THHN/THWN vs THHN-2/THWN-2

[malachi constant]

I am a specifying engineer that uses a spec template (MasterSpecs). The most recent version of MasterSpecs that I downloaded has THHN-2/THWN-2 listed for conductors, as opposed to what I'm used to seeing listed (THHN/THWN). What's the difference? About five minutes of googling didn't get me anything. The '11 NEC doesn't seem to even acknowledge THHN-2.

To put it another way, when should someone specify THHN/THWN vs "-2"? And vice versa?

My hunch is there is a very small performance improvement to go to the "-2", and also a very small cost difference, small enough that it doesn't really matter one way or the other what you spec.

 

[jumper]

THWN-2 and other -2 wires are rated at 90 degrees wet or dry.

 

[malachi constant]

Would a good approach to a spec be to allow both THHN/THWN and THHN-2/THWN-2?

 

[jumper]

Depends if you need the amperage of the THWN-2 for derating.

BTW, there is no THHN-2, the -2 ending is for wire used in wet locations.

 

[Dennis Alwon]

Most thhn wire is now dual rated, which means it is thhn/thwn. The difference is the label without the "-2" is only rated 75C where used in a wet area. If de-rating is not an issue , then either one would be fine.