Neher McGrath: 75C vs 90C (600V)

[publicgood]

I am performing duct bank calculations on a 480V feeder with THWN conductors. I’d expect I may utilize the 90C rating for de-rate calculations, as long as it is not less than the 75C rating ampacity to maintain termination ratings.

However, I ran into an article that speaks to 100% rated circuit breaker assemblies... which is what I have. Does this hold any weight? I suspect not.

“There is typically some discussion about utilizing the 90 C rating of the conductors. NEC Section 110.14 (C) states: “Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustments, corrections or both.” This application typically isn’t applicable for 100% rated breakers, where, in fact, 90 C cable is required. But it must be sized per the 75 C ampacity. These 100% rated breakers use the wire as a heat sync to serve continuous loads at the full rating of the breaker.”

https://www.csemag.com/articles/heating-calculations-for-electrical-duct-bank/

 

[electrofelon]

for 100% rated breakers, where, in fact, 90 C cable is required. But it must be sized per the 75 C ampacity.

That is my understanding. I believe you can still start at 90 for your derating.

 

[publicgood]

After further consideration...maybe it doesn’t make sense to use the 90C for conductor derates with 100% rated CB assemblies. Providing an example from an Eaton whitepaper. The breaker gains additional ventilation and benefits from use of 90C wire heat sink ability...but the conductors are smaller without any design difference in the middle of the circuit.

https://www.alliedelec.com/images/Products/mkt/pb/eaton/Pdfs/ullistedmccbs.pdf

 

[JFletcher]

The benefits come for sizing the ocpd for continuous loads, and in wire sizes where there may be a two wire size difference in required ampacity... An example is a feeder that requires 430 amps. At 90°, one set of 500 MCM Cu is good. at 75 degrees, one needs 750 MCM Cu. If there is existing conduit that would be very difficult to impossible to replace to accommodate the larger wires, or as in a recent thread here, a calculation mistake, being able to use the 90* C column may save your bacon there too.

 

[ron]

Once the temperature is calculated, we do not allow the hottest duct to exceed the listed equipment assembly termination temperature, which at low voltage is 75 deg. almost 100% of the time.