Three-phase motor trips the breaker only when the line is fully loaded

What we told them

A motor that only trips under full load with a clean bench test is usually suffering voltage imbalance, not a motor fault. Measure phase-to-phase voltage at the terminals while the whole line is running, not at idle. A 2% imbalance produces disproportionate current rise and heat. In this case one phase was sagging because an unrelated heater bank had been added to the same supply years earlier. The motor was the victim, not the cause.