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#90859 - 12/15/04 09:51 PM Wire sizing 60 or 75 degree  
sgreany  Offline
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Joined: Jul 2003
Posts: 26
I learned that if I terminate a wire with insulation rated for an ambient temperature rating of 75 degrees C. to somthing like a breaker rated 60 degree C. I have to size the wire to the 60 degree C. Colum of 310.16

Can someone explain this this to me? How dose the terminal change the physical properties of the insulation?

I have asked this quistion since the code change went into affect and no one can seem to give me a streight answer. It seems to me someone confused ambient temp. with accual operating temp becuase thats the only way it makes sense.


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#90860 - 12/15/04 10:28 PM Re: Wire sizing 60 or 75 degree  
electricman2  Offline
Member
Joined: Nov 2003
Posts: 269
Thomasville, NC USA
Lets use an example. A 8AWG copper conductor with 75C insulation has an ampacity of 50A. This means that with 50A current in the conductor its temperature could reach 75C. If connected to a terminal with a 60C rating the excess temperature would be detrimental to the 60C termination.


John

#90861 - 12/16/04 06:41 AM Re: Wire sizing 60 or 75 degree  
Happy Birthday iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Great description electricman2. [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#90862 - 12/16/04 08:58 AM Re: Wire sizing 60 or 75 degree  
sgreany  Offline
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Joined: Jul 2003
Posts: 26
Yes I understand that. I want to know why as is in the rule is to install GFCI protection in bathrooms, the reason is becaus of the presence of water creates a higher shock risk, or the rule is anything installed outside has to be sunlight resistant. why, because sunlight can cause some materials to deteriorate and fail.

I think someone misunderstood the term ambient temp. when thay created this rule and it should be re evaluated.


#90863 - 12/16/04 09:04 AM Re: Wire sizing 60 or 75 degree  
sgreany  Offline
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Joined: Jul 2003
Posts: 26
also thats not correct, the ambient temp rating by definision in the code is the enviroment temp. where the wire is installed, this wire can safley operate without the insulation breaking down applying the apropriate ambient temp. corection factor of course. At 75 deg. C. salt water will boil, thats kindof a warm instalation dont you think?


#90864 - 12/16/04 09:32 AM Re: Wire sizing 60 or 75 degree  
dmattox  Offline
Member
Joined: Oct 2004
Posts: 265
Anaheim, CA
The code isn't saying that wire will be installed in an ambient temperature of 75 degrees C, its saying that wire can be allowed to reach 75 degrees C. And table 310.16 gives you the amperages that will achieve that temperature assuming an ambiant temperature of 30 degrees C.

If the ambiant is higher, ofcourse it will take less amperage to reach the threshold temperature of the wire so thats what the correction factors at the bottom of the table are about.

As far as GFI's outside, the code does not require a GFI installed outside, but a GFCI protected outlet. And that is purely for shock protection due to water.


#90865 - 12/16/04 10:19 AM Re: Wire sizing 60 or 75 degree  
sgreany  Offline
Member
Joined: Jul 2003
Posts: 26
Perhaps you are correct, but if you read through 310.13 and the notes the wording and language indicates that operating temp. is intended as ambient temp. Also, I learned in trade school that the temp rating indicated max ambient temp. for that particular insulation. Thats probably my source of confusion. The code should, in the futer, clerify this. Perhaps a call to a wire MFG. will shed some more light on the subject, as I am still not shure what the intent of maximum operating temp. Read it carfully it could go either way.


#90866 - 12/16/04 01:55 PM Re: Wire sizing 60 or 75 degree  
Happy Birthday iwire  Offline
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Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
sgreany

The question you asked has absolutely nothing to do with the ambient temperature.

electricman2 nailed it, there is nothing for the NEC to reevaluate, the rule is a good rule.

First read carefully the name of Table 310.16

Quote
Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)


The rating of 8 awg in the 75 C column is 50 amps.

Now if the ambient temperature is 30 C as the table is designed around and if the cable or raceway has three 8 AWGs each loaded to 50 amps the temperature of the conductors will approach 75 C.

75 C is 167 F and yes that is warm but it is within the design limitations of any 75 C insulation.

Connect this conductor that is close to 75 C to a 60 C rated breaker and the heat transfer from the conductors to the breaker is very likely to cause the breakers thermal overload to trip the breaker.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#90867 - 12/16/04 02:08 PM Re: Wire sizing 60 or 75 degree  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,099
Estero,Fl,usa
If you look at the bottom of 310.16 you see "correction factors". That is an additional derating you must use if the ambient is above 30c.

As for termination temp, it is usually not really a factor since virtually any breaker above 30a has 75c terminations. 240.4(D) pretty much takes care of those 30a and below.
I say "usually" because I am sure you can find cases where this is not true but they are "unusual" ;-)


Greg Fretwell

#90868 - 12/16/04 03:33 PM Re: Wire sizing 60 or 75 degree  
tdhorne  Offline
Member
Joined: Mar 2001
Posts: 345
Maryland, USA
iwire Wrote:
Quote
sgreany

The question you asked has absolutely nothing to do with the ambient temperature.

electricman2 nailed it, there is nothing for the NEC to reevaluate, the rule is a good rule.

First read carefully the name of Table 310.16

quote:Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)

The rating of 8 AWG in the 75 C column is 50 amps.

Now if the ambient temperature is 30 C as the table is designed around and if the cable or raceway has three 8 AWGs each loaded to 50 amps the temperature of the conductors will approach 75 C.

75 C is 167 F and yes that is warm but it is within the design limitations of any 75 C insulation.

Connect this conductor that is close to 75 C to a 60 C rated breaker and the heat transfer from the conductors to the breaker is very likely to cause the breakers thermal overload to trip the breaker.

Bob
I believe that the actual problem with the temperature rating of terminals is whether they will continue to conduct the current safely rather than the effect the heat would have on adjacent parts. When the testing laboratories test terminals they subject them to repeated thermal cycling. The temperature rating of the terminal is the maximum temperature it can be repeatedly subjected to without loosening or failing physically which would cause arcing and destructive heating of the terminal and the conductor insulation. In other words you can only allow a certain temperature rise in a terminal before the stresses induced by thermal expansion will cause the terminal to fail.
--
Tom H


Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison

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