TNMaster
Industrial
- May 25, 2018
- 7
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.
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.