I have recently had several discussions about only using the 90°C column for conductor sizing when fill and/or ambient require derating.
Tell me if I've got this right.
Here's an example scenario: A piece of equipment has an MCA of 72 amps. The unit and supply conductors will be in a 125°F ambient. If ambient temperature did not require a derate, #4 THHN would be selected since it has a 75°C ampacity of 85 amps. However, the ambient is 125°F and a derate is going to be required. The derate factor for THHN is .76 in a 125°F ambient. Going by the 90°C column and applying a .76 derate, the same #4 THHN has an ampacity of 72.2 amps. The #4 THHN is still adequate because going by the 90°C column to derate, the conductor ampacity exceeds the MCA by 0.2 amps. A small margin, but still acceptable, correct?
One point of confusion that I had is when the derated ampacity falls below that of the 75°C column such as in this example.
Does the derated ampacity need to meet or exceed the 75°C column ampacity or can it be utilized up to its derated ampacity below the 75°C column ampacity.
If it falls below the 75°C column ampacity and is utilized up to that derated ampacity, will it exceed the temperature limitations of 75°C rated terminations?
What if the wire is derated from the 90°C column and the derated ampacity is above that which is in the 75°C column? Can the wire then still only be utilized up to it's 75°C rating despite the derated ampacity being higher than that of the 75°C column ampacity?
Thanks!