Grounding Ring Size

[hugop973]

Hi, I need your input to understand correctly the way of sizing the grounding ring. Ground Ring not less than 2 AWG Cu per NEC.

I worked in a design of an industrial building with 3,000 A service, the ground ring that encircle the building was spec 4/0 and have not received comments on that. I'm not 100% sure but I guess IEEE recommends either 3/0 or 4/0

I came across some other projects where the ground ring has been sized 500 MCM or 750 MCM, etc. What is the theory behind that? Am I missing something here ?

Thanks for your help

 

[cuky2000]

Sizing ground conductor has to do with the capacity of the bare conductor to withstand the maximum ground fault short circuit current without fusing or annealing the ground conductor. I addition of the magnitude of the fault current, the duration to clear the fault and the corrosive of the soil to impact the degradation of the ground conductor for the life expectancy of the facility.
Usually conductor size of 2/0 and 4/0 for industrial building application is adequate. However, it is recommended to verify if the conductor will withstand the fault. Check if you project allows the use of Ufer ground (rebar) to supplement the ring bare ground conductor described in your post.

 

[resqcapt19]

The NEC rule is based on the fact that with systems operating at 600 volts or less, it is unlikely that the earth can carry enough current to damage the #2 conductor, and that these systems should not be relying on the grounding electrode system to clear faults. For the Ufer grounding electrode, the NEC only requires a #4 grounding electrode conductor.