Okay, let's break down this grounding electrode conductor (GEC) conundrum. You're dealing with a common, but potentially problematic, practice, and it's good you're questioning it.

The Practice You Describe

You're describing a system where multiple grounding electrodes (e.g., ground rods, building steel) are connected to an external copper ground bar separate from the service equipment enclosure, and then a single, sized conductor (usually 3/0) runs from that external ground bar to the service equipment. This 3/0 is being used as the GEC. This setup is also being replicated in other electrical rooms, with similar external ground bars connecting transformer GECs.

Is it Legal? (Generally, NO, but with a significant BUT) The Problem with the "Traditional" Interpretation of the NEC

The way you initially described the system, it sounds like the sole connection between the grounding electrodes and the service equipment is via that single 3/0 conductor from the external ground bar. This generally violates NEC 250.24(C)(1) which requires the GEC to be connected within the service equipment, or the bus bar. The code is very specific that the GEC must connect directly to the equipment grounding conductor (EGC) system at the service equipment, or be connected to a bus specifically designed to connect to the grounding electrode system.

Why the Code Requires Direct Connection

The intent is to ensure the lowest possible impedance path from the fault back to the source. This means a direct, sized connection from the grounding electrodes to the service equipment.

The BUT - The '250.64(C) Exception' and the Possibility of Legality

This is where the nuance comes in. NEC 250.64(C) addresses the continuous GEC and enclosures and permits the following:"...the grounding electrode conductor shall be permitted to be spliced by listed irreversible compression-type connectors listed for the purpose or by the exothermic welding process.

Grounding electrode conductors connected to ground rings, concrete-encased electrodes, and ground rods shall be permitted to be spliced by exothermic welding, listed connectors, or clamps.

Interior metal water piping shall be permitted to be spliced by exothermic welding, listed connectors or listed clamps"In other words, this section is addressing splices, so as long as the splice is listed or welded the GEC is still compliant.

So, where does that leave us? The question comes down to how the GEC is 'spliced' to the external ground bars. It would need to be done in such a way that it meets the code definition, while remaining properly sized for the entire GEC system, including the electrodes.

Key Considerations to Make It (Potentially) Compliant


GEC Sizing

The size of the conductor running from the external ground bar to the service equipment (your 3/0 example) MUST be sized according to 250.66 based on the size of the service entrance conductors. It's not enough to just arbitrarily run a 3/0. You need to size it appropriately, it just acts as a splice to a GEC that runs to the electrodes.
2.

Listed Connectors

All connections to the external ground bar must be made with listed connectors suitable for the purpose (i.e., grounding and bonding connectors). Bolted connections alone are insufficient in most cases. Compression lugs and exothermic welds are generally favored for reliability.3.

Permanent and Secure

The external ground bar and all connections must be mechanically secure and protected from physical damage.4.

Inspection Approval

Crucially, the AHJ (Authority Having Jurisdiction) must approve the installation. The AHJ is the final arbiter of code compliance. If they don't like the setup, it doesn't matter what anyone else thinks.

Why the External Ground Bar is Still Problematic (Even if "Legal")



Increased Impedance

While compliant to code, relying on multiple bolted connections outside the service equipment inherently introduces impedance (resistance) to the ground fault path. This can reduce the effectiveness of the grounding system in clearing faults quickly.

Maintenance

External connections are exposed to the elements and are more prone to corrosion and loosening over time, requiring regular inspection and maintenance.

Lack of Redundancy

If that single conductor from the external ground bar to the service equipment fails, you lose the entire grounding electrode system.

Your Better Approach (The One You're Thinking Of)

You are absolutely right in your thinking! This is generally the preferred and more robust approach:1.

Connect Grounding Electrodes Directly to the Service Equipment

Run separate GECs from each grounding electrode (ground rod, building steel, etc.) directly to a grounding bus inside the service equipment enclosure. Size each GEC according to 250.66, based on the individual electrode's requirement (smaller for ground rods, larger for building steel).2.

Bonding Jumper to External Ground Bar

Then, if you need to extend the grounding electrode system to other locations, run a properly sized bonding jumper (also sized according to 250.66, based on the service entrance conductor size) from the service equipment's internal ground bus to your external ground bar. This makes the external ground bar an extension of the service grounding system, not its primary connection.3.

Extend to Other Electrical Rooms

From that external ground bar, you can then run bonding jumpers to other electrical rooms to connect to their grounding electrode systems (transformer GECs, for example).

250.30(A)(5) and (A)(6) - Separately Derived Systems (Transformers)

Yes, 250.30(A)(5) and (A)(6) specifically address the grounding of separately derived systems (transformers, generators, etc.). They permit the connection of the GEC for the transformer to a ground bus within the same building or structure as the separately derived system, but it MUST be connected to the grounding electrode system.

Key takeaway: A separate, dedicated grounding electrode conductor must run from the transformer's neutral point (X0) to a grounding electrode (or the building's grounding electrode system).

In Summary

The "typical" installation you describe is potentially problematic and relies on very careful execution and inspection to be compliant. It also introduces potential reliability issues. The BEST practice is to connect GECs directly to the service equipment's internal ground bus and then extend the grounding system with bonding jumpers to other locations. 250.30(A)(5) and (A)(6) allow you to use ground buses in separate electrical rooms for transformer GECs, but those buses must be properly connected to the grounding electrode system. ALWAYS consult with your local AHJ for their specific requirements and interpretations.

I strongly advise you to review these points with a qualified electrical engineer and your local AHJ to ensure your installations are safe, compliant, and reliable. Grounding is critical for safety, and it's worth getting it right. Flag for review