I came across an old thread (2005)

[TNMaster]

I came across an old thread (2005) discussing the benefits of XHHW over THHN.
A lot of reference was made to the temp rating of the wire in regard installation and de-rating.

I know the NEC states that no component of a system can be rated higher then the lowest rated component.
There was a question on my MASTER's Exam many years ago where this came into play. I was questioned after the exam because it is rarely answered correctly.
Since no one manufactures a connector rated higher than 75C, THHN can never be rated 90C in a practical application, wet or dry. The NEC ampacity chart must be used with this in mind.
I have never selected THHN wire size based on 90C column in the chart. I have always reduced the THHN rating to the 75C column which sometimes requires larger AWG.
If any of the system connectors are rated 60C, the ampacity rating must be reduced accordingly as well.

I would be interested in hearing comments on this from an engineering perspective.

 

[dpc]

I'm not quite sure what your question is. It's basically correct that, in the US, low voltage conductors can generally not be rated higher than the temperature rating of the terminations. So in practice, this means that we use the 75 deg ampacity (sometimes 60 deg) ampacity regardless of the 90 deg C ampacity of the wire.

In situations where a conductor must be derated due to factors such as high ambient temperature, it is permitted to derate based on the 90 deg C ampacity if applicable.

Also, 100% rated molded case breakers must be connected with 90 deg C rated wire.

 

[waross]

From a practical point of view;
I always used THHN because of the ease of pulling when retrofitting old installations.
The temperature rating is not the only factor.
Also, you can't go very far at maximum 90 degree ampacity before the voltage drop gets you.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TNMaster]

Not so much a question as my perspective based on 40+ yrs in the field and my understanding of the NEC.
Was just wondering how that measured up to the EE perspective. Was not aware of the point on the molded case breakers.
Goes to show there is always something to be learned. That point was not mentioned in the older blog regarding use of XHHW.
There is an installation occurring at my facility where #6 stranded XHHW is being used instead of THHN, simply because there was a lot of it on site for another job.
Just got me wondering.
Thanks for the comments.

 

[jghrist]

The 90° ratings can be useful if you have to derate the cable. The factors in derating like number of cables in conduit do not affect the temperature of the connectors.

 

[TNMaster]

The cable derating does not affect the temperature of the connectors, it's true.
My point is that the temperature of the connectors affects the cable rating in regard to the NEC ampacity of conductors chart. Therefore, THHN cannot be used at its 90C rated amperage, even when derating is or is not otherwise required. It must be used at the corresponding size 75C or 60C rated amperage depending on the temperature rating of the connector chosen. This is a requirement 9of NEC.
Therefore, even if derating is not otherwise required by length of run, conduit fill, ambient temperature, etc., THHN can never be used at its rated ampacity in the chart.
This mistake is made by a great majority of those using the chart to size wire, including most EEs I have known. The only exception to this is runs of 24" or less.

 

[wroggent]

Maybe give this thread a read

https://www.eng-tips.com/viewthread.cfm?qid=437011

 

[TNMaster]

Interesting read for sure but it just underscores my point. A lot of reference is made to the ampacity tables in NEC which in my long experience, is usually read incorrectly.
According to the NEC and its commentary (the one written by 3 of the six gentlemen who actually sit on the committee), THHN cannot be used at the ampacity listed in that table for the given gauge but must be used at the amperage allowed for 75C 0r 60C, whichever connector is used. The only exception is in a circuit supplying fire fighting equipment. Remember that everything in the NEC is determined from a fire prevention perspective. If there is already a fire, everything changes.

 

[bacon4life]

Engineering is about more than just looking up a number in table. We design a system using limited resources to provide adequately optimized solution. In any system, the ratings of each component must be evaluated to see if that component is the limiting component. Getting the question wrong just means the person was only thinking about the wires, not the entire system that includes wires, terminations, conduit, adjacent conductors and ambient air temperature.

The difference might be terminology. I agree you cannot directly read from the 90C column. However, do you mean you have never used the 90C column as a starting point to begin applying derating factors for high ambient temperatures or adjacent conductors?

 

[waross]

You can use larger gauge pigtails for connections.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TNMaster]

@ bacon4life
You said the magic word, system. Most electricians and EEs I have encountered in my career have oversimplified the ampacity table in using it to select wire.
I joined this forum to improve my knowledge because after 47 years of industrial maintenance and over 20 years as a licensed master, I still believe there is something to be learned.

I have completed many projects that were engineered, and corrected quite a few (far too many, in my opinion)that were "engineered".
I was testing the consensus against my understanding of the NEC.
I have been exposed to so much "code knowledge" in my career which was not; that it concerns me as a professional and as a caring individual.