Okay, a voltage imbalance like that with a good incoming supply and a dedicated neutral is a serious issue that needs careful and methodical troubleshooting. Here's a breakdown of potential causes and a plan to investigate

Understanding the Problem
Voltage Imbalance Symptoms

The 90V/150V split on a 240V system is significant. It indicates a major impedance difference between the two legs of the circuit. This can stress equipment and potentially damage it.

Dedicated Neutral

You've wisely eliminated a shared neutral as the culprit. This rules out some common causes of imbalance.

400A Service, Multiple Panels

This is important because problems could be either at the 200A panel or somewhere else on the 400A service affecting the 200A panel.

Possible Causes (in order of likelihood/ease of checking)


Loose Connections

The most likely culprit.



Main Breaker Connections (200A Panel)

Turn off the main breaker and carefully check the tightness of the connections on both the line and load sides of the main breaker. Use a torque screwdriver to ensure they are within the manufacturer's specifications. Look for signs of arcing, burning, or discoloration. Even if they look tight, re-torque them. This is the first thing to do.

Neutral Connection (200A Panel)

Double-check the tightness and condition of the neutral connection where your new dedicated neutral lands in the 200A panel. Also, inspect the neutral bus bar itself for corrosion or loose screws.

Service Entrance Connections

This is more dangerous and might require utility involvement. If you suspect a problem at the meter or the connection from the utility, contact them immediately. Do not attempt to work on the service entrance without the utility's involvement.2.

Faulty Main Breaker (200A Panel)

A failing main breaker can introduce resistance on one leg, causing the imbalance.

Testing

With the main breaker off, use an ohmmeter to measure the resistance through the main breaker on each leg. Compare the readings. A significant difference indicates a faulty breaker. Replace it.

Caution

This test requires the breaker to be off and the power removed.3.

Internal Panel Damage

Rare, but possible. Look for signs of physical damage, arcing, or corrosion inside the 200A panel.

Inspection

Carefully inspect the bus bars, wiring, and components within the panel for any damage. If you find anything suspicious, consult with a qualified electrician experienced in panel repairs.4.

Mis-wired Branch Circuits in the 200A Panel

Someone may have connected the neutral of one circuit to the hot leg of another, creating an imbalance.

Voltage Check at Branch Circuits

Methodically check the voltage from each hot leg to neutral at each branch circuit breaker. If you find one or more circuits that are significantly off (e.g., close to 0V), that's a major clue. Turn off the circuit and investigate the wiring thoroughly. You should see close to 120 volts from each leg to ground/neutral, and close to 240 volts from leg to leg.5.

Large, Imbalanced Loads

Even with a good panel, a massive imbalance in load can slightly affect voltages. However, a 90V/150V split is far beyond what a normal load imbalance would cause. It's unlikely this is the sole cause, but worth considering.

Measure Current

Use a clamp meter to measure the current on each leg of the main feed to the 200A panel. A large difference could be a contributing factor. Determine if they are running unusual/large loads on a single leg.6.

Problems in Other Panels Fed by the 400A Service

Although less likely with a dedicated neutral, a severe fault or ground fault in another panel downstream of the 400A service could potentially influence the voltage at the 200A panel. It's more likely to trip a breaker, but it's worth investigating if the problem persists after checking the 200A panel thoroughly.

Isolate Panels

To test this, you could sequentially turn off the main breakers to the other panels fed by the 400A service and see if the voltage at the 200A panel improves.7.

Transformer Issues (Very Unlikely, But Possible)

If the 400A service is fed by a transformer that has issues (winding fault, etc.), this could cause a voltage imbalance. This is rare.

Utility Involvement

This would require the utility to investigate.

Troubleshooting Steps


Safety First

Turn off the main breaker to the 200A panel before doing any work inside. Use proper PPE (insulated gloves, safety glasses). Verify the power is off with a reliable voltage tester.2.

Check and Re-torque Connections (as above). Start with the main breaker.3.

Measure Resistance of Main Breaker (as above).4.

Inspect Panel (as above).5.

Check Branch Circuit Voltages (as above).6.

Measure Current on Main Feeds (as above).7.

Isolate Other Panels (as above).8.

Contact Utility (if needed).

Important Considerations



Qualified Electrician

If you are not completely comfortable working with electrical panels, hire a qualified electrician. This is not a job for amateurs.

Documentation

Document everything you do, including voltage readings, resistance measurements, and any visual observations. This will help you track your progress and communicate the problem to others.

Tile Installer

While they may have contributed by overloading a circuit, the voltage imbalance suggests a pre-existing or underlying problem that was exacerbated by the load. Don't automatically blame them.

In Summary:Start with the loose connections*. That's the most common cause. Be methodical, be safe, and don't hesitate to call a qualified electrician if you're unsure. Good luck! Flag for review