Thats probably about 10,000 amps unfused off the xformer . . .
If it is a normal subdivision with no electric heat homes, it is a 25 kVA transformer, will not put out more than 7700 amperes, and will drop most of it going through the secondary and service drop. However, from the looks of the arc, you are correct, it is probably coming from a 50 or 75 kVA.
The transformer is probably fused to protect the line and will not blow until the fault is pretty solid at the bushings or there is an internal fault. We use a 15 T fuse on a 25 kVA transformer at 7.62 kV. The T rated fuse will carry 150% indefinitely. Do the math and you will see that the transformer is not protected. :smile: