Rule 26-258: 125% of rated secondary current
150,000 / 208 / 1.732 x 1.25
primary conductors are also 125% in the same rule. It's the same calculation except 600 volts is substituted for 208 volts.
The same rule allows you to reduce the size to the demand providing... etc.
Overcurrent rules are in 26-256. Watch the ampacity of panels and splitters fed from transformers. They also must be rated for 125%.
For our Southern friends, 600/347 volts is common in Canada.
