Conductor Temperature Rating

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Danbig

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Ok, so this may be a very basic quesiton, but what exactly do Temperature Ratings of Conductors from table 310.16 mean?

I know the entire table is based on an ambient temperature of 30? C, and I understand the derating for higher ambient temperatures.

Do the temperature ratings for the conductors ever come into the picture? Are 90? C rated conductors able to operate without failure in environments up to 90? C?
 
Danbig said:
Ok, so this may be a very basic quesiton, but what exactly do Temperature Ratings of Conductors from table 310.16 mean?

?
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it is the amount of Amps needed on a wire (depending on the size, and material(copper/aliminum)) needed to heat that wire to the particular temperature.

it means if you take a #8 THHW(that is in the 90deg column) and you put 55A on it, the tempature of that wire would reach 90degC(194degF)

but if you put 40A on the same wire then the temp of that wire would reach 60degc(140degF)

therefore a #8TW with 55A on it would reach 90degC BUT the wire is only rated for 60degC so damage to the wire(insulation) would probably be the end result(not good)

hope that helped
 
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Yes. Read the 3rd column in table 310.13.

Also, look at the correction factors at the bottom of 310.16. The allowable ampacity starts to drop of rapidly as you approach the temperature rating of the insulation.You'll also notice there are "--" in the 60?C and 75?C columns. You can't exceed the limit of the insulation.
 
Be aware that these tables make assumptions about the rate of heat dissipation and thermal insulation around the conductors.

What the tables tell you is that for a given ambient temperature _and_ a given set of thermal characteristics for the installation, when the conductor is operated at the number in the table (as adjusted for ambient) it will reach the temperature at the top of the column.

The heat dissipation values are quite conservative, and the load calculations are generally quite liberal, so generally one doesn't actually have conductors at 60C, let alone 90C. However some circumstances (conductors embedded in foam insulation, for example) the thermal insulation can be better than the table assumes, and conductor temperatures will be higher.

The temperature rating of a conductor is also somewhat statistical in nature. When a conductor is operated at its temperature rating, you expect it to last for a given amount of time (20000 hours is the number used for motor design; I don't know what is used for building wire). Operation at higher temperature won't instantly destroy the wire, but will make it age faster; operation at lower temperature will help the wire last longer.

-Jon
 
Adding to what has been posted:

Regarding copper conductors.
It is really not the conductor so much as the insulation types. Notice for the same size conductor, that different insulation types have different temperature ratings.
Depending of which table you read, copper melts a around 1970F-1990F.
Those temps are much higher than the temps of 140F, 167F and 194F



Here is a pretty interesting link to a Periodic Table.

www.chemicalelements.com
 
Pierre C Belarge said:
Here is a pretty interesting link to a Periodic Table.

www.chemicalelements.com
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If you notice, the elements we consider most conductive all have a single electron in the outermost energy level (aka valence shell). That's the free electron that is easily displaced by another, which allows the element to conduct electricity.
 
winnie said:
The heat dissipation values are quite conservative, and the load calculations are generally quite liberal, so generally one doesn't actually have conductors at 60C, let alone 90C.
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I was thinking that 194? F seems high for a #8 THWN running 55 amps.
 
480sparky said:
That's because #8 THWN is rated 75?C/167?F with an ampacity of only 50.
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My mistake. I meant to say THHW. In any case, I'd be interested to know if anyone has actually observed a conductor generating anywhere near this much heat. I think I would be worried if I felt a conductor at over 100? F. Wouldn't that be hot to the touch?
 
Dan
Next time you get a chance, take a receptacle circuit and identify it in the panel so you know which conductor/breaker to test. Next, load that circuit up (drills, heaters, etc...) and let the loads operate for a while. Then go and test the temperature of the conductor and breaker -especially the termination. You will notice it is most likely quite warm and you possibly will not be able to even touch it.
 
If you were to take _3_ number 8 conductors in THHN insulation, bundle them together, install them such that they were surrounded by exactly the sort of thermal insulation assumed by table 310.16, place the installation in a 30C ambient environment, and then run 55A on a continuous basis through all three conductors, then the conductor temperature would end up pretty darn close to 90C.

I'd say exactly 90C, except that the table is clearly rounded to 5A values :)

-Jon
 
Let me offer a different way to look at it. The table is not saying that at a given ambient temperature (e.g., 30C), and under the specified set of local conditions (e.g., 3 conductors in conduit), a conductor of the given size (e.g., #8) and material (e.g., copper) and insulation system (e.g., THHN) carrying the tabulated amount of current (i.e., 55 amps) WILL reach the temperature at the top of the column (i.e., 90C). Rather, it is saying that that temperature (i.e., the rated temperature of the insulation system) will not be exceeded.

Set up the experiment, if you wish. There is no assurance that you will read 90C. But there is assurance that you won?t read 91C.
 
Charlie,

I think that we are in practical agreement.

I claim that given sufficient _thermal_ resistance between the conductors and ambient, the conductor temperatures will reach the temperature rating of the insulation.

However I believe that tables are very conservative with respect to the assumed thermal resistance values. In the real world with practical installations, thermal resistance is generally be lower than the tables assume, and thus conductor temperatures will be lower than the tables suggest.

A caution is in situations where thermal insulation is much better than the usual case. We had a discussion in the past year or so about NM cables passed thorough firestopped holes in insulated top plates, or some such, where conductor temperatures were _very_ high.

-Jon
 
Danbig said:
My mistake. I meant to say THHW. In any case, I'd be interested to know if anyone has actually observed a conductor generating anywhere near this much heat. I think I would be worried if I felt a conductor at over 100? F. Wouldn't that be hot to the touch?
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I once read a "rule of thumb" or "finger" relating to drive belts slipping, tension. etc. It said that if you keep your hand on the belts for more than several seconds, it was <140F. That depends on your tolerance for heat but 100F is barely warm, just over body temperature.
 
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