Breaker size for 6 awg

[Span]

Condition is main 200A panel with 6 awg feeder to supply a sub-panel what size breaker allow to use? Assuming ambient 45 C, THHN, GC argue for 70A but I

said it only allow for 60A.

My argument from 110.14.C The temperature rating of conductor not to exceed the lowest temperature rating of any connected termination. Most of the breaker ambient rating are 40 C, and 60/75 C wire. Am I right or wrong?

 

[Dennis]

It would depend on the temp rating of the all pieces of equipment including conductors on the circuit. For instance if the wire is NM cable then the max your ocpd can be would be based on the 60C column. If you are run in conduit then you limitation would be 75C . The 90C of the wire is only used for de-rating.

So in short the circuit rating is based on the weakest link in the circuit. All of today's panels and breakers are rated 75C and if the #6 conductor after de-rating is still rated 65 amps (75C column) then you can use a 70 amp ocpd as long as the calculated load is not more than 65 amps.

If you wire is rated 60C as NM cable would be then the #6 will only be rated 55amps so it could be on a 60 amp ocpd as long as the calculated load is not more than 55 amps.

 

[Span]

Thanks Dennis, it helps.