THHN v.s. THWN

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Cletis

Senior Member
Location
OH
I have a few questions

What is the actual difference between the two when it relates to water resistance? Say you take both of them and set in the bottom of an aquarium, how long would it take to penetrate the thhn v.s. the thwn ?? What are the coating differences ? links ?

Why do they put 90C rated thhn in NM which is rated for 60C ?

I guess I"m looking for two things, will thhn really decompose and short out if in really high moisture environment? how long ?

I"m guessing it's some sort of extra layer of thermoplastic (which lowers ampacity) but I can't find via google yet.
 
augie47

augie47

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as far as the second part of the question, I believe if you search you will find that the insulation on NM conductors is not actually a THHN conductor. It is an "insulated conductor" meeting the specifications of the listing agency.
In IAEI meetings it has been stated that the 60? rating has more to do with the rating of the devices where NM is normally terminated.
 
augie47

augie47

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Similar to THHN now being marketed as THHN/THWN, NMC, which is approved for damp locations, is now marketed as NMC/UF (dual rated).
 
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big john

Senior Member
Location
Portland, ME
Looking at UL 83, it appears that the difference is that THHN only has to pass a dielectric withstand test after 6 hours of immersion at 15?C.

Whereas conductors with a "wet rating" like THWN have to withstand a long-term immersion test at their conductor rating of 75?C for a minimum of 12 weeks.

Ah, nuts. Just realized I wasted my time answering a Cletis question.
 
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Cletis

Senior Member
Location
OH
I think I know whats going on now, either there are billions of feet of UF in stock at the warehouses and they need to clear out or someone hasn't talked to someone yet and tell them the increasing need for "damp" location romex yet. I'll make a call to the higher ups at southwire tomorrow and report back here if I get any good answers
 
kwired

kwired

Electron manager
Location
NE Nebraska
Cletis said:
I think I know whats going on now, either there are billions of feet of UF in stock at the warehouses and they need to clear out or someone hasn't talked to someone yet and tell them the increasing need for "damp" location romex yet. I'll make a call to the higher ups at southwire tomorrow and report back here if I get any good answers
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Correct, if there is no demand for it why would suppliers buy it? The little demand that does exist is taken care of with UF which typically is readily available.
 
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curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
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Electrical Contractor
What exactly are you trying to accomplish? Why do "they" need to get rid of all the UF cable and replace it with NMC? I don't think all the manufactures and wholesale suppliers are going to change their product line because you want something different.

Type UF cable can be used as a substitute for NMC cable. Read the last sentence here.
http://www.southwire.com/products/CUUFB.htm
 
Dennis Alwon

Dennis Alwon

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curt swartz said:
What exactly are you trying to accomplish? Why do "they" need to get rid of all the UF cable and replace it with NMC? I don't think all the manufactures and wholesale suppliers are going to change their product line because you want something different.

Type UF cable can be used as a substitute for NMC cable. Read the last sentence here.
http://www.southwire.com/products/CUUFB.htm
Click to expand...

IMO, nm should be rated damp to avoid a lot of issues with it. Why are we allowed to install NM in a crawl space that is dripping wet yet we cannot install nm in a conduit outside.

In our area there are many outdoor panels many of which has branch circuits that go down about a foot or 2 and turn into the crawl space via a 2" pvc pipe. So to be compliant you would have to change over and install a jb and then splice every cable or run uf homeruns-- but that is a different issue altogether
 
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Cletis

Senior Member
Location
OH
I highly agree with the above statement. This constant transitioning from nm-b in basement to a jbox, then UF out brick, then UF to first can, then 1st can above finished ceiling in outside ceiling in nm-b etc and so forth. That coupled with which inspector O' Day is going to show up and how he/she interprets damp location is highly annoying. This can all be solved with either changing the rating of nm-b to damp or modifying it or making some nmc readily available. When your doing a 1000 jobs a year going from basements to gazebos to carports to crawlspaces etc... if there was just one wire to cover it all that would make life easier, faster, and more profitable (I can't stand UF, especially in the winter). I know we can just run UF everywhere but it seriously is not fun to work with and skin even if you have the right method. It increases my probability of paying out another workmans comp claim from a 2" gash in a finger as well. There has to be a huge need for this, it can't be just me that's having this constant problem.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Cletis said:
I highly agree with the above statement. This constant transitioning from nm-b in basement to a jbox, then UF out brick, then UF to first can, then 1st can above finished ceiling in outside ceiling in nm-b etc and so forth. That coupled with which inspector O' Day is going to show up and how he/she interprets damp location is highly annoying. This can all be solved with either changing the rating of nm-b to damp or modifying it or making some nmc readily available. When your doing a 1000 jobs a year going from basements to gazebos to carports to crawlspaces etc... if there was just one wire to cover it all that would make life easier, faster, and more profitable (I can't stand UF, especially in the winter). I know we can just run UF everywhere but it seriously is not fun to work with and skin even if you have the right method. It increases my probability of paying out another workmans comp claim from a 2" gash in a finger as well. There has to be a huge need for this, it can't be just me that's having this constant problem.
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I have never seen NMC, what if it isn't much better to strip then UF?

My guess is it maybe doesn't have absorbent fillers (paper) like NM does, but still has a similar outer jacket to NM.
 
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Cletis

Senior Member
Location
OH
kwired said:
I have never seen NMC, what if it isn't much better to strip then UF?

My guess is it maybe doesn't have absorbent fillers (paper) like NM does, but still has a similar outer jacket to NM.
Click to expand...

Basically, I want romex (just like SEU). I don't see why romex couldn't mimic SEU with the same ratings ??
 
GeorgeB

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
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Retired
George Stolz said:
Are you still buying something that's not dual rated? Do they still sell that? Is it cheaper or something? Why not just eliminate the question?
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It's been reported before that the blue and orange big box stores sell thhn without the dual rating. I looked last time and confirmed that is true. It is a Southwire product.

Aside, as a controls guy more than power, I prefer MTW. As a "consultant" doing commissioning and debugging, not an electrician, my opinions matter little, but this is just an observation.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Cletis said:
Basically, I want romex (just like SEU). I don't see why romex couldn't mimic SEU with the same ratings ??
Click to expand...
One property of type SE cable that is different is the concentric bare conductor opposed to a single solid or stranded conductor. But you could use SE cable in place of NM cable - but typically no demand except for a few common service or feeder sizes so you probably not finding it in say 12-2 versions. I do find it occasionally in old barns as 10-2 in the old cloth covered SE cable, but I'd guess the demand for such size has greatly diminished from what it may have been 80 or 90 years ago.
 
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GoldDigger

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GeorgeB said:
It's been reported before that the blue and orange big box stores sell thhn without the dual rating. I looked last time and confirmed that is true. It is a Southwire product.

Aside, as a controls guy more than power, I prefer MTW. As a "consultant" doing commissioning and debugging, not an electrician, my opinions matter little, but this is just an observation.
Click to expand...
I was one of those who reported single rated THHN at big box, but it turned out that the TWNN part was just in fine print in a different part of the label.
 
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