Hard Time Understanding 110.14 C

[payneeco]

Using Mike Holt Understanding the NEC Volume 1 to Study for Electrical Exam. Here is the question: What is the minimum size secondary conductor required for a single phase 1.5 KVA 480 to 120 V transformer that is protected with a 5A fuse on the primary. I understand the solution until it gets to step 4 and they use 60 degrees c to size wire on table 310.16. This gets me a lot because I don't fully understand 110.14 (C)(1) ....which unfortunately is referenced a lot in a broad range of questions. Can someone help me understand when to use 60 C versus 75 C versus 90 C. I'm looking for ways to master just this one small thing. I'm thinking it would go a long way on the exam if I just fully understood this one thing.

 

[augie47]

If the device at either end of the conductor such as a breaker or receptacle is rated 60 deg then you are limited to the 60 deg ampacity of the conductor. For example if you are using a THWN conductor rated 40 amps at 90 deg and 20 amps at 60 deg you would be limited to the 20 amp rating if the termination was dated 60 deg