# Wire Derating Question



## Pharon (Jan 20, 2014)

I believe the 75 degree C rating is for wet locations and the 90 degree rating is for dry locations.


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## Pete m. (Nov 19, 2011)

Most, but not all, equipment terminations are rated for 75 degrees celcius.

110.14(C) allows us to use the higher temperature rating (90 degrees) for ampacity adjustments.

As an example you could have 9 current carrying 12's in a raceway and still utilize 20 amp OCPD's because you start your adjustments using the 90 degree ampacity.

So, depending on what the question is asking determines what column of 310.15(B)(16) you use.

Pete


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## Pharon (Jan 20, 2014)

Yeah, I don't think that's it, at least not what he's asking. Otherwise why not have all 90 degree wire put in the 75 degree column (i.e. RHH, MI, etc.).

This shows different ampacity ratings for THHW based on wet vs. dry:

http://www.polarwire.com/flex-building-wire.pdf


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## Meadow (Jan 14, 2011)

I think its time the whole wire sizing method be rebuilt from the ground up.


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## Meadow (Jan 14, 2011)

Pharon said:


> Yeah, I don't think that's it, at least not what he's asking. Otherwise why not have all 90 degree wire put in the 75 degree column (i.e. RHH, MI, etc.).
> 
> This shows different ampacity ratings for THHW based on wet vs. dry:
> 
> http://www.polarwire.com/flex-building-wire.pdf



Thanks! I was wondering what MTW looked like. Lookn good :thumbup:


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## jmileysr (Aug 18, 2015)

Pharon. That is what I was looking for. Where is it stated in the NEC book though?


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## Pharon (Jan 20, 2014)

Good question - I have no idea. I'd go fishing for it now but I have to catch a train.


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## Barjack (Mar 28, 2010)

Pharon said:


> Yeah, I don't think that's it, at least not what he's asking. Otherwise why not have all 90 degree wire put in the 75 degree column (i.e. RHH, MI, etc.).
> 
> This shows different ampacity ratings for THHW based on wet vs. dry:
> 
> http://www.polarwire.com/flex-building-wire.pdf


Isn't it possible that at some point some wire manufacturers made THHW/THWN rated only at 75C?


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## Dennis Alwon (May 9, 2009)

Pharon said:


> I believe the 75 degree C rating is for wet locations and the 90 degree rating is for dry locations.


That depends on the insulation. Table 310.104 shows Thwn-2 good for 90C in dry and wet locations


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## Dennis Alwon (May 9, 2009)

jmileysr said:


> Pharon. That is what I was looking for. Where is it stated in the NEC book though?


Where is what stated???


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## telsa (May 22, 2015)

Barjack said:


> Isn't it possible that at some point some wire manufacturers made THHW/THWN rated only at 75C?


IIRC

THHN 90

(*T*hermoplastic *N*ylon) (hence: TN )

Above ground/ dry ( Each "H" stands for 'hot' as in a bump UP from 60 degrees) 

[ As if TN (60) and THN (75) ever existed. ]

Then a SECOND rating was appended to the same wire -- for wet usage.

THHN // THWN the first 90 in dry air // the second 75 when wet.

Strictly speaking THHN is not rated for wet conditions... if that's the ONLY legend it has. You won't find any such wire. (If you ever could.)

Instead you see dual ratings on a given conductor.

THWN-2 is rated 90 for wet or dry -- making the prior THHN legends obsolete -- but those are still laid on, anyway.

This latter legend also is co-printed with MTW

MTW 105 -- the NEXT bump up in heat tolerance.

So you'll run into today's ubiquitous: 

THHN, THWN-2, MTW [ 90 // 90 // 105 ]

The latter MTW @ 105 requires dry conditions -- as in peckerheads. 

(Motor Termination Wire )

No, it's not in the tables, and you can't use 105 degrees in any de-rating calculation under the NEC. It's more of a NEMA standard... for motor-heads. It's their way of pushing this class of conductor for motor termination usage.


*"XHHW* is an alphabetism or initialism which stands for "XLPE (cross-linked polyethylene) High Heat-resistant Water-resistant." 

This type of insulation is pervasive with aluminum conductors// feeders -- but can be had with copper, too. (Not normally stocked -- at least out my way.)

Even XHHW has been supplanted by XHHW-2. [ ie 90 when wet ]

Such test questions have been crafted by (I must assume) non-electricians -- as the only stuff you have a chance to touch for new installations is the fully rated 90 degree wire. ( Setting aside Romex, which is on its own performance track.)


{ Poco's throw all of these ratings out the window -- and will tolerate much higher temperatures and loadings than the NEC will. But then, their stuff is in free air or buried, quite unlikely to burn ones house down. }

%%%

As to the query: where is it stated in the NEC? 

It's stated in the legend on the wire itself.


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## Pharon (Jan 20, 2014)

Dennis Alwon said:


> That depends on the insulation. Table 310.104 shows Thwn-2 good for 90C in dry and wet locations



Yes but the question asked was about THHW and XHHW. So far what I've found is that those insulations have different ratings depending on whether they are in wet or dry locations. Not sure why, though. 

I think there used to be an NEC table in Section 310 that elaborated on this back in 2002 but not sure if they took it out or just moved it in subsequent editions.


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## Dennis Alwon (May 9, 2009)

Pharon said:


> Yes but the question asked was about THHW and XHHW. So far what I've found is that those insulations have different ratings depending on whether they are in wet or dry locations. Not sure why, though.
> 
> I think there used to be an NEC table in Section 310 that elaborated on this back in 2002 but not sure if they took it out or just moved it in subsequent editions.


I stated the table info--310.104 is where it was moved to.


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## Dennis Alwon (May 9, 2009)

Pharon said:


> Yes but the question asked was about THHW and XHHW. So far what I've found is that those insulations have different ratings depending on whether they are in wet or dry locations. Not sure why, though.


Almost all xhhw wire today is xhhw-2 which is listed 90c wet and dry. Same with thwn-2 but thhw is rated 75C wet and 90c dry


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## Pharon (Jan 20, 2014)

Yeah, well exam questions aren't always known for their practicality.


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## Pharon (Jan 20, 2014)

Dennis Alwon said:


> I stated the table info--310.104 is where it was moved to.


My apologies - I missed this post yesterday - that's what I get for trying to follow this thread on my phone... I see it now, Table 310.104(A) - it's all right there in black and white.


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## jmileysr (Aug 18, 2015)

Got it. Thanx all!


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## Meadow (Jan 14, 2011)

Pharon said:


> Yeah, well exam questions aren't always known for their practicality.



Which I never liked. Nearly all THHN is also dual rated THWN.


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