Let's break down the grounding and neutral conductor sizing for your 3000 amp service.

Question 1: Grounding Electrode Conductor (GEC) and Equipment Grounding Conductor (EGC) Main Panel GEC (250.66)

You are correct. With 3000 amps of service entrance conductors and paralleled conductors, you need to look at Table 250.66 based on the size of your largest ungrounded conductor. Since you are using 350kcmil, the required GEC is a 3/0 copper.

Subpanel EGC (250.122)

This is where it gets tricky with parallel conductors. The code states "Where ungrounded conductors are installed in parallel in multiple raceways or cables, as permitted in 310.10(H), the equipment grounding conductor, where used, shall be installed in parallel in each raceway or cable. Each parallel equipment grounding conductor shall be sized based on the overcurrent protective device protecting the circuit conductors in the raceway or cable in accordance with Table 250.
122."

Determining the OCPD

Your subpanel is fed from a 3000 amp overcurrent protective device.

Size per Table 250.122

Looking at Table 250.122 for a 3000 amp OCPD, you need a 400 kcmil copper EGC.

Your interpretation

Yes, you are correct! You need a 400 kcmil copper EGC in each of the ten conduits feeding the subpanel.

Does This Make Sense?

At first glance, it seems excessive, but here's the rationale:1. Fault Current Path

The primary purpose of the EGC is to provide a low-impedance path for fault current to return to the source. This low impedance is essential for the overcurrent protective device (OCPD) to operate quickly and clear the fault, minimizing damage and potential hazards.2.

Parallel Conductors and Impedance

When you parallel conductors, the impedance of the entire grounding path becomes critical. If you only had one large EGC, a fault in one of the conduits would have to travel through all the conduits to reach that one EGC, significantly increasing impedance and slowing down the OCPD response. By placing an adequately sized ground in each raceway, you are keeping the impedance low for each of the parallel runs.3.

Inductance

In AC circuits, inductance can impede current flow, especially at higher frequencies (like fault currents). Having multiple EGCs helps to reduce the inductive reactance of the ground fault path.4.

Code Compliance

The code is written to ensure safety, even if it seems redundant in some cases. By strictly following 250.122, you're creating the safest possible installation, even with the added cost of the EGCs.

Question 2: Neutral Conductor Size Minimum Size (220.61)

While you state you have zero neutral load, the code still addresses the minimum neutral conductor size for services. 220.61(B) states, "Where the service or feeder has no neutral load, the minimum size of the neutral conductor shall be the minimum size as listed in Table 250.102(C)(1) of this Code." This table refers us to 250.24(C)(1) which tells us to use 250.66, which would require a 3/0 for the main service based on the size of your ungrounded conductors, even with no calculated load.

Parallel Neutral

Similarly to the ground, if you have parallel neutral runs, you need to have a neutral in each run.

1/0 is insufficient

It is unlikely that a 1/0 would meet the code requirements for a neutral conductor in this case.

Important Considerations and Recommendations



Engineering Judgment

While code compliance is mandatory, an electrical engineer can perform calculations to verify the adequacy of the grounding system. An engineer might be able to demonstrate that a slightly smaller EGC (than the literal interpretation of 250.122) would still provide adequate fault current protection. This would require detailed calculations and documentation. However, getting this approved by the AHJ can be difficult.

AHJ Approval

Always consult with your local Authority Having Jurisdiction (AHJ) for their specific interpretation and requirements. They have the final say on what is acceptable.

Professional Installation

A 3000 amp service is a complex installation. Ensure that a qualified and licensed electrician performs the work, and that all applicable codes and standards are followed.

In Summary:

You are likely correct in your assessment of the grounding conductor requirements. It may seem excessive, but it's based on ensuring a low-impedance fault current path for safety.The neutral conductor must still meet the minimum requirements for the service size according to 220.61. Always consult with an electrical engineer and your AHJ for the most accurate and compliant solution. Flag for review