The primary current of the 150 KVA transformer is about 180 amps.
Table 450.3(B) allows the primary overcurrent protection to be 250% of the primary current when secondary protection is provided.
180 x 2.5 = 450 amps.
On the secondary side we are allowed 125% and we can round up to the next standard size OCP.
The secondary current of a 150 KVA transformer is 416 amps.
They could have gone higher than 400 amps on the secondary side.