven , could please post a separte subject with examples for short ciruit calculations stating for example from tarnformer , MCC panel , Sub Panel up to the load ( Motor )

calculations shall include the short circuit calculations for transformers , bus bar ,circuit breaker , cables and the motor

 

thanks a lot

 

answered 3/26/2013 waelroustom 2
waelroustom

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