NM cable. WHY 60 degree.

[Strife]

Now, if I got this correctly, one can use 90 degrees ampacity for derating purposes, but has to use the 60 degrees for OCPD when it comes to NM.
This really puzzles me.
I always thought (and I know I'm right), that the degrees limitations are because the wire will get THAT hot when fully loaded (IE #12 loaded at 30A will get 90 degree hot), and 75 or 60 degrees because no termination is rated for 90degrees. So the problem at the termination is not the wire, but the terminal, which will melt if getting hotter than 75 degrees.
The middle of the conductor is not the end of it.
So why exactly do we have to use 60 degrees for NM? I can see if the insulation is rated for 60 degrees(in which case it'd be the middle of the wire which couldn't use the 90 degree ampacities).
Why exactly can NM get 90 degree hot in the middle, but can not get 75 degree hot at the termination?
And please, no answers like:"because Southwire rated their NM as such". Southwire wants to sell more #6 than #8 so that's not a valid answer.

 

[petersonra]

There is no good reason for it. Like many similar things, it just is.

 

[jwelectric]

a picture is worth a 1000 words

 

[Strife]

Sorry, but that's the the middle of the wire.
NM's supposed to be good at getting hot 90 degree in the middle, but not 75 degree at the terminal.
In that picture the heating occured because of lack of derating, not because the OCPD was not sized at 60.
I can count 9 pieces there, and looks like might be LEAST 4-5 more what I can't see, so that least 28 current carrying conductors. WAY past even the 60 degree termination OCPD. And you can see clearly how the overheating fades away as the cables have some breathing room going to the right side.
So that's failure to derate properly, not the 60 degree rule. Even at 60 degree rule, that bundle would have overheated when fully loaded.

a picture is worth a 1000 words

 

[Strife]

There is no good reason for it. Like many similar things, it just is.

RIGHT.
So much for the "going green" hypocrisy. For all it's worth, there's a LOT more copper wasted because "it just is" #6 instead of #8.
Let's go green as long as it cost more money, but who cares about the extra amount of a very limited resource? It just is.
And people wonder why I'm so skeptical of these "save the planet" (insert expletive here).

 

[augie47]

My thought has always been that the 60? limitation on the NM is due to the 60? rating on most devices (such as UL Listing RTDV for receptacles).
The UL listing in combinatiion with NEC 110.14 would require 60? termination, hence the 90? insulation with a 60? termination.

 

[Strife]

Why is it then that I can run a conduit with #8 Thhn/THW(whatever) for a 50A receptacle, but I need #6 for NM?

My thought has always been that the 60? limitation on the NM is due to the 60? rating on most devices (such as UL Listing RTDV for receptacles).
The UL listing in combinatiion with NEC 110.14 would require 60? termination, hence the 90? insulation with a 60? termination.

 

[jumper]

Didn't the old NM have 60C conductors?

 

[M4gery]

Why is it then that I can run a conduit with #8 Thhn/THW(whatever) for a 50A receptacle, but I need #6 for NM?

Even better, why is it that one day I can run 50 Amps thru a #8 SE cable but the very next day it's illegal and wrong to do the exact same thing?

 

[jumper]

Even better, why is it that one day I can run 50 Amps thru a #8 SE cable but the very next day it's illegal and wrong to do the exact same thing?

2011 amended this a little.

 

[cadpoint]

Why is it then that I can run a conduit with #8 Thhn/THW(whatever) for a 50A receptacle, but I need #6 for NM?

Those are all three different insulation materals, they tests the wire first per the AWG size then they put the different types of insulation on the wire then test that. You can see most of this in Chapter 9 Tables.

Frankly (probably wrong) NM in a cable assemble and they also test that together not seperatly (sp).

 

[Hv&Lv]

A couple of interesting articles...

http://www.ecmag.com/index.cfm?fa=article&articleID=8547

http://www.ecmag.com/?fa=article&articleID=8918

 

[George Stolz]

So why exactly do we have to use 60 degrees for NM?

