Conductor sizing for 40 degree C breaker

[pjlittlefield]

How do I properly size conductors that will be supplied from an ITE (old) circuit breaker with a temperature rating of 40 degrees C? Ampacities in 310-16 are for 60, 75, and 90 degree C.

The existing 150 amp, 3 pole circuit breaker will be feeding an air conditioning unit with a name plate Max Overcurrent of 150 amps.

 

[don_resqcapt19]

I don't think that is the termination temperature rating. It is more likely the maximum permited ambient temperature.
Don

 

[charlie b]

I agree with Don. To size the conductors, you use the 75C column (presuming you have a conductor that itself has a rating of at least 75C).

 

[templdl]

Part of the UL489 test requirements states that "all breakers shall be calibrated to carry their continuous rating in an ambient temperature of 40degC." This is said to be "ambient compensated."
It is possible to get 50degC calibrated breakers but they wil not be UL listed.
I don't know if they are still available but there have been "ambient compensating" breakers which have a double bimetalic element instead of just the one which readjusted the thermal element to changes in ambient temperature.