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?

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).]

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).]

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.

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).]

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.

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.

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.

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).]

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).]

If you look at my proposal to the 2002 7-178
you see they reference a submission to the 84 code when they say no. I suppose nobody told CMP7 they are using 90c conductors in NM now. In the early 80s it was TW

Solar I really do not understand your thoughts. That could be a problem on my end.

But here is my issue, no matter what column we start with the derating the final ampacity can not exceed that of the 60 C column.

The heat in the wire is a function of the current flow, the wire resistance, and the ambient temperature.

The power produced by the current flow is I squared R, we all remember. This is presented as heat in the wire. If there are a lot of other wires in the same conduit, this heat adds up; if the wires are run through a boiler room or through a hot attic, the power-loss/heat-gain is added to an already hot wire, maybe heating it beyond it's rating.

The rating of the pastic that makes the sheathing on NM, must not be rated 90 degree, is all I can think of that would cause the NFPA folks to not allow the 90 degree conductors to be loaded so that they would be heated beyond 60 degrees.

One thing that is confusing, is: we call the conductors 60-75-90 degree insulation, but we are talking Celsius, not Farenheight. 90 degree C equals 194 degrees F, way beyond a hot day in the desert.

Current NMSC is rated for 90 degrees in Canada but our ampacities are derived a little differently. Older NM cables were 60 and 75 degree wire.

It may have to do with installation practices and the thickness of the insulation- NM is unique in having such a thin jacket and having direct contact with combustible material. I don't see why it would be difficult to have put on a 90C jacket, but heat transfer THROUGH that jacket may pose a fire risk unto itself.

OK, suppose that you have 8/3 NM that you are using for three-phase branch circuits.

If you have one of those 8/3's out in the open, its ampacity is determined by the 60 degree column, which is 40 amps. You fuse the circuit at 40 amps, and the temperatures stay within the 60 degree limits.

Now, suppose you run three of those 8/3's in a raceway. You must derate. You are allowed to derate using the 90 degree column. Nine CCCs derate to 70% of their value, so 70% of 55 amps in the 90 degree column is 38.5 amps. That's less than the 60 degree ampacity of 40 amps, so we set our adjustable breakers ( just for pedagogical purposes) to 38.5 amps.

The temperature in that raceway can rise to the 90 degree temperature limits.

Thus, since 334.80 explicity allows using the 90 degree column for derating, it appears that protecting the jacket from temperatures higher than 60 degrees is not the reason for the ampacity limitation. (I could be wrong--the CMP might have had a schizophrenic moment and written conflicting sentences right next to each other. )



[This message has been edited by SolarPowered (edited 02-16-2007).]

The reason that NM is rated at 60 degrees is that the standard that it is tested at is a 60 degree standard.

One could design an NM type of cable, test it at the 75 or 90 or 105 degree standard and have it so rated. It would not be NM - NM implies the 60 degree test standard.

But then, why does 334.80 allow derating using the 90 degree values? As I show above, this allows the cable to heat to the limit for 90 degree insulation. If the cable is only tested to 60 degrees, and therefore in theory possibly dangerous above 60 degrees, then allowing derating using the 90 degree values makes no sense.

That's why I think this rule is more to protect what's around the cabling than the cabling itself. If the conductors in a single run of NM (not bundled) were allowed to get to 90C, it could potentially create dangerous levels of heat in the sap-soaked 2x4 studs it's run through, etc.

Now, if we assume that when NM is bundled together, then only the outer cables of the bundle need be subject to the 60C restriction. The inner cables of the bundle will be the only ones unable to dissipate enough heat and thus the only ones subjected to the higher temperatures, and may reach 90C without harming anything.

...anyone else want to grasp at straws? I can't think of any other good reasons 90C derating would be allowed.

Solar I appreciate your reply and I am still willing to concede I may be wrong here.

This is the point I have an issue with.


I do not believe that to be true.

It seems to me it should no increase past the 60 C temp range.

Let me ask this, if that where true and the conductors where going into the 90 C temperature range what electrical device could I terminate them to?

Breakers, switches and outlets are not generally rated over 75 C.

Bob

Bob,

Consider this as an example from a different direction: Suppose that you have three #8 THHN CCCs in a pipe. Further suppose that they are connected to 90C terminations so that they can run using the 90C table values. By table 310.16, we can use these at 55 amps.

Can we agree that the purpose of the 55 amp limit is to keep the wires below the 90C temperature limit?

Then, suppose that we add more conductors. Say, bring the total number to 20 CCCs. Because of the thermal resistance of the conduit and surrounding materials, the temperature inside the pipe would heat up to more than the 90C limit if we ran these 20 #8's at 55 amps. So, we must derate to keep the temperature below 90C. Table 310.15(B)(2)(a) says that we have to derate by 50%, to 27.5 amps. By doing so, we keep the temperature in the pipe below the 90C limit.

