Transformer Class of Insulation

Modern oil filled transformers are almost always rated for 55/65 degree centigrade above ambient. The really old and a few very poor quality transformers are rated only at 55 degree, but before the 1960's most transformers were capable of 65 degree rise over ambient. This was due to "thermally upgraded insulation" which was all that was available to me in designing transformer back then. Few manufacturers were using the old insulation. Some transformers were rated 65 degree only but the savings were marginal, since the only savings were calculations amounting to 5 minutes work of an engineer with out computers or calculators and the stamping of number on the nameplate. You referred to insulation class and that is incorrect as applied to transformers. I have seen the insulation class pertaining to motors stamped on the nameplate of some poor quality transformers, but the is not properly applied. The insulation is one thing, but the oil is another. Research was done to try to push the temperature higher, but cost and losses in the transformer showed that 65 was optimal. With a 65 rise which is the average rise, the hot spot is more than that, 80 degrees. With a standard 40 degree maximum the hot spot will be 120C and the alarm or trip may be set above that with an AWR hot spot guage/alarm which reads a combination of the top oil temperature and a heated well corresponding to the current in the winding via a CT (current transformer)

Bottom line, the temperature rating 55/65 must be on the nameplate if it meets ANSI or NEMA specification for the USA. I am sure that most other countries require the same information. At least they did when I was designing, we shipped transformers all over the world. What size transformer are you talking about? Really small distribution transformers may not have the same requirements about nameplates, and are frequently 65C only.