THHN / THWN wire temperature rating

[cppoly]

THHN is rated for 90 degree C and THWN is rated for 75 degree C but both of these ratings can be listed on the same wire. Just want to make sure I have this right, if installed indoors you can use the 90 degree column for ampacity adjustment but if installed outdoors you must use the 75 degree column?

 

[infinity]

That is correct. Most modern THWN is rated for 90 degrees C with the -2 after the THWN.

 

[cppoly]

Thanks.

Next question would be why it is this way. Would be helpful to know.

 

[MAC702]

Without knowing or caring about the chemistry of the insulation, the powers that be must have decided that THHN is an indoors insulation, but THWN is good for outdoors. So if outoors, you'd have to ignore any THHN rating and use the THWN rating. But with THWN-2, it no longer matters, practically, for us.

 

[Dennis Alwon]

As Trevor stated all the Thwn wire is usually rated thwn-2 which means it is rated 90C in both dry and wet locations.

I would love to know why a wire with just a thwn rating is rated 90c dry location but only 75 in wet locations

 

[LarryFine]

It's my understanding that the presence of water impedes, rather than improves heat dissipation.

 

[Carultch]

That is correct. Most modern THWN is rated for 90 degrees C with the -2 after the THWN.

Not all wire of any given manufacturer in this family is rated that way. I've seen datasheets where the smallest sizes do not carry the "-2" dual rating, and are only rated as THHN/THWN, without also being THWN-2. Meaning 75C wet, 90C dry.

 

[infinity]

Not all wire of any given manufacturer in this family is rated that way. I've seen datasheets where the smallest sizes do not carry the "-2" dual rating, and are only rated as THHN/THWN, without also being THWN-2. Meaning 75C wet, 90C dry.

Yup that's why I said most.