Okay, let's break down the grounding electrode conductor (GEC) sizing for your separately derived system (SDS) generator. The correct approach depends on the specific part of the NEC you're using, but generally, you'll be referring to Article 250.66.Here's the process and reasoning

Understanding the Basics
Separately Derived System (SDS)

Your generator is an SDS because it's providing power independent of the utility service.

Grounding Electrode Conductor (GEC)

This conductor connects the generator's neutral (bonded neutral in this case) to the grounding electrode (e.g., ground rod, building steel). Its purpose is to provide a low-impedance path for fault current back to the generator, allowing overcurrent protective devices (breakers) to trip and clear the fault.

Sizing the Grounding Electrode Conductor (GEC) - NEC 250.66

According to NEC 250.66, the size of the grounding electrode conductor is based on the size of the derived phase conductors. It is NOT based on the size of the overcurrent protective device (breaker).Here's the breakdown for your situation:1. Identify the Largest Phase Conductors

You have two feeders: 1600A Breaker: 4 sets of 600 kcmil conductors. 400A Breaker: 1 set of 600 kcmil conductors. Clearly, the 1600A feeder with 4 sets of 600 kcmil conductors has the largest combined phase conductor size.2.

Calculate the Equivalent Circular Mil Area of the Largest Phase Conductors

Multiply the circular mil area of one 600 kcmil conductor by the number of parallel conductors: 600 kcmil = 600,000 circular mils 4 x 600,000 circular mils = 2,400,000 circular mils3.

Use NEC Table 250.66 to Determine the GEC Size

Look up 2,400,000 circular mils in Table 250.66. You'll find that this value falls in the range requiring a 3/0 AWG copper grounding electrode conductor.

Therefore, the grounding electrode conductor for your generator should be sized as 3/0 AWG copper, based on the total circular mil area of the 1600A feeder conductors.Important Considerations and Clarifications Aluminum GEC

If you use an aluminum or copper-clad aluminum GEC, you'll need to use Table 250.66's column for aluminum/copper-clad aluminum and select the appropriate size. It will likely be larger than 3/0 AWG.

Enclosure Bonding

Don't forget to properly bond the generator frame/enclosure to the neutral point of the SDS (typically where the neutral is bonded). This bonding conductor needs to be sized according to 250.28(D)(1).

Grounding Electrode System

Ensure that you have an adequate grounding electrode system in place (e.g., ground rod, building steel, concrete-encased electrode).

Local Codes

Always check with your local electrical inspector or authority having jurisdiction (AHJ). They may have specific requirements or interpretations that supersede the NEC.

Generator Manufacturer instructions

Check for any manufacturer's instructions related to grounding, as they may have specific requirements for their equipment.

Why Not the Sum of All Feeders?

You might be tempted to add the conductor sizes of both the 1600A and 400A feeders. However, 250.66 is clear: the GEC is sized based on the largest ungrounded conductor* of the derived system.

In summary, based on the information you provided and the NEC 250.66, a 3/0 AWG copper grounding electrode conductor is the correct size. The #2/0 awg provided by the engineer is undersized. Always verify with your local inspector and consult the generator manufacturer's documentation. Flag for review