ECN Forum Forums Code Related Discussion NEC & other Code issues Why is NM restricted to 60 deg ampacity?

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