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

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.

BigMax

In the USA we call it the "equipment grounding conductor". Its size ,is as stated above, is based on the OCP (over current protection) on the phase conductors serving said equipment. We follow table 250.122 from the NEC to size the ECG.

50% of your current load

the earth as to be half the conductor size over 16mm

I disagree ....the earth wire should be greater or equal to the current carring conductor... It should offer less resistance to the flow of current or the fault current might take a different path ...

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.