Okay, let's break down the grounding conductor sizing for your 1500/2000kVA, 480V (no neutral) system with an underground run to an indoor switchboard. This is a common scenario, but the details are crucial for code compliance and safety.

Understanding the Configuration Transformer

1500/2000kVA outdoor transformer, likely 480Y/277V on the secondary (even if you aren't using the neutral).

Supervised Industrial Installation

This is important as it can influence some aspects of the code interpretation and allowances.

Secondary Conductors

Run underground from the transformer to an indoor switchboard. PVC conduit is used for this run.

Switchboard

Indoor switchboard operating at 480V (no neutral connection inside the switchboard).

Grounding

Neutral is bonded to the transformer enclosure at the transformer location only. No neutral conductor is run to the switchboard.

Grounding Conductor

You need to run a grounding conductor (Equipment Grounding Conductor - EGC) inside the PVC conduit with the phase conductors.

Key Question: Is this a Service or a Feeder?

This is the critical point. Since you are bonding the neutral at the transformer and not extending a neutral to the switchboard, and you only have the 3 phase conductors plus a grounding conductor being extended to the switchboard, this installation is considered a feeder NOT a service.

Code Sections to Consider


NEC 250.66 (Grounding Electrode Conductor Size)

This section applies to the grounding electrode conductor which connects the grounded conductor (neutral) to the grounding electrode system (e.g., ground rods, building steel).

This section is NOT applicable in your case because you are not extending a neutral to the switchboard. The GEC size is irrelevant to what runs to the switchboard.2. NEC 250.122 (Equipment Grounding Conductor Size)

This section applies to the equipment grounding conductor (EGC), which is used to bond non-current-carrying metal parts (conduit, enclosures, etc.) back to the source, providing a low-impedance path for fault current.

This IS the relevant section for your situation.Sizing the Equipment Grounding Conductor (EGC) per NEC 250.122

Since this is a feeder, you need to size the EGC according to NEC 250.122. The size of the EGC is based on the overcurrent protective device (OCPD) protecting the feeder conductors. Here's how that applies in your case: The OCPD

Your primary overcurrent protection for this feeder is going to be inside the switchboard. Find the highest-rated overcurrent device (circuit breaker, fuses) that protects the entire feeder (i.e., the main OCPD in your switchboard that is fed by the conductors from the transformer).

NEC 250.122 Table

Use NEC 250.122(B) and Table 250.122 to determine the minimum size of the EGC based on the OCPD rating. The table provides the minimum copper or aluminum wire size for the EGC.

Example

If the main breaker in your switchboard is rated at 2000A, Table 250.122 would require a minimum #3/0 AWG copper EGC or a 500 kcmil aluminum EGC.

Important Considerations and Clarifications



No OCPD at the Transformer

You mentioned that there is no overcurrent device at the transformer. This is not uncommon in supervised installations, but it is CRITICAL that the OCPD in the switchboard provides adequate protection for the conductors between the transformer and the switchboard. Verify that the ampacity of the conductors feeding the switchboard is sufficient for the breaker installed in the switchboard. If the transformer conductors are too small for the breaker in the switchboard, you may need a step down breaker or fuses on the line side of the switchboard main breaker to protect the feeders.

PVC Conduit

Because you are using PVC conduit, an equipment grounding conductor must be installed with the phase conductors. Metallic conduit could act as a grounding path, but PVC provides no grounding capability.

Continuous Lengths and Terminations

Ensure that your EGC is installed in continuous lengths (no splices unless properly made) and is securely terminated at both the transformer enclosure (where the neutral is bonded) and the switchboard enclosure.

Supervised Industrial Installation

While "supervised industrial installation" can allow some flexibility (e.g., under certain conditions, you might be able to use engineering judgment for some ampacity calculations), it does not generally override the fundamental grounding requirements of NEC 250. Always err on the side of caution and comply with the code.

Local Codes and AHJ

Always consult with your local Authority Having Jurisdiction (AHJ) to confirm their interpretation of the code and any local amendments. They have the final say.

In summary, your grounding conductor from the transformer to the switchboard should be sized according to NEC 250.122, based on the rating of the overcurrent protective device (OCPD) in your switchboard that protects the feeder conductors.Recommendation

Given the large kVA of your transformer and the potential for high fault currents, I strongly recommend:1.

Consult a Qualified Electrical Engineer

Have an electrical engineer perform a short-circuit study to determine the maximum available fault current at the switchboard. This will help you ensure your grounding system is adequate and that your equipment is properly rated.2.

Consider Redundancy: For critical applications, consider running a larger EGC than the minimum required by the code for additional safety and improved fault clearing.This information is for guidance only and should not be considered a substitute for professional engineering advice. Always consult with a qualified electrical engineer and follow all applicable codes and regulations. Good luck with your installation! Flag for review