12.47 kV is the distribution line voltage (High side voltage). It could be 3 wire Delta 12.47 kV (no ground wire connection) or 4 wire WYE 7.2 kV (12.47/sqrt(3)=7.2) line-ground, 12.47 kV line-line). It depends on how you connect the transformer.
The low side winding of each single phase...