Transformer Losses And Impedance @ 75 Deg C?

My guess would be
20 deg C ambient
55 deg C transformer temperature rise
=75 deg C

then what about if transformer temperature rise is 50 deg C, or average ambient temperature is 32 deg C ?

Electrical characteristics change with temperature. Specifying a test temperature insures that temperature is not a factor in determining performance of the device.

" Specifying a test temperature insures that temperature is not a factor in determining performance of the device."-- how? in transformer temperature plays a vital role in determining the performance (overloading and Hot-spot temperature)

Nice try Lyn but you're obviously not reaching this guy. Let me try to answer this.

Excessive heat is the bane of any transformer. The series resistance of the copper windings increases with temperature. Breakdown voltages of insulation drops with higher temperatures. So even though most transformers will not routinely experience high temperatures, testing them at higher temperatures will more likely reveal a flaw than testing them at expected temperatures. However, one should not test a transformer at so high of a temperature that transformer failure is guaranteed. 75°C happens to just be an easy to remember number using the standard temperature scale the world uses today that that is both hot but not too hot. Had we agreed to use the Newton scale for temperature instead then transformers would likely be tested at the slightly cooler 25° N temperature.

Thanks! Maybe I should have just said it's the reference temperature for these tests.