You've hit on a common point of confusion regarding parallel EGCs in MC cable. Let's break down the requirements

Understanding the Core Rule 250.122:
This section dictates the minimum size of the equipment grounding conductor (EGC) based on the size of the ungrounded conductors. Table 250.122 is the starting point. For a 350 kcmil conductor, the table requires a 4/0 AWG EGC. This table is a baseline. You can never go smaller than what it requires unless an exception applies.

Parallel Conductors 310.10(G)

This section addresses the specific requirements for conductors run in parallel. You MUST comply with all sub-sections of this article when running in parallel. (5) is important because it talks specifically about parallel EGCs.

MC Cable and 250.122(F) - Your Key Point

You are correct that 250.122(F) refers to raceways, cable trays, and auxiliary gutters,

NOT directly to MC cable. This is a critical distinction. While it indirectly affects MC cable installations through other code sections, it doesn't offer specific guidance within 250.122(F) for MC cables.

The Specific Language of 310.10(G)(5)

This is the key to your question. It states the following:>"(5) Equipment Grounding Conductors. Where conductors are installed in parallel, equipment grounding conductors shall be installed in parallel where required in 250.130(C), 250.146(D), or 250.178(C). 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. Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables where installed in parallel that the combined circular mil area of the sectioned equipment grounding conductors in each cable complies with Table 250.122."

Understanding the meaning



250.130(C), 250.146(D), or 250.178(C) - If these sections require the EGC to be installed in parallel, then they must be. Each parallel EGC must be sized based on the overcurrent protective device protecting the circuit conductors and Table 250.122. Here's the critical point for you: "Sectioned equipment grounding conductors smaller than 1/0 AWG shall be permitted in multiconductor cables where installed in parallel that the combined circular mil area of the sectioned equipment grounding conductors in each cable complies with Table 250.122." This means that you can use conductors smaller than 1/0 (14 AWG - 8 AWG) as long as the combined EGC conductors in the cable are equal to or greater than the requirement in Table 250.122.

Answering Your Question



No, a 1/0 AWG cannot be used in parallel in this scenario based on 310.10(G)(5). The permissive language you're referring to only applies to conductors smaller than 1/0 AWG when inside a multi-conductor cable. The language does not provide allowance for a 1/0. You must meet the requirement of 250.122, and run the 4/0.

In summary


Base Requirement

Table 250.122 dictates that a 350 kcmil conductor requires a 4/0 AWG EGC.
2.

MC Cable Restriction

310.10(G)(5) only allows for sectioned EGCs smaller than 1/0 AWG to be used in parallel within a multi-conductor cable, provided the combined circular mil area is equal to or exceeds the requirement of Table 250.122.
3.

Your Situation

Since you have a 1/0 AWG EGC, you cannot use this exception. You must comply with the base requirement of Table 250.122 and have a 4/0 AWG EGC (or an equivalent combination of smaller conductors smaller than 1/0 that meet or exceed the circular mil area of a 4/0, within the MC cable).

Practical Implication:

This means you'd need to: Use MC cable with a 4/0 AWG EGC (or a suitable combination of smaller-than-1/0 EGC conductors within the cable that meets the required circular mil area).It's a subtle but critical point. While the idea of equivalent circular mil area is valid in some circumstances, this specific section of the code has a defined exception for conductors smaller than 1/0 AWG in multiconductor cables only. Flag for review