NM cable. WHY 60 degree.

[Smart $]

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 that's like saying every criminal carries a gun illegally. What about the criminals that carry a gun legally? What about the criminals that have no use of a gun?

An apprentice is a good example of an unqualified person that should care about code.

 

[Hv&Lv]

I would have to guess a combination of articles, including 110.14, 240.4(D), and 334. also annex E for additional information for article 334. Since NM-B can be fastened directly to wood members, My guess is there cannot be any heat due to overload available to aid in combustion.
How hot does a 12 guage wire get carrying 20 amps for a while? (just curious about this)

 

[Strife]

NO,
I said you can make all the rules, criminals will still find a tool to kill.
In Florida a felon is not allowed to own a gun, so there shouldn't be any criminal carrying a gun legally. You think there isn't?
HA HA

And that's like saying every criminal carries a gun illegally. What about the criminals that carry a gun legally? What about the criminals that have no use of a gun?

An apprentice is a good example of an unqualified person that should care about code.

 

[Smart $]

NO,
I said you can make all the rules, criminals will still find a tool to kill.
In Florida a felon is not allowed to own a gun, so there shouldn't be any criminal carrying a gun legally. You think there isn't?
HA HA

So what about the criminals that haven't been convicted? Are you saying there are none that carry a gun legally? Yes, I'm certain there are persons, both criminals and not, that carry a gun illegally... but there is likely both that carry legally. The point is that not all scenarios fit into a black and white classification system no matter how hard you try.

 

[rbalex]

Is anyone aware of a manufacturer that uses anything other than THHN for their NM-B?

 

[M4gery]

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?

Isn't that the reason why we need to put neutrals in all switchboxes now?


(as of 2011 code, that is)

 

[K8MHZ]

Is anyone aware of a manufacturer that uses anything other than THHN for their NM-B?

Isn't NM-B rated for 90 degrees?

You were the first to mention NM-B. Inspector told me that NM and NM-B had different temp ratings and that NM-B was good for 90.

 

[rbalex]

Isn't NM-B rated for 90 degrees?

You were the first to mention NM-B. Inspector told me that NM and NM-B had different temp ratings and that NM-B was good for 90.

I don't do residential and NM (and NM-B) isn't really in my skill set (ask Don ) But, as I understand it, NM-B is just about all that is available in the "NM" world today - and it's still limited, with various adjustments, to 60C per 334.80.

I also haven't seen anything other than THHN as the basic wire. I was asking whether anyone knew of any other insulation being used because it may explain at least part of the OP question. About 15 years ago NFPA was on the warpath about "products of combustion." Since THHN and the jacket of NM is primarily PVC it has some very nasty burn properties.

 

[augie47]

I don't do residential and NM (and NM-B) isn't really in my skill set (ask Don ) But, as I understand it, NM-B is just about all that is available in the "NM" world today - and it's still limited, with various adjustments, to 60C per 334.80.

I also haven't seen anything other than THHN as the basic wire. I was asking whether anyone knew of any other insulation being used because it may explain at least part of the OP question. About 15 years ago NFPA was on the warpath about "products of combustion." Since THHN and the jacket of NM is primarily PVC it has some very nasty burn properties.

It is my understanding that the listing for NM conductors is not specifically the same as THHN. The insulated conductors for NM are not marked.

This is Southwire's product spec sheet:
http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheetOEM122

 

[rbalex]

It is my understanding that the listing for NM conductors is not specifically the same as THHN. The insulated conductors for NM are not marked.

This is Southwire's product spec sheet:
http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheetOEM122

I believe your understanding is correct; I'm refering to Section 334.112 that requires "The insulated power conductors shall be one of the types listed in Table 310.13(A) that are suitable for branch-circuit wiring or one that is identified for use in these cables. Conductor insulation shall be rated at 90?C (194?F)." I note the FPN Includes cables with the "-B" mark as compliant withthe requirement.

Edit add - it also appears that Southwire is essentially using an unmarked THHN.

