Has anyone heard of derating CT’s for higher temp’s? Is there a standard associated with this? Or where would this have come from?
We currently use the nameplate for the CT limit, and do not derate for higher temp’s.
What is a CT?
Current Transformer, or Combustion Turban, depending on your conversation. Here I am asking about Current Transformers.
Oh, good, we use it for Cooling Tank. 
What? Really? I thought all this time we were talking about Coefficient of Thrust! At higher temperatures the CT for many rotors (helicopters, aircraft propellers, Wind turbines) can be substantially lower (air viscosity increases).
OK enough fooling. I’m actually sure this is about a current transformer, too. I don’t think it should be derated for a nominal range of operation… but if you use it in extremes other effects could come into play. What CT are you working with, and what temperature are you talking about?
Mostly breaker CT’s, and our temperature has only exceeded 100 F once in recent records. But our above sea level living might come into play.
Apparently, there is something new in IEEE C57.13. I don’t have a copy yet to see what it is.
I have been going over posts that I didn’t see when they were posted and came across your question.
Siemens has a very good explanation complete with graphs and calculations.
See their site:
“Current transformers have a thermal-rating factor, as specified in the instrument transformer standard, IEEE Std C57.13. Due to the number of questions Siemens receives about the thermal-rating factor (TRF), it appears there is confusion what the practical meaning of the TRF is.”