thhn wire and thwn wire is often sold as the same wire. is there really a difference?

I'm not sure what exact differences you're looking for, but other than the listing, I can't really think of any.

THHN is listed for temperatures not exceeding 90C in dry locations,

THWN is listed for temperatures not exceeding 75C and is also listed for use in wet locations.

Most of these types of wire insulations are dual listed as THWN/THHN. I've never personally seen just the one listing.

THHN or THWN is a thermoplastic insulation, usually PVC.

As Tom listed, there is several differnces. What is important is what what you application is. Is the environment wet (THWN) or damp (THHN) and ambient temperture rating needed. A big one would be coduit fill. Given that THWN is rated at 75 degrees, wire derating becomes a bigger issue sooner then THHN. I typically use only THHN and XHHW. These two cover pretty much most typical applications so stocking is less of an issue at the warehouse and service rig. Much if not all the THHN I have used is THWN rated as well so either way, I am covered. I never specifically needed THWN so I have not really look at THWN in detail.

I had this discussion with a co-worker recently. I think that the conductors in Romex are truly rated as THHN, while the spools of "THHN/THWN" are really type THWN. If you compare the two, THWN has a thicker clear nylon covering than the conductors in Romex cable.

My theory is only a guess though. I think that manufacturers sell it as dual-rated to save the expense of making two types of wire, not to mention the cost for contractors to stock both types. Unless there was a significant cost difference, like back in the TW/THW days, then it might be worth making and stocking both types.

On a side note, does anybody remember opening a fresh box of TW and getting a sniff of it? Man, I used to love that smell.

I have seen a lot of "well conductor" with the thick rubbery insulation that was singularly THWN rated.

Greg, are you thinking of that red/black/yellow stuff that looked like miniature triplex? I thought that was type THW.

All I know it that I remember trying to pull some of that through a piece of 1/2" EMT and it was the hardest work I think that I ever did. Man, that insulation was thick.

The stuff I saw was red, black and green twisted and I am sure it said THWN. My neighbor was using it on his dock (old irrigation man) and was trying to pull it in 3/4" RNC.
I gave him a bottle of yellow 77 to get it to go.

Thanks to all for you responses. I am running through 1 1/4" conduit to a boat dock sub panel. The boat takes a 30 amp double and there will be an additional 15 amp single for a recepticle and one other 15 amp single for a switched water pump. I am coming out of the main with a 50 amp. the sub panel will be mounted on the dock post. I hope this helps explain why I am asking. Thanks again.

The wire is simply dual rated. They made a waterproof wire that can stand 90c. Just bear in mind, when you use THHN/THWN in a wet location you have to follow all the THWN rules, particularly the 75c ampacity rule.
... No I can't explain that.

OOH OOH! I know the answer to that one. Water (wet location) holds heat better the air thus it can not disapate the heat.

Am I correct in thinking that I should use the thhn/thwn #6 wire for my application described in the thread above? I want to do it right so I don't have to worry about it later. Thanks guys.

This is always a plus

How longer is your feeder?

With a 50a breaker you could use 8ga copper for this feeder at the 75c ampacity. It sounds like you will have fairly diverse loading. The other issue might be voltage drop if this is a long run.

I decided to go with 6ga because the feeder is just about 100ft. My main concern was if the thhn/thwn copper was the correct type to use. I have had a lot of people tell me to just use the uf in the conduit if I was worried about the wet location, but that just wouldn't be right.

don't forget to upsize your grounding conductor too since you are upsizing for Voltage drop (See note Table 250.122).