Table 250-66

[bigo]

I have a 480 VAC, 3 PH MCC with feeder cables of 3 parallel 500 MCM/phase. One of my buckets feeds a 45 KVA transformer; 480-208/120 vac, 3 PH. The primary feeders to the Xfr are 3-#4. The secondary cables are 3#4/0. The circuit breaker protecting the transformer is a 70A,3P breaker.

What's the size of the grounding conductor I need to run from the transformer's center tap?
Do you base the size on the feeder to the MCC? the feeder to the transformer? or the size of the secondary cables? Why?

[charlie b]

I interpret your question as relating to a grounding electrode conductor, and not an equipment grounding conductor. Thus, my answer is #2 copper, based on the secondary conductors being in the range of “over 3/0 through 350.” But I started in 250.30(A)(2)(a): “...sized in accordance with 250.66 for the derived phase conductors....”