Okay, let's break this down. You're troubleshooting a voltage problem on a 480V to 208V step-down transformer connected to a 120/208V high-leg delta service. Your primary (480V) voltage is reading high (560-565V), and you've already tried adjusting the taps.Here's a structured approach to help you diagnose and solve the issue

Understanding the System
High-Leg Delta

A 120/208V high-leg delta system is a 3-phase system where one phase (the "high leg" or "wild leg") is significantly higher in voltage to ground than the other two phases. Typically, the voltage from the high leg to neutral is around 208V (approximately 1.732 120V).

Delta-Wye Transformer

You likely have a delta-wye transformer to step down the 480V to 120/208V. The delta side is connected to the 480V, and the wye side provides the 120/208V.

Transformer Taps

Transformers have taps to compensate for variations in the incoming voltage. You've already tried adjusting them, which is good.

Possible Causes and Troubleshooting Steps1. Incoming Voltage Issues



Check the 480V Source Voltage

The first and most crucial step is to thoroughly investigate the 480V supply.

Measure All Three Phase-to-Phase Voltages

Use a quality multimeter to measure the voltage between each pair of the three phases on the 480V side at the source, not just at the transformer. This will tell you if the imbalance is coming from the utility. Look for:

High Voltage

Is the utility providing excessively high voltage? (This is unlikely but needs to be ruled out.)

Voltage Imbalance

Are the phase-to-phase voltages significantly different from each other? A large imbalance can cause problems.

Verify Phase Rotation

Ensure the phase rotation is correct. While incorrect rotation usually doesn't increase voltage, it can cause other issues. Use a phase rotation meter.

Utility Consultation

If you find that the incoming voltage is consistently high or unbalanced, contact the utility company immediately. They are responsible for providing voltage within acceptable limits.2.

Transformer Problems



Transformer Nameplate

Verify the nameplate voltage ratings of the transformer. Make sure it's designed for 480V on the primary (delta) side and 120/208V on the secondary (wye) side.

Transformer Connections

Double-check that the transformer is wired correctly according to the manufacturer's instructions.

Delta Side

Confirm the delta connections on the 480V side are properly made.

Wye Side

Confirm the wye connections on the 120/208V side are properly made, including the neutral point (X0).

Transformer Taps (Revisited)

Even though you've tried adjusting the taps, revisit this step after you've confirmed the incoming voltage is correct. Use the tap that brings the secondary voltage closest to 120/208V. If your incoming voltage is consistently high, a lower tap setting will be necessary.

Transformer Health

Is the transformer new or used? If it's used, have it tested to ensure it's not failing internally. A winding short or other internal fault can cause voltage abnormalities.3.

Secondary (120/208V) Load



Load on the 120/208V Panel

Are you measuring the 212-215V with a load connected to the 120/208V panel? If so, disconnect the load temporarily and measure the open-circuit voltage. A heavily loaded panel can influence the voltage readings.

Balanced Load

Ensure the load on the 120/208V panel is as balanced as possible across the three phases. A highly unbalanced load can contribute to voltage imbalances and potentially affect the transformer's output.4.

Neutral Connections



Neutral at Transformer

Make sure the X0 terminal on the transformer is properly grounded. A floating neutral can cause all sorts of unpredictable voltage issues.

Important Considerations Safety

Work on electrical systems is dangerous. Always de-energize the equipment and follow proper lockout/tagout procedures before working on any electrical components. If you are not a qualified electrician, hire one.

Metering

Use a true RMS multimeter for accurate voltage readings, especially with non-linear loads.

Documentation

Consult the manufacturer's documentation for the transformer and the car charger.

Grounding

Ensure proper grounding throughout the system. A good ground is essential for safety and proper equipment operation.

Specific Questions to Answer What is the kVA rating of the transformer? What are the tap settings available on the transformer? What is the actual phase-to-phase voltage at the 480V source? What is the make and model of the transformer and the car charger? What is the load connected to the 120/208V panel (other than the car charger)?Why a Neutral May (or May Not) Be Necessary 480V Side

You are correct; a delta-connected primary does not require a neutral. You only need the three phase conductors and a ground.

208V Side Whether a neutral is required depends on the load. Since you mentioned a car charger, it may not require the neutral.By systematically checking these items, you should be able to isolate the cause of the high voltage on your transformer. Start with the incoming 480V source voltage. It is the most likely culprit. Please let me know if you have any questions. Flag for review