cable temp rating design for heat reduction at CB connection

[Patrick Gorrell]

Does higher temp rated cable have slightly more copper? If no, then how does higher temp rated insulation reduce teh temp rise at a CB connection limited to 40C? Higher rated insulation intuitively resists deterioration at higher heat levels but is it a fuction of heat dissapation along the cable length lowering connection heat?

If yes, there's more slightly more copper in a 75C vs 60C, it makes sense a higher temp cable with more copper/same ampacity generates less heat.

 

[hbiss]

No, the insulation has a higher temp rating that's all. A higher temp insulation doesn't mean that the conductor can carry more current or produce more heat, it comes into play when it comes to derating and the environment where it's used.

-Hal

 

[electrofelon]

Somewhat along the same lines, maybe you heard this and aren't quite remembering it right:. 100% rated breakers require 90 degree rated conductors used at the 75 degree ampacity. Actually pretty much just academic because nearly all conductors are rated 90 degrees now.

 

[Julius Right]

In my opinion, if in a part of a cable run path the ampacity is different [less than required] some time we have to increase the conductor cross section area. We also may calculate the ampacity in a part of this run for more than 90oC conductor temperature [ let's say 105oC if the insulation rating permits this].In any case we have to reduce the ends temperature up to allowable temperature of breaker terminals.
For instance, in my opinion, a cable tray of 10 ft. long [as per NEC art 310] may be used in order to reduce the temperature from 105 to 90 [and even less].