NEC Article 110.14(C)

[realolman]

... The 90? wire is allowed to be smaller awg because the insulation dissipates heat better, and also the insulation does not get as soft at the higher temperature as the lower temperature stuff.

Because the "better" insulation allows a smaller awg, the conductor is going to get hotter at maximum amperage, and this is the reason that it can't be used with terminations of a lower rating.

Why, then is it OK to splice the 75? conductor to the 90? conductor which will be hotter, when it is the heat that the 75? conductor cannot "handle" that makes it the lower temperature conductor in the first place?

I hope that made some sort of sense.:smile:

 

[JWCELECTRIC]

I'm not sure I completely understand your point but the reason for THHW in both columns is because in dry location it is rated 90 and in wet it is located 75 :smile:

I am just looking for clarification for the 90 degree column and to eliminate a junction box at each end of two points where you can down size the cables between the jboxes per Iwire's sketch (post #5). I guess it can't be done as I discribed (end of a long week & confussing myself). But if I had 4' length of larger 75 deg cable (per UL testing) inside larger panel, terminated on breaker, then could I butt splice to smaller 90 deg cable inside panel run in conduit between the two points?

 

[Twoskinsoneman]

I am just looking for clarification for the 90 degree column and to eliminate a junction box at each end of two points where you can down size the cables between the jboxes per Iwire's sketch (post #5). I guess it can't be done as I discribed (end of a long week & confussing myself). But if I had 4' length of larger 75 deg cable (per UL testing) inside larger panel, terminated on breaker, then could I butt splice to smaller 90 deg cable inside panel run in conduit between the two points?

As I read it that is fine. You can have a 6inch 75(degree) wire on the breaker wire-nutted to a 90(degree) wire right in the cabinet and then make your run to some other place that it is legal to make splices in and wire-nut it to the 75(degree) wire for your termination....

It is legal because the conductor does not "exceed the lowest temperature
rating of any connected termination",

 

[dejeud]

There's a nice article on this by someone who sounds pretty well up there in the electrical field.

http://ecmweb.com/mag/electric_middle_wire_isnt/

I'll try to explain it myself also.

The reason we use different size conductors, protection and the most of the NEC hoopla it's one and one only: HEAT caused by the ampacity that runs thru that wire.
A #12 wire rated at 30A, 90C mean that when there's 30A running thru that wire it'll get 90C hot. Now, when you land this wire on a terminal rated for only 75C or 60C and the wire gets 90C hot, you got a problem, the terminal will cook. This takes care of the first part.

However, as the article I mentioned above is titled, the middle of the wire it's not its end. When the wire is in conduit or or a box or a gutter, there's other heat limitations that apply. When you have 6 conductors getting 90C hot, the heat will accumulate so NEC limits the heat each wire will create by limiting the amps each wire will carry,see the derating table for 3 or more conductors. Same reasoning applies to the temperature corrections. This is the second part.

Let's say I want to size the wire for a 40A circuit landing on a breaker with a 60C lug rating and 6 conductors in the raceway.

1: "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device."
That means that in sizing my wire I have to look up at the 60C ampacity column which is #8.

2: However if I use a 60C rated wire and derate for the 6 conductors, I'll get only 32A allowed. So if I am to use 60C rated wire I will have to use a #6 wire.This is where the second part of the article comes in play:

Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both.

So if I look at 90C rated #8 wire I see it's 55A. This part of NEC allows me to use the 90C rated #8 and derate from the 55A value when making corrections for the number of conductors.

This has been a never ending discussion and sadly, even after the code made it clearer, a majority of people still claim that if you have 6-#12 in a conduit you have to derate from the 20A value and not the 30.

NEC 2008 Article 110.14 (C):
The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperatur ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both.

I read it many times but I did nor quite understand what it want to mean?
I will appreciate if anybody can explain thie article with examples.

Thanks,
Mamun

 

[Smart $]

...

Why, then is it OK to splice the 75? conductor to the 90? conductor which will be hotter, when it is the heat that the 75? conductor cannot "handle" that makes it the lower temperature conductor in the first place?

...

It not entirely about a conductor's insulation properties. It's about the heat generated from current passing through the conductor due to resistance—I?R loss. Heat generated equals temperature increase and the conductor is connected to terminals and device that also have temperature ratings. A device which exceeds its nominal operating temperature range may not operate as designed properly. A terminal has a temperature rating because through time, the expansion and contraction due to the heat generated by the current and lack thereof passing through it will degrade the connection, and become worse at an exponential rate through continued cycling.

 

[dejeud]

No, NO, NO, and No.
It's not because the heat dissipates better, it's because the insulation is rated to withstand up to 90C hot. Therefore NEC only allows 30A on a #12 wire rated at 90C because, more or less, that's how hot that #12 will get when you have 30A running thru it.

Really people, please just read this article in its entirety, I've been battling this for 20 years now and I'm baffled at the lack of knowledge that exist when it comes to the most important part of electricity.

http://ecmweb.com/mag/electric_middle_wire_isnt/

... The 90? wire is allowed to be smaller awg because the insulation dissipates heat better, and also the insulation does not get as soft at the higher temperature as the lower temperature stuff.

Because the "better" insulation allows a smaller awg, the conductor is going to get hotter at maximum amperage, and this is the reason that it can't be used with terminations of a lower rating.

Why, then is it OK to splice the 75? conductor to the 90? conductor which will be hotter, when it is the heat that the 75? conductor cannot "handle" that makes it the lower temperature conductor in the first place?

I hope that made some sort of sense.:smile:

 

[MikeTX]

a question

a question

If circuits >100A will be limited to 75C and possibly lower, why are there so many cable types with high temperature ratings? Seems like we are buying the higher rated cables, but throwing money away since the cables will eventually be code limited to a fraction of their ratings. In other words, why buy a 150C or 200C cable when a 75C or 90C cable does the same thing? :-?