derating wire with temp

[kevin schlickau]

Question
What is the size breaker you would use and rating of wire for a #6 THHN in a temp of 105F and one lug is unknow.
I have been getting two answer from other electrician on this problem. Please show math.
Thank you

 

[augie47]

#6 THHN is a 90° insulation conductor rated 75 amps in that column105°F is a .87 myltiplier for an ampacity of 65 amps.
If you can not confirm the termination temperature to be 75°C or higher, you would need to trminate at the 55 amp rating (60°) and 250.4 would allow you to use a 60 amp breaker.(per 110.14)
(In all probability it is lilely a 75°C termination and you would be allowed a 65 am p load on a 70 amp breaker)

 

[kevin schlickau]

So the wire now can only hold 60 amp

 

[augie47]

The"wire" is not necessarily the limiting factor. 110.14 notes that on circuits 100 amp or less you are limited to the 60° rating of the conductor.
It does allow for the 75° rating IF the terminations are rated 75° which, in real life, is very common. So your THHN wire with an adjusted amapcity of 65 amps (due to ambient) is allowable on a 65 amp load but without knowledge that the equipment termination in 75° you are limited to the 55 amp amapcity and a 60 amp breaker.

 

[kevin schlickau]

That is hard to rap my brain around because, you really didn't have to derate the ampacity of the wire. The wire derating is to carry 65 amp but its on a 55 amp breaker because for 60F lug so nothing really happened.

 

[augie47]

That is hard to rap my brain around because, you really didn't have to derate the ampacity of the wire. The wire derating is to carry 65 amp but its on a 55 amp breaker because for 60F lug so nothing really happened.

That's true in this case. The 60° rule by 110.14 is the limiting factor so derating the 90° conductor did not effect the outcome but it's good to make sure the ambient doesn't limit you more.
Had it been a TW conductor, the ambient adjustment would have been the limiting factor.