The answer is yes, you absolutely need a ground conductor (equipment grounding conductor, or EGC) ran with the current carrying conductors (L1 and L2) to the primary side of the transformer. This is for safety and to ensure proper operation of overcurrent protection devices in the event of a fault.Here's why and the relevant code references: Safety

The equipment grounding conductor (EGC) provides a low-impedance path back to the source (the switchboard) for fault current. This allows overcurrent protection devices (circuit breakers or fuses) to quickly trip and de-energize the circuit in the event of a ground fault. Without an EGC, a ground fault can result in dangerous voltage on the transformer enclosure and connected equipment, posing a shock hazard.

Overcurrent Protection

Overcurrent protection devices (OCPDs) are designed to trip when a fault current flows. The EGC ensures a high enough fault current so the OCPD can trip quickly and clear the fault.

Specifics for Your Situation (480V to 120/240V, 50kVA Transformer)


Size of the EGC

The size of the EGC is based on the rating of the overcurrent protection device protecting the circuit. The National Electrical Code (NEC) (NFPA 70) specifies these requirements. Refer to

NEC Table 250.122 to determine the proper size.2. Type of EGC

The EGC can be a wire (copper or aluminum), a rigid metal conduit (RMC), intermediate metal conduit (IMC), electrical metallic tubing (EMT), or other approved raceway.3.

Installation

The EGC must be installed with the circuit conductors in the same raceway, cable, or trench. This ensures that the impedance of the fault current path is minimized.4.

Separately Derived System

A transformer is considered a separately derived system.

NEC Article 250.30 specifies the grounding requirements for separately derived systems. This includes bonding the transformer enclosure to the EGC and establishing a grounding electrode system (GES) for the secondary side of the transformer.

Important NEC Articles to Consult



NEC Article 250: Grounding and Bonding

This is the core article covering all aspects of grounding and bonding. Pay close attention to the sections on equipment grounding conductors (EGCs) and separately derived systems.

NEC Article 450: Transformers

This article covers the installation requirements for transformers.

NEC Article 300: Wiring Methods

This article covers the general requirements for wiring methods, including the installation of EGCs.

Example

Let's say the primary side of your 50kVA transformer is protected by a 70A circuit breaker. According to

NEC Table 250.122, you would need at least a #8 AWG copper EGC.

Conduit vs. Cable



Conduit

If you're using conduit (RMC, IMC, or EMT), the conduit itself can serve as the EGC if it's properly installed with approved fittings to ensure electrical continuity. However, it is considered best practice and sometimes required to run a wire EGC inside the conduit even when the conduit itself could be used. This ensures a reliable ground path, especially in environments where corrosion or mechanical damage could compromise the integrity of the conduit.

Cable

If you're using a cable assembly, the cable must include an equipment grounding conductor.

Disclaimer

This information is for general guidance only. Electrical work can be dangerous. You must consult the current edition of the National Electrical Code (NEC) and other applicable local codes and regulations. A qualified and licensed electrician should perform all electrical work to ensure safety and compliance.*In conclusion:

DO NOT operate the transformer without an properly sized EGC. It is a critical safety component. Failing to install it creates an unsafe situation. Flag for review