THHN/THWN conductors

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cppoly

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A conductor labeled THHN/THWN has a 90 degree rating right? This 90 degree rating applies to both wet and dry locations?
 
cppoly said:
A conductor labeled THHN/THWN has a 90 degree rating right? This 90 degree rating applies to both wet and dry locations?
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Not if it's just THWN... 75?C in wet locations. For 90?C in wet locations, it has to be rated THWN-2.
 
No. THWN/THHN is rated 75 wet, 90 dry.

However THWN/THHN-2 is rated 90 wet or drive. I am willing to be you have THWN/THHN-2
 
iwire said:
I am willing to be you have THWN/THHN-2
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Unless it is smaller than #8. I have not found any manufactures that make THWN-2 is sizes smaller than #8. All 14, 12 & 10 is plain THWN and only rated at 75C in wet locations. I have always wondered if this is just by choice or a UL thing.
 
How does the environment change the insulation rating though? You have a 90 degree THHN/THWN conductor indoors but when you take it outside the insulation rating changes to 75? I don't get it
 
cppoly said:
How does the environment change the insulation rating though? You have a 90 degree THHN/THWN conductor indoors but when you take it outside the insulation rating changes to 75? I don't get it
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Its not really indoors vs. outdoors, its wet vs. dry. I don't know for a fact the reason, but it most likely has to do with the temperature at which insulation degradation occurs at different levels of moisture content. Thermoplastics are not totally impervious to moisture.
 
Smart $ said:
Its not really indoors vs. outdoors, its wet vs. dry. I don't know for a fact the reason, but it most likely has to do with the temperature at which insulation degradation occurs at different levels of moisture content. Thermoplastics are not totally impervious to moisture.
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That makes sense.
 
Besoeker said:
Well, yes. but what is the difference?
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I don't know the exact difference, but its likely the chemical composition of the insulation materials. Both have PVC insulation with a nylon sheath/jacket, and AFAIK the same thickness. Compliance is per UL Standard 83. Many conductors are dual rated THHN/THWN, if not THHN/THWN-2, sometimes triple rated including MTW under a different UL Standard.
 
curt swartz said:
Unless it is smaller than #8. I have not found any manufactures that make THWN-2 is sizes smaller than #8. All 14, 12 & 10 is plain THWN and only rated at 75C in wet locations. I have always wondered if this is just by choice or a UL thing.
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Uh ... you must not be shopping in the right place. I've got #10 and #12 THWN-2 on the van right now. Got it from my local electrical supplier by asking for "number 10" and "number 12" without any elaborations. Pretty sure Home Depot sells the same stuff, marked THHN or THWN-2 or a couple other things.
 
Got another question. If THHN/THWN conductors from inside a building feed equipment on the roof, what ampacity is used for derating 75C or 90C? Would this follow the 10% rule for lower amapacity? For instance if the distance of the conductors being outside is less than 10% of the conductors being inside then the 90C ampacity can be used? I don't think this is mentioned in the code so I'm taking a guess.
 
cppoly said:
Got another question. If THHN/THWN conductors from inside a building feed equipment on the roof, what ampacity is used for derating 75C or 90C? Would this follow the 10% rule for lower amapacity? For instance if the distance of the conductors being outside is less than 10% of the conductors being inside then the 90C ampacity can be used? I don't think this is mentioned in the code so I'm taking a guess.
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Contrary to Rob, I believe it can apply... but you would still be limited to the 75?C ampacity equivalent for termination temperature limitation of 110.14(C), as the maximum ampacity.
 
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