Speaking with Square D engineer, their theory is that is was not high inductive load (high amperage) that caused the melting, but a high resistitive load (example of a space heater), and that was caused by a defective compressor at the weakest link. They also stated that the odds of 2 different breakers not tripping under load is highly unlikely. If it was truly a high inductive load that the wire would not have melted at the far end.