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Joined: Jul 2004
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Does anyone know why NM cable is singled out to be restricted to 60 degree ampacity? AC and MC can use the 75 degree ampacity; what is "special" about NM?

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Besides NEC article 334.80 telling us we have to... I would guess because of the non-metallic sheathing compared to the metallic jacket of an AC or MC cable.

[This message has been edited by ShockMe77 (edited 02-15-2007).]

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To clarify my question, I understand that 334.80 is where it says this in the NEC. What I am wondering is if anyone here can explain to us what the thought process was that led those who write the Code to decide that this restriction was necessary for NM, but not for other types of cable. For example TC, which is rather similar to NM, has no such restriction.

[This message has been edited by SolarPowered (edited 02-15-2007).]

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If you look at article 110.14 C . It tells you that most wiring terminals and lugs for smaller wiring are rated only at 60 degree c. Even though there might be other reasons this is reason enough.

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I don't think that's it, because #10 and smaller is already restricted to the 60 degree ampacity or less by the limits to 30, 20 and 15 amps. Thus the 60 degree limitation only has relevance for #8 and larger, where you certainly do find lots of 75 degree terminations.


[This message has been edited by SolarPowered (edited 02-15-2007).]

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I think it is because the NM outer sheathing has been rated for 60 degrees; similiar to the requirement for the plastic bushing to have the same or greater temperature rating than the conductors passing through.


Earl
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That also doesn't appear to be the reason, as you are permitted to use the 90 degree column for derating. That will permit the cable to get hotter than allowing the 75 degree column for fusing would allow.

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That will permit the cable to get hotter than allowing the 75 degree column for fusing would allow.

No, I do not believe so as even though the derating can start at 90 C the final derated ampacity can still not exceed the 60 C rating.


Bob Badger
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Let me rephrase that. Derating using the 90 degree ampacity is designed to keep the temperature of the wire below the temperature limit of the 90 degree insulation. If you have a bundle or a raceway with, say, 20 CCCs, you could get up to the 90 degree limit, even with ampacity that's less than the non-derated 60 ampacity.

Whereas, if you were to use the 75 degree column for ampacity, in a context that doesn't require derating, the wire will never heat to more than the 75 degree limit.

Thus I contend that it doesn't appear that protecting the jacket from overheating is the reason for the restriction to 60 degree ampacity. And again, why the limit for NM, and not for TC cable? What is the difference between these two cable types that requires the difference in treatment?


[This message has been edited by SolarPowered (edited 02-15-2007).]

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To expand on an earlier post of mine, this rule is irrelevant for #10 and smaller wires, because those are already limited to 30, 20, and 15 amps, which are at or less than the 60 degree ampacity. So in almost all cases where this rule applies, we are talking about either the feed for a stove or cooktop, or to a subpanel.

So, does anybody have any recollection about what you might have read or heard about the CMPs' deliberations on this subject?

[This message has been edited by SolarPowered (edited 02-15-2007).]

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