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A quench test of a 13 T and 46 kA Nb3Al cable-in-conduit conductor was performed. Thermo-hydraulic quench back (THQB) was observed clearly for the first time in a large magnet when the temperature was set at 0.5 K lower than the current sharing temperature and normal zone propagation velocity reached 70 m/s. These results suggest possibility of the THQB should be considered in a design of a large magnet system in case that the operating temperature is considerably close to the current sharing temperature. Calculation results by one-dimensional stability and quench simulation code are compared with the experiment. Although the calculation results show some discrepancy with the experiment they support the conclusion that the THQB took place. Some improvements of the model seem necessary.