Okay, let's break this down and address the owner's preference for running the grounding electrode conductor (GEC) inside.

First, the NEC basics



NEC Article 250 covers grounding and bonding. You're right to focus there. GEC (Grounding Electrode Conductor)

This is the wire that connects the service equipment (meter base and transfer switch in your case) to the grounding electrode system (ground rods).

Here's the analysis of your proposed plan


Connecting to the Meter Base and Transfer Switch

Connecting the GEC to both the meter base and the transfer switch is generally acceptable and often required for good grounding. Make sure the connection is done with listed connectors.2.

Running Inside

This is where the concerns arise.



Protection from Physical Damage

NEC 250.64(B) addresses physical protection of the GEC. Running it inside along the framing does NOT automatically provide this protection. The GEC would need to be adequately protected from physical damage along its entire run. The GEC needs to be as short and as straight as practical for the path chosen.

Material of GEC

This conductor needs to be suitable for the application. Generally, a bare copper conductor is used.

Single Conductor

This can be run exposed, but as mentioned it needs to be protected.

Sizing

It is imperative that the GEC is properly sized based on the service entrance conductors. Using a conductor of inadequate size would violate NEC requirements. This is covered in NEC 250.66.

Continuous Run

The GEC should be a continuous run without splices (or be irreversibly spliced with a listed method), unless it's busbar running the full length of the building, or it is electrically bonded at the splice with listed splice kit.3.

"Rat Wall" Penetration

Drilling through the rat wall to reach the ground rods could be okay, but it creates another point where the GEC is vulnerable. You'd want to properly seal the penetration to prevent rodent entry and ensure no sharp edges can damage the wire.

Why your traditional method (separate conduit) is generally preferred



Easy Protection

Rigid metal conduit (RMC) or intermediate metal conduit (IMC) provides excellent physical protection for the GEC. It meets the requirements of NEC 250.64(B).

Clean Installation

Conduit provides a neat and professional-looking installation.

Code Compliance

Running the GEC in conduit is a straightforward and commonly accepted method, making code compliance easier to demonstrate.

Alternatives and Considerations



EMT Conduit

If the area is not subject to severe physical damage, you could consider using Electrical Metallic Tubing (EMT) inside to protect the GEC. However, remember that EMT offers less physical protection than RMC or IMC.

Guarding

If you absolutely must run the bare wire along the framing, you would need to consider installing guarding to protect it from physical damage.

Local Codes

Always check with your local electrical inspector. They may have specific requirements or preferences regarding grounding electrode conductors.

In conclusion

Running the GEC inside could be permissible, but it requires careful planning to ensure physical protection. Running the conductor down the inside framing is unlikely to meet code requirements, unless the building is protected with a raceway. If possible, it's advisable to continue with a raceway or conduit down the exterior of the building.

Recommendations


Consult with the electrical inspector. Explain your plan and get their feedback before you start the work.2.

Consider EMT

If the owner is against RMC or IMC on the exterior, explore using EMT conduit inside for the entire run.3.

Prioritize Protection

If you must run the bare wire, implement robust guarding to prevent damage.4.

Re-emphasize the Value of the Traditional Approach

Explain to the owner that the traditional method provides the best protection, easiest code compliance, and often the most cost-effective solution in the long run.5.

Document Everything:* Regardless of which method you choose, thoroughly document the installation, including wire sizes, connections, and protection methods.Good luck with the project! Safety is always paramount. Flag for review