Because the CMPs are afraid of NM, and penalize it accordingly.

 

[Smart $]

... but the terminal, which will melt if getting hotter than 75 degrees.
...

No, the terminal wll not melt (at least not initially). What happens is the termination loses integrity over time as it heats up beyond its rating then cools down. The terminal "melts" when the connection has lost integrity to the point where arcing occurs.

I believe the 60? rating is because NM is mostly used in residential applications, where terminal temperature ratings are more likely to be 60?C, where circuits are more likely to be overloaded, and where many installations are performed by non-qualified persons that don't understand terminal temperature limitations.

 

[kwired]

No, the terminal wll not melt (at least not initially). What happens is the termination loses integrity over time as it heats up beyond its rating then cools down. The terminal "melts" when the connection has lost integrity to the point where arcing occurs.

I believe the 60? rating is because NM is mostly used in residential applications, where terminal temperature ratings are more likely to be 60?C, where circuits are more likely to be overloaded, and where many installations are performed by non-qualified persons that don't understand terminal temperature limitations.

There is more than terminal temperature limitations that they don't understand. As has been said about code that may be geared toward these individuals - if they don't understand it does it really matter if there is a code addressing it or not? I don't know why NM is limited to 60 Deg but think it is more than just non qualified persons doing installations.

 

[kwired]

Why is it then that I can run a conduit with #8 Thhn/THW(whatever) for a 50A receptacle, but I need #6 for NM?

If your conductor were TW it would need to be #6.

 

[Smart $]

There is more than terminal temperature limitations that they don't understand. As has been said about code that may be geared toward these individuals - if they don't understand it does it really matter if there is a code addressing it or not? I don't know why NM is limited to 60 Deg but think it is more than just non qualified persons doing installations.

Well, as in most cases of qualified vs. non-qualified, there is a vast gray area in between, not to mention countless degrees of both.

And I did mention a few reasons, not just being installed by non-qualified persons.

 

[Strife]

Hold on there, the copper itself(even with insulation) is rated 90 degrees. So the copper can get 90 degree hot without losing integrity(isn't that what 90 degree rating means?). Without insulation the temp rating will be even higher actually. So the problem at the termination is the rating of the termination, and not the copper in the NM(Am I missing something here?).
Now if you tell me a 50A oven outlet is rated at 60 degree, I'll buy it, allthough I never seen an oven outlet melted.

No, the terminal wll not melt (at least not initially). What happens is the termination loses integrity over time as it heats up beyond its rating then cools down. The terminal "melts" when the connection has lost integrity to the point where arcing occurs.

I believe the 60? rating is because NM is mostly used in residential applications, where terminal temperature ratings are more likely to be 60?C, where circuits are more likely to be overloaded, and where many installations are performed by non-qualified persons that don't understand terminal temperature limitations.

 

[Strife]

What makes you think the unqualifed person CARES about the code? or the NM limitations for that matter?
So we're creating a rule to protect from someone who doesn't care about the rule?
WOW
Isn't that like telling a criminal he can't own a gun?

and where many installations are performed by non-qualified persons that don't understand terminal temperature limitations.

 

[Smart $]

Hold on there, the copper itself(even with insulation) is rated 90 degrees. So the copper can get 90 degree hot without losing integrity(isn't that what 90 degree rating means?). Without insulation the temp rating will be even higher actually. So the problem at the termination is the rating of the termination, and not the copper in the NM(Am I missing something here?).
Now if you tell me a 50A oven outlet is rated at 60 degree, I'll buy it, allthough I never seen an oven outlet melted.

Correct. However, the current running through the copper in the NM is what generates the heat that raises the temperature at the termination. If you run enough current through the copper to make it 90?C, the termination temperature will also be 90?C. If the termination is rated 60?C or 75?C, the current through the wire is limited to that which causes its temperature to exceed the temperature rating of the termination.