Okay, let's break down what's likely happening and how to troubleshoot this. The symptoms you describe point to a grounding/bonding issue and potentially a phase issue with your 480V generator connection. Since you are using it to step down instead of up it changes some of the troubleshooting.Here's a structured approach to understanding and fixing the problem

1. Understanding the Expected Behavior:
Backfeeding Basics

Transformers are generally bidirectional. A transformer designed to step up voltage can also step down voltage if the connections are reversed.

Transformer Voltages



Original Configuration (Step-Up)

240V/480V Primary (input) to 480V/240V Secondary (output). This is a split single phase.

Reversed Configuration (Step-Down)

480V/240V Primary (input) to 240V/480V Secondary (output).

Your Goal

You want to input 480V 3-phase on the high-voltage side of the transformer and get 240V split single phase out of the low-voltage side.

2. Analyzing Your Symptoms



Input

480V between each phase. Okay so far.

Output (Weirdness)

240V between the hot legs on the 240V side, but 22V to ground on one leg and 240V to ground on the other. This is the key indicator of a problem. This suggest you are not getting a neutral out of the transformer correctly.

3. Potential Causes and Troubleshooting Steps



Grounding/Bonding (Most Likely Culprit)



Generator Grounding



This is critical. Is your generator properly grounded? The generator's frame must be connected to a grounding electrode (ground rod) at the generator location, and if used as a separately derived system, needs to be bonded correctly. Transformer Grounding (X0)

The neutral point (often labeled "X0" or similar) on the low-voltage (240V) side of the transformer must be properly bonded to ground. This is often done in the panel where the 240V is being fed. The utility neutral is usually connected here when you are not using the generator. When you switch to generator power this neutral must be correctly grounded to allow the correct voltage to ground. If this neutral isn't properly bonded, you won't have a stable 120V-to-ground reference on each leg.

Inspect the Bonding Jumper

Look for a thick copper wire connecting the transformer's X0 terminal to the ground busbar in the panel. Ensure it's secure and properly sized.

Check for Ground Loops

Are there any unintended ground paths that could be interfering with the system? This is less likely but possible.

Phase Rotation/Wiring Errors (Less Likely, but Check)



Incorrect Phase Rotation

If you are inputting 3-phase 480V, it may have phasing implications for the output. But with only two output wires it shouldn't matter.

Miswiring

Double-check that you haven't accidentally swapped any input or output wires. Follow the transformer's nameplate and wiring diagram.

Transformer Issues (Less Likely, but Check)



Winding Fault

It's possible (but less likely) that the transformer has a winding fault. This would require more specialized testing (insulation resistance test, turns ratio test) with a megohmmeter.

Overheating

Is the transformer overheating when you run the generator? If so, stop immediately.

Generator Output Issues



Voltage Stability

Is the generator maintaining a stable 480V output under load? Check the generator's voltage regulator.

Frequency

Is the generator producing the correct frequency (60 Hz in the US)?

4. Detailed Troubleshooting Steps


Verify Generator Grounding

Make absolutely sure the generator's frame is connected to a grounding electrode. If it's a portable generator, this is usually a ground rod driven into the earth.2.

Inspect Transformer X0 Ground Connection

Carefully examine the connection from the transformer's neutral point (X0) on the 240V side to the ground busbar in the load center. Make sure it's a solid connection with no corrosion.3.

Check Voltage to Ground

With the generator running and connected, measure the voltage from each of the 240V output legs to the ground busbar in the panel. You should see approximately 120V on each leg.4.

Verify Wiring

Double-check that the 480V input is connected to the correct terminals on the high-voltage side of the transformer, and that the 240V output is connected to the correct terminals on the low-voltage side. Refer to the transformer's nameplate and wiring diagram.5.

Neutral Conductor

There should be a neutral conductor going from the transformer to the panel.

5. Safety Precautions



De-energize before working

Always disconnect power at the generator and the main breaker before working on any electrical connections.

Use proper PPE

Wear appropriate personal protective equipment (safety glasses, insulated gloves).

Qualified Electrician

If you are not comfortable or experienced with electrical work, hire a qualified electrician to perform the troubleshooting and repairs. Working with electricity is dangerous.

Example Scenario & Solution

Let's say you find that the transformer's X0 neutral wasn't connected to the ground busbar in the panel. Here's what would happen:

Without a Neutral Bond

The 240V output legs would "float" relative to ground. One leg might be close to ground potential (hence the 22V reading), while the other leg would be much higher (closer to 240V). This is because there's no stable reference point for the voltage.

Solution: Properly connect a properly sized grounding conductor from the transformer's X0 terminal to the ground busbar in the panel. This provides a stable neutral reference, and you should then see approximately 120V from each leg to ground.By following these steps and carefully inspecting the grounding and wiring, you should be able to identify and resolve the issue. Good luck, and be safe! Flag for review