The same logic applies to NM. When we derate using the 90C table values, we are derating to keep the temperature below the 90C temperature limit.


Let's start out with a case that's a bit artificial. Suppose you have your cables starting at the panel, running for 20 feet with adequate separation. Then, for some reason, you have to bundle them for 20 feet, and then they run for another 20 feet with separation before they reach the load.

In this case, it's easy to see that the 20-foot bundled section can get hot, but the terminations will be cooler. So the bundle might be at the 90C limit, but you'd be fine with 75C terminations.

More realistically, the bundling or well-filled conduits normally occur right at the panel. (And I'm talking here about all wire types. This case is particularly true for THHN in pipe.) It appears that normal practice is to assume that the length of wire inside the panel, before it gets to the breaker, is sufficient to allow the wire to cool enough for a 75C termination.

I'm not sure that's really what happens, but it appears that this assumption doesn't cause problems in practice, so no one has tried to make the code more restrictive in this area.

I suspect that the reason it works out OK is the derating tables are worst-case, but the worst case just doesn't happen in practice. For example, you can run 20 #10 THHN derated to 20 amps in one pipe. In theory, if there were 20 amps on all 20 conductors, you could get up into the 90C range. In reality, you just don't have that much load on all of them. If they are, say, office receptacles, the average load might be a third to half of the 20 amps. So, in practice, the wires don't get hot enough that they're toasting the 75C terminations on the breakers. Thus, no one has bothered to change the code to fix a theoretical problem that isn't being seen in practice.


[This message has been edited by SolarPowered (edited 02-21-2007).]

I disagree with your assumption. In some cases the terminations might only be rated 60 C.

So we would have to drop 30 C or 86 F in a short distance.

Even with 75 C terms we would have to drop 43 F in a short distance inside an enclosure that would certainly be heating up.

In your other example with the NM I would say that obviously the bundled section will be warmer than the unbundled section but no section will exceed 60C.

I am still willing to admit I am wrong here but sure would like to here some other views or explanations.




[This message has been edited by iwire (edited 02-21-2007).]

Sorry that I've not been following this thread, but the original question happens to be one that really bugs me. It seems to me that the CMP did one of those 'back handed' code changes where one part of the code is adjusted to correct an error that really should have been dealt with someplace else. An example is 310.15(B)(6), where a better load calculation would probably make more sense. In this case, the CMP is correcting an error in ampacity calculation; but rather than saying that NM cables with 90C temperature ratings have _different_ ampacity than THHN conductors in conduit with 90C temperature ratings, they simply say that NM cables must be used at their 60C ampacity.

The physics of ampacity calculation is given by the Neher-McGrath equation, http://www.calcware.com/cwnmcalc2.htm . This equation is explicitly permitted to be used for ampacity calculations, under 310.15(C) if you have suitable 'engineering supervision'.

What the Neher-McGrath equation gives us is a relationship between the conductor temperature, the surrounding ambient temperature, the electrical resistance of the conductor, the thermal resistance between conductor and ambient, and the current being carried by the conductor. The conductor temperature is not set only by the current and ambient conditions, but is also set by the thermal resistance to ambient. Presumably any other sources of heat (other conductors) in the vicinity would be considered somehow as part of the ambient.

The tables permitted under 310.15(B) are based upon the Neher-McGrath equation with certain assumed parameters of thermal resistance and insulation characteristics. These parameters are never stated explicitly, however it is quite clear from the way the values of table 310.16 change with ambient temperature and permitted insulation temperature that the Neher-McGrath equation is being used.

This tells me that if a conductor has an ampacity of X amps, and I push X amps through that conductor, then if the presumed conditions of thermal resistance hold, then the conductor will heat up to its maximum temperature ( the temperature value at the top of the column). If I take three #8 THHN conductors, and bundle them together into a conduit, and run 55A through each of them, and if the ambient temperature is 30C, and <b>if the thermal resistance of my experimental setup matches that assumed when table 310.16 was generated</b> then the temperature of these conductors would hit 90C.

In particular, if the _derated_ ampacity of a conductor is based upon the 90C rating, then I expect the conductor to heat up to 90C when used at its ampacity. Going back to the example above, three #8 THHN conductors, in a conduit, in a 50C ambient condition. I run 45A through these conductors. I expect a conductor temperature of 90C, even though the conductors are being used at less than their normal 75C ampacity.

Unfortunately, I've not worked 310.15(B)(2)(a) into this thought process Take 3 conductors at full capacity and call the heat output 1. 6 conductors at 0.8 capacity would have a total heat output of 1.28, 9 conductors at 0.7 would have a total of 1.47, 20 conductors at 0.5 would have a total of 1.67, 30 conductors at 0.45 would have a total of 2.03 and 40 conductors at 0.4 would have a total heat production of 2.13. Clearly the assumptions about thermal dissipation capability have to change as the number of conductors in a raceway goes up.

