George
Look at the top of table 310.16. It gives you the information related to the table. If you have #12 TW with 25 amps continuous(3 hrs)with an environment temp of 30C then the conductor temp will rise to 60C. If the
conductor is #12 THWN(90C rating) with a
continuous load of 25 amps with the environment of 30C, the conductor temp will rise to 75C. If the load is 30 amps the #12 will rise to 90C. The type of insulation makes the difference. The code only allows #12 to have a breaker rated at 20 amps so the temp would be lower than 60C.
