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Cable Sizing Vs Starting Current Of Dol Motor


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Can somebody help me in this.

I recently facing difficulty to do sizing of my DOL motor.

The motor is 75KW,415V,

What is the factor that i need to consider beside the max. load current when sizing the cable?

ie. Starting current of the motor,Short circuit current of the motor etc.



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Dear Leeyinkeat,


The distance between your motor starter and the motor is another variable which can effect the cable sizing.


If it is within 25 metres, then i would propose 70 sq mm copper.


I would suggest a soft starter for your application, there are may in the market but i use ATS48 series of Telemecanique.








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Hello leeyinkeat


There are two major factors to consider:

  1. Thermal
  2. Voltage Drop
The thermal factors determine the current rating of the cable. There are standards that cover this, and they do differ depending on the country that you are in. You are pretty safe to take the standard rating of the cable and match that to the motor rating plus 20%. In your case, the motor would probably have a current rating in the order of 130 - 135 amps, so select the cable based on a continuous rating of 156A.

The voltage drop is often the major determining factor and in many regions, the maximum acceptable voltage drop is 5%. If you select the cable for a maximum voltage drop during start of 5%, it will probably be much larger than the cable size required for continuous running. At the minimum, you should minimize the voltage drop at run to less than 2 - 3% as this is "power" loss in KW.


Best regards,

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Please read the topic in following link



For checking the impacity of cable, here is a useful calculator. Down load it.




When you would open the page, you would find the download option at the end of page. After down loading go to Mechanical > Wires

"Don't assume any thing, always check/ask and clear yourself".

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Thank you for the useful info.

But yet i still have a question.

For this motor installation, the starting current for the motor would goes up to 1000Amp due to it is using DOL starter.

And we know most of the time the motor failed on the high surge.

So, do i need to size the cable up to the starting current of the motor?

Thank you.


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Dear leeyinkeat;


It’s dependent upon the starting time of motor. If motor starts under heavy load such as flywheel coupled motor where the starting time is too long, we have to select the proper sized cable which may bear the too high starting current for a long period. If we select the cable according to our normal practice, the cable would heat up during starting the motor. According to my experience, if the starting current of motor is greater than 15sec, the cable should be doubled in size as we select the cable under normal load application.


Now, what is the starting time of your motor?


"Don't assume any thing, always check/ask and clear yourself".

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