You've hit on a valid concern. Here's a breakdown of why the supplier's statement is concerning and what considerations you need to ensure a safe and code-compliant installation

Why the Supplier's Statement is Problematic
Fault Current Path

In a single-phase transformer installation, a fault on the primary side must have a reliable, low-impedance path back to the source (the utility transformer) to allow sufficient fault current to flow and trip the upstream overcurrent protective device (OCPD), such as a fuse or circuit breaker. Without this low-impedance path, a ground fault can linger, posing a significant safety hazard:

Overheating

The fault current, if limited by high impedance, can cause equipment overheating and potential fire.

Electrocution Hazard

Touch voltage could rise to dangerous levels.

NEC Requirements

The National Electrical Code (NEC) has specific requirements for equipment grounding and bonding to ensure a safe and effective fault current path.

How Faults are Typically Cleared in Transformer Installations

There are two main ways to provide a fault current path on the primary side of a transformer:1. Equipment Grounding Conductor (EGC)

This is the most common and recommended approach. You run a dedicated EGC (typically copper or aluminum) alongside the primary phase conductor from the utility source to the transformer. This EGC connects to the transformer's enclosure and is bonded to the grounded neutral conductor at the source (utility transformer).2.

Metallic Conduit

If the primary conductors are installed in metallic conduit (rigid metal conduit, intermediate metal conduit, or electrical metallic tubing), the conduit itself can serve as the EGC, provided it meets specific requirements in the NEC regarding its electrical continuity, and it is properly grounded/bonded.

Considerations and Next Steps1. Confirm Utility Requirements

The utility might have specific requirements for grounding that take precedence. Contact the utility company and get their official stance on the primary-side grounding for this transformer installation in writing.2.

Review the NEC

Consult Articles 250 (Grounding and Bonding) and 450 (Transformers) of the NEC for specific requirements. Pay close attention to:

250.4(A)(5) and (B)(4)

These sections emphasize the need for a low-impedance path for fault current.

250.118

This section lists acceptable types of EGCs.

250.130

Methods of grounding fixed equipment.

450.10

Grounding transformers.3.

Impedance of the Utility Ground

If the fault current path relies on the earth, and not on a direct metallic ground connection, there will be a very high impedance path.4.

Determine the Appropriate EGC Size

The size of the EGC is based on the rating of the overcurrent protection device (OCPD) protecting the primary conductors (refer to NEC Table 250.122).
5.

Consider Conduit as an EGC

If using metallic conduit as the EGC, ensure all couplings and connections are wrench-tight and listed for grounding purposes. Bond the conduit to the transformer enclosure and to the grounded neutral at the source.6.

Distance to Utility and Impedance

A "fairly long run" is a key factor. Longer runs mean greater impedance in the fault current path. This reinforces the need for a dedicated EGC alongside the primary conductor. The utility can perform a fault current study to determine the available fault current at the transformer location. This will help determine the required size of the EGC.7.

Documentation and Approval

Document your chosen grounding method in your electrical plans and obtain approval from the local electrical inspector before starting the installation.

In summary, the supplier's advice is incomplete and potentially unsafe. You must provide a reliable, low-impedance fault current path back to the utility source. A dedicated EGC is generally the best practice, and the NEC requires it in many situations. Consult with the utility and the electrical inspector to ensure a safe and code-compliant installation. Flag for review