This is a tricky situation, and you've already done some good troubleshooting. Let's break down the likely causes and solutions

Likely Causes (Ordered by Probability):

1. Transformer Inrush Current

You're absolutely right, transformer inrush current is the most likely culprit. Dry-type transformers can draw massive inrush currents (8-12x the rated current) for a very short period (milliseconds) when energized. This is due to the core not being initially magnetized, and the inductance acting like a short circuit for a brief moment. This inrush current is enough to trip the 800A breaker, especially with its trip unit set at its lowest setting.2.

Pre-Existing Fault in Downstream Panel/Wiring (Less Likely, But Important to Rule Out)

While you checked for opens and shorts in the transformer cabling, a subtle fault in the 400A panel or its downstream wiring could be present. This would only present itself during the initial surge.3.

Faulty "Spare" Transformer

While you were told the spare was good, its history is unknown. It's possible there's a shorted turn or other internal fault exacerbating the inrush current issue, making it trip the breaker more aggressively. (This is more probable considering the previous transformer failure due to lightning.)4.

Overly Sensitive 800A Breaker

While less likely, the 800A breaker could be overly sensitive due to age or internal damage.

Troubleshooting and Solutions (Prioritized)


Address the Inrush Current

This should be your first step.



Temporarily Adjust Trip Unit Setting (Cautiously)



Carefully increase the instantaneous trip setting on the 800A main breaker (MGP36800). Since you have a range of 2-10 times the max trip setting (which should be approximately 800A your adjustment setting #2), increasing the instantaneous setting is safe but you are reducing the protection.

Important Safety Note

Only adjust this temporarily for testing purposes. Record the original setting and immediately return it to the original setting (the number 2) once you've resolved the issue. Document the changes you make. It is also recommend that an experienced licensed professional perform this adjustment.

Staggered Energizing (Less Effective with Solid-State Trip Units)

In older breakers, staggering the closing of the breaker contacts might help. But with the SqD I-Line breakers, that is likely not a practical option.

"Pre-Charge" the Transformer (Difficult, Potentially Risky)

This is theoretically possible by using a Variac or other adjustable voltage source to slowly ramp up the voltage to the transformer. However, this is complex, requires specialized equipment, and could be dangerous if not done properly. I would not recommend this unless you have extensive experience with such procedures.2.

Isolate the Load



Disconnect Loads in 400A Panel

Turn off ALL breakers within the 400A, 120/208V distribution panel, including the main breaker. Energize the transformer and see if the 800A breaker still trips. If it doesn't trip, the problem is likely a load or wiring issue downstream of the transformer, within the 400A panel or its connected loads. Then you can systematically turn on breakers (one at a time) in the 400A panel to identify the offending load/circuit.

Disconnect Transformer Secondary

If the 800A breaker still trips with everything disconnected in the 400A panel (including the main breaker), the problem is likely the transformer itself, or in the wiring directly between the 225A breaker and the transformer primary. Disconnect the transformer secondary conductors from the 400A panel. Re-energize the 225A breaker. If the 800A breaker still trips, the transformer is almost certainly the problem.3.

Test the Transformer



Megger/Insulation Resistance Test

Use a megohmmeter (insulation resistance tester) to thoroughly test the insulation resistance of the transformer windings to ground and between windings. This can reveal hidden insulation breakdown that could be contributing to the inrush current issue. A low reading (close to 0) indicates a problem and likely confirms the transformer is faulty. Consult a qualified electrician familiar with transformer testing for proper procedure and interpretation of results.

Turn-to-Turn Test

A turn-to-turn fault is difficult to detect, but it can significantly increase inrush current. Specialized equipment is required for this test (e.g., a TTR - Transformer Turns Ratio tester).4.

Investigate the 800A Breaker



Breaker Testing

If you've ruled out the transformer and downstream issues, have the 800A breaker tested by a qualified electrician or testing service. They can verify its calibration and functionality.

Procedure


Safety First

Work de-energized whenever possible. Use proper PPE (Personal Protective Equipment). If working near live parts is unavoidable, follow strict safety procedures.2.

Isolate the 400A panel, per #2 above. Record Results.3.

If still tripping, Isolate transformer secondary, per #2 above. Record Results.4.

If still tripping, and if you have the necessary expertise and equipment, perform the Transformer tests in #3 above. Record Results.5.

If still tripping and you've ruled out the transformer and downstream issues, call a qualified electrician or electrical testing service for assistance in testing the 800A breaker.

Important Considerations



Lightning Damage

Lightning strikes can cause subtle damage that doesn't immediately manifest as a dead short. The "spare" transformer may have been affected by the same event, even if it wasn't directly hit.

Document Everything

Keep detailed records of all tests performed, settings adjusted, and results obtained. This will be invaluable for troubleshooting and for future reference.

Consult a Professional:* If you're not comfortable performing any of these steps, or if you're not getting clear results, contact a qualified electrician or electrical engineer for assistance. Working with electrical equipment, especially at these voltage and current levels, can be dangerous.Let me know the results of your isolation testing, and we can further refine the troubleshooting process. Good luck! Flag for review