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earth wire size fro motors


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Hello tahira,


In general, the earth conductor should be large enough to 'safely' carry the prospective short circuit current. This really means that the earth conductor must be of sufficient cross sectional area to safely (ie without excessive temperature rise) carry the short circuit current until the branch protection device (ie circuit breaker/fuse) clears.


In practice, how do you apply this information?


Here in Australia, I would refer to Australian Standard AS3000, which lists common Active conductor sizes with minimum applicable Earth conductor sizes. In general, the size ratio varies by between 30-50% or so. Unfortunately there is no simpe fixed ratio.


An alternative (and more complex, as per AS/NZS3008) approach is to calculate the size required based on prospective short circuit currents, expected fault duration and protective device clearing times. Most of this information is not readily available, therefore reference to AS3000 is easier (for me).


My suggestion therefore would be to refer to your local standards.


I hope this is helpfull.



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  • 3 months later...
Originally posted by tahira

how to calculate the SWG for earth wire , it also depend on load ?

some body tell me the general rule i.e earth wire size half the cables used for equipment.plz tell me in detail

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  • 7 months later...
  • 2 years later...

Hi Tahira;


We are working at a new project and following JAAKKO POYRY standards. We also select the main earth conductor for machines the half size of power cable. If power cable is, for example, 120mm², then we select 70mm² earth conductor.


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

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