I am forced to assume that 310.15(B)(2)(a) was derived in a fashion similar to table 310.16, and that if I have a set of conductors with derated and _adjusted_ ampacity of X, and I run X amps through these conductors, that the temperature of the conductors would rise to the 'temperature rating' of the conductor as used in table 310.16 prior to derating and adjustment.

This problem with this whole approach, however, is that the thermal resistance in the real world is very unlikely to match that used to derive table 310.16. The thermal resistance numbers are presumably relatively conservative. However if, for whatever reason, the thermal resistance in a given situation were _higher_ than that assumed for table 310.16, then the temperature of the conductors would end up higher than expected.

My first guess is that the CMP decided to deal with the issue of 'NM cable buried in thermal insulation' by simply saying 'use the 60C ampacity'. This is probably a much simpler approach than having to develop an entire different set of tables for 'NM cable buried in thermal insulation' and trying to deal with the enforcement nightmares. But it hides the reality that a 90C conductor is a 90C conductor, designed to be used at up the point where the wires are actually at a temperature of 90C, near to the boiling of water.

The above leads me to a different guess. NM cable is generally permitted in flammable construction where the fuel load of the plastic cable is nothing compared to that of the rest of the building. I guess the thought is that 'wood is already flammable, why worry about a bit of plastic'. The kindling temperature of wood is very much dependant upon the water content of the wood. Heat wood up to 90C for an extended period of time, and you boil the water off and make it easier to ignite. Perhaps the 60C restriction is used to protect the building materials that NM is ordinarily used with.

-Jon

But you just explained that using the derating rules the temperature will reach 90 C.

I appreciate you jumping in here, I am trying to soak this in but I am not convinced that a conductor loaded to its rated capacity 60 C under the conditions given will normally reach 60 C.

I am willing to bet that there is some headroom left there for safety.

Bob

Bob- I'm watching to see what the answer is to the question you've asked. I've asked the very question of UL and they don't come back with any answers. After I'd asked a coupla different times from a coupla different UL reps at our IAEI meetings I quit asking - figured they thought it wasn't an important enough to justify an answer. I like you, suspect that there is some fudge room and the actual tempetature reached is probably 10° or so below the insulation rating. What is clear to me is it's the insulation on the wire that we are concerned about not the copper wire. A #12 copper wire will actually carry about 100a. in the uninsulated state. Don't know where I read that information but it stuck with me.

As I said, I was only guessing as to the why of the 60C rating.

I am certain that the ampacity values are based upon the temperatures used at the top of table 310.16.

I agree that there must be serious 'fudge factors' built in to the tables, however IMHO these fudge factors are in the thermal resistance values assumed.

If the fudge factor were in the temperature (meaning that they would say 90C but really target 80C, for example) then the correction factors at the bottom of the table would be different.

Say you have 10 12/2 cables bundled together, carrying 15A per circuit (so that you have 20 current carrying conductors, 30A derated to 15A by 310.15(B)(2)(a). I believe that in the thermal resistance circumstances built in to 310.16 and 310.15(B)(2)(a) that the temperature in the core of the bundle would hit 90C, and that if the real temperature is lower this is because the thermal resistance is lower.

-Jon

CMP7 says the answer is in a report to the 84 code (ROP to the 2002 code 7-178). Maybe someone with the ROP from that cycle can see what they are talking about.

Don't know if this helps but the following is from the 1981 NEC Handbook commentary on Section 336-2:
Types NM and NMC may have conductors rated 60'C (140'F), 75'C (167'F), or 90'C (194'F) for use in different ambient temperatures. Cables with conductors rated at 75'C (167'F) are designated type NMH or NMCH, and thoase with conductors rated 90'C (194'F) are designated Type NMHH or NMCHH. The ampacities of nonmetallic-sheated cable types, regardless of the conductor tempurature rating, are those of 60'C (140'F) conductors.

The commentary from the same section in the 1984 Handbook:
Prior to the 1984 Code, Types NM and NMC could have conductors rated 60'C (140'F), 75'C (167'F), or 90'C (194'F) for use in different ambient tempuratures. Cables with conductors rated at 75'C (167'F) were designated Type NMA or NMC-A and those with conductors rated 90'C (194'F) were designated Type NMB or NMC-B. The ampacities of nonmetallic-sheathed cable types regardless of the conductor tempurature rating, are those of 60'C (140'F) conductors.

I believe all the NM cable on the market now is Type NMB. The ROP and ROC from 1981 and 1984 would seem to be the place where the answer to the 60' ampacities mystery can be found.
Alan--

When there was a mix of ampacities out there this probably made sense (like in 84) but now it is hard to find any 60c NM unless you are digging in an old wall. I am not sure why this still remains, hence my proposal.
I was never happy with the answer but it is my fault that I didn't ask for more from them in the comment phase. My bad ... sorry.