 

[kwired]

And that's like saying every criminal carries a gun illegally. What about the criminals that carry a gun legally? What about the criminals that have no use of a gun?

An apprentice is a good example of an unqualified person that should care about code.

An apprentice although unqualified is supposed to be supervised by someone who is.

I would have to guess a combination of articles, including 110.14, 240.4(D), and 334. also annex E for additional information for article 334. Since NM-B can be fastened directly to wood members, My guess is there cannot be any heat due to overload available to aid in combustion.
How hot does a 12 guage wire get carrying 20 amps for a while? (just curious about this)

Ever open a panel in a dwelling on a hot summer day when the A/C has been running all day?

The A/C circuit will be the only one that is hot, the breaker will also be pretty warm. Most other dwelling loads are not continuous enough to develop this kind of heat.

One thing that has not been brought up in this thread is that the breaker uses the conductor as a heat sink to dissipate heat away from the breaker. The first few inches of the conductor will be hotter than the rest of the conductor.

Isn't NM-B rated for 90 degrees?

You were the first to mention NM-B. Inspector told me that NM and NM-B had different temp ratings and that NM-B was good for 90.

NM-B can be derated from the 90 degree values, but final conductor size must still be as large as the size from the 60 degree column. This is still same thing that must be done with THHN/THWN, it can be derated from 90 degree values but final size must still be at least as large as 60 or 75 degree column depending on termination ratings.

 

[nosparks1]

My understanding of the 60 degree rating was due to the construction spec of the conductors lack of heat dissapation thru the sheathing and in the 1970's they went to the 90 degree insulation on the conductors to reduce to adverse affects of the excessive heat on the 60 degree conductor insulation when out of the jacket in outlet boxes for lighting applications.

 

[Smart $]

An apprentice although unqualified is supposed to be supervised by someone who is.

So no apprentice ever wired his buddy's new house on the side???

 

[kwired]

So no apprentice ever wired his buddy's new house on the side???

Nope, that never happens:lol:

Sad thing is often his family or buddies kind of take advantage of him so they can save a buck more so than he willingly gets into things he is not necessarily sure about.

The ones that care about doing things right will ask their boss, coworkers, this or other forums, questions when they are not sure about something.

The biggest thing they never think about is liability if something should go wrong, even if it is a few years after the installation was done. But then there are lots of people moonlighting in other professions that have the same issue.

 

[K8MHZ]

NM vs. NM-B

Inspector told me that NM-B came around because, if you look in the directions, most, if not all lighting fixtures specify they be connected to 90 C wire. That would be pretty tough with plain old NM.

Made sense to me.

 

[zappy]

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.

It protects the DIY homeowner who puts a larger fuse in because he needs more power:roll:

 

[Hv&Lv]

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.

I can take 20 NM cables and tie them together in a tight bundle with cable ties and derating will not apply as long as there is spacing between the cables on either side of the bundle.

Isn't this what the other post was about?http://forums.mikeholt.com/showthread.php?t=141632&page=2

 

[hurk27]

If I can remember it right, is the temp rating of a conductor is a combination of things but the one that has the biggest effect is the heat conductivity of the conductor to remove the heat at a terminal, a larger wire will remove more heat from the terminal like a heat sink, If you have notice that over loaded circuits fail more often at terminals then any where else.

 

[Smart $]

... a larger wire will remove more heat from the terminal like a heat sink, ...

That's true as long as the wire is not the source of the heat... which is why we have 110.14(C). But for a given amount of current, a larger wire will generate less heat.

 

[kwired]

If I can remember it right, is the temp rating of a conductor is a combination of things but the one that has the biggest effect is the heat conductivity of the conductor to remove the heat at a terminal, a larger wire will remove more heat from the terminal like a heat sink, If you have notice that over loaded circuits fail more often at terminals then any where else.

That is also why a 100% continuous rated breaker does not require derating the conductor 25% for continuous loads. This breaker doesn't have additional heat to sink into the conductor.