Sub feed cable

[Dansos]

We are installing an outdoor disconnect for a 200 A service in a residential home. We will be running 4/0 AL from disconnect to the main panel inside. I have always ran 200 A sub feed cable ( I believe 4/0-4/0-4/0-2/0 AL)
My question is..... why does the “ground” running from disconnect to panel need to be sized differently than our “earth grounds” (#6 Cu)??
Can you site the NEC on this please? My helper asked me why and I did not have a definitive answer for him. I always assumed we sized equipment grounds on the size of the feeder cable but is the fourth wire in a sub feed an equipment ground? I am confused on this subject.
Thank you in advance.

 

[LarryFine]

Yes, the EGC in the feeder is an equipment ground, and it is based on the size of the 200a main breaker.

The rod GEC never needs to exceed #6 cu because that is considered to be the limit of earth conductivity.

 

[Dansos]

Thanks for the clarification on that.

 

[mopowr steve]

Because the ground running between the disconnect and panel is an “equipment ground” which needs to be sized to effectively carry a larger amount of current (to trip OCPD) than that of your “grounding electrode conductor” that lands on a ground rod. Earth/rod resistance no matter where you are on Earth is usually high enough to restrict the amount of current that the GEC will ever deliver.