You're on the right track with 250.30(A)(5) and understanding the challenge of providing an effective ground path in your precast concrete building. Let's break down your proposal and address the potential issues

Understanding the Code and Your Proposal
250.30(A)(5) "Common Grounding Electrode Conductor." This section explicitly allows for the use of a single conductor to serve as both the equipment grounding conductor (EGC) for the feeder and the grounding electrode conductor (GEC) for the separately derived system (SDS) provided it meets the size requirements for both. This is your key provision. Your Proposal

You're suggesting upsizing the 480V EGC feeding your transformers to also serve as the SDS's GEC. This conductor would run from the originating panelboard (either service equipment or subpanel) to the transformer and be bonded to the transformer's X0 terminal and the building's grounding electrode system (GES) at the transformer location.

Analysis and ConsiderationsArguments in Favor of Your Proposal (Code Compliant Potential)



Code Compliance

As you pointed out, 250.30(A)(5) directly addresses this scenario. If the EGC is properly sized, this is a code-compliant method.

Simplicity

Using a single conductor can simplify the installation and reduce material costs.

Potential Issues and Points to Carefully Consider


Sizing the Conductor

This is critical. You must size the conductor to meet the requirements of both the EGC and the GEC.

EGC Sizing (NEC 250.122)

The EGC size is based on the rating of the overcurrent protection device protecting the 480V feeder to the transformer. Use Table 250.122 to determine the minimum required size.

GEC Sizing (NEC 250.30(A)(6))

The GEC size is based on the size of the derived ungrounded conductors (the conductors carrying 208V on the secondary side of the transformer). Use Table 250.66 to determine the required size.

The larger of the two (EGC or GEC) MUST be used for the entire length of the conductor.2. Continuous Run

Ensure the conductor is a continuous run from the originating panel to the transformer X0 and connection to the GES. No splices within the raceway are permitted.3.

Grounding Electrode System Connection

The most critical part of this plan. You must establish a reliable, low-impedance connection to the building's grounding electrode system at the transformer location. Since you cannot rely on the rebar, you'll need to:

Existing Grounding Electrode(s)

Look for existing grounding electrodes (e.g., a ground rod, ground ring, or metal water pipe that qualifies as a grounding electrode) near the transformer location. Connect the upsized EGC/GEC to one of these. The "nearest available grounding electrode" per the code should be used.

Supplemental Grounding Electrode(s)

If no suitable grounding electrode is readily available, you'll need to install one or more supplementary electrodes. Common options include:

Ground Rod(s)

Drive one or more ground rods. Be mindful of soil conditions and the resistance-to-ground requirement (25 ohms or less if using a single rod, or supplemental if using multiple rods).

Ground Plate

Bury a ground plate.

Concrete-Encased Electrode (Ufer Ground)

If possible, this is often the most effective option, but it requires embedding a conductor within the concrete foundation (usually not feasible in precast). This is more likely if there's a poured-in-place concrete slab at the transformer location.4.

Enclosure Bonding

Ensure the transformer enclosure itself is properly bonded. This is typically done by bonding the enclosure to the EGC.5.

Mechanical Protection

Protect the GEC/EGC conductor from physical damage. Consider using conduit or other appropriate protection, especially where exposed.6.

Documentation

Document your grounding system design clearly on the electrical drawings. This is important for future maintenance and modifications.7.

Inspection

Be prepared to thoroughly explain your grounding system design to the electrical inspector. Highlight the code sections you are complying with (250.30(A)(5)).

Addressing Your Specific Questions

"Can I increase the size of the wire-type EGC in the primary (480 volt) feeder so that it is equal to the size of the GEC required by the step-down transformer and use one conductor for both the 480 volt EGC and the transformer GES bonding conductor?"

Yes, assuming it meets all the criteria outlined above: proper sizing based on both EGC and GEC requirements, continuous run, proper connection to the grounding electrode system, and enclosure bonding. "If the 480 volt feed for a given transformer originates in a sub-panel, is it suitable to use the EGC from the service equipment all the way to the transformer as the GEC for the transformer as long as the size of the EGC for its entire length from service equipment to transformer is equal to or larger than the transformer GEC size requirement?" Yes, this is generally acceptable. The key is that the conductor must be sized appropriately for its entire length and must be continuous. It essentially becomes part of the grounding electrode conductor system extending from the service entrance.Alternative Solutions (If your primary proposal proves problematic)



Isolated Grounding Electrode System

Install a dedicated grounding electrode system specifically for the transformers. This might involve multiple ground rods, a ground plate, or a combination. This could be more expensive, but might be easier to implement in certain situations. The primary benefit is that you don't have to upsize the feeder EGC.

In summary: What you're proposing is a code-compliant solution, provided* you meticulously follow the sizing and installation requirements. The biggest challenge will be ensuring a reliable connection to the building's grounding electrode system. Careful planning, proper documentation, and clear communication with the electrical inspector are essential for a successful installation. Flag for review