How to size grounding electrode conductor for service entrance switchgear that contains a MV transformer?

[careful]

Does 250.66 still apply (largest conductor is #3/0), or does 250.122 need to be accounted for in some way?

 

[wwhitney]

Assuming there is a service disconnect in the switchgear that is on the primary side of the transformer, and the transformer is wired as an SDS, then you have two separate GEC requirements: one for the service grounded conductor, and one for the transformer secondary grounded conductor (or for an ungrounded system, the GECs connect to the EGC system). You can satisfy those separately, or if you want to use one GEC for both purposes, use the larger size.

Cheers, Wayne

 

[careful]

Assuming there is a service disconnect in the switchgear that is on the primary side of the transformer, and the transformer is wired as an SDS, then you have two separate GEC requirements: one for the service grounded conductor, and one for the transformer secondary grounded conductor (or for an ungrounded system, the GECs connect to the EGC system). You can satisfy those separately, or if you want to use one GEC for both purposes, use the larger size.

Cheers, Wayne

Thanks for clarifying. Do you know why there is no limit to size in .122 but .66 only goes up to 3/0? The transformer is large enough that the EGC will surpass the #3/0 GEC requirement, but ultimately that EGC is part of the pathway to the electrodes.

 

[wwhitney]

Thanks for clarifying. Do you know why there is no limit to size in .122 but .66 only goes up to 3/0?

I'll take a stab at the answer: the GEC's role is to stabilize system voltage relative to earth and to deal with overvoltage from lightning or utility error/accidents. Given the impedance of the earth, there's not much point in going larger than 3/0. The GEC plays no role in normal fault clearing.

Whereas the EGC does play a role in fault clearing, so as the ungrounded conductors continue to increase in size (lower impedance), the EGC should similarly increase in size (lower impedance) correspondingly, to ensure sufficiently high fault currents for speedy OCPD operation.

Cheers, Wayne