I am currently working on a theater with a 4KA service. Service conductors from pad mount transformer = 8 sets 4 conductors 750MCM. After consulting table 250.66 and the following notes,
I am confused on how to determine the grounding electrode conductor.The EE calls for 250MCM copper. Is this correct?

According to 250.66 it would be 3/0 copper or 250 MCM alum/copper clad alum. I don't see where there could be an argument for 250 copper UNLESS THE ENGINEER SPEC'D IT!! Since you said it was the EE, he gets to specify. They love to oversize stuff.

I agree that 250 copper is larger than required by the NEC for the service conductors you have.

That said there is no problem with oversize conductors as long as you picked that up on the bid.

The higher the job cost the more profit you should make.

Using a larger cable than is necessary will also create a choke. It actually slows down the reaction to ground (when you want a fault to follow a wide dispersion pattern). This may be the case where you are working... or may not, sometimes people do stuff that does not make sense and appreciate you asking, sometimes they are offended, how's your relationship with the EE?

Just something to consider.

750 mcm 75% on 310.16= 475 amps
475x 8=3800amps
Read 240.4c over 800amps wire size MUST be equal to OR greater then. Also Read 220.22 about Neutral size ,You might be able to Dn. size it.
Grounding Electrode conductor Sizes per 250.66 3/0 Minumum.

Service actually consists of (1) 2500 amp CB, (1)1600 amp CB and (2) 100 amp CB so the 8 sets of 750MCM is not based on a 4KA main breaker. Sorry I should have stated that in my original post.I will check into downsizing the neutral, but I doubt the EE will allow it without a deduct.