To add comments to JBD’s post, if it were a state requirement it would probably be found at a site like:
http://www.energy.ca.gov/title24/ In roughly the 15-300kVA range for low-voltage drytypes, there are three “grades” or temperature-rise ratings for transformers: 150°C, 115°C and 80°C, with the idea that lower operating temperature yields, I guess, lower losses. As mentioned, transformer efficiencies are based on no-load [iron] losses and load [copper] looses. It’s not hard to imagine the CEC again trying to legislate laws of physics.
