310.16

[George Stolz]

So, let's imagine this inspector is a good one, operating under the assumption that the attic in question is going to reach 185?F.

How do you proceed? How do you find "the number"?

(Yes, I have 80 acres and a shovel too, but I'm overlooking that option.)

 

[pierre]

George


The multiplier for the ambient temperature correction is .41.

It looks like you would hypothetically be changing the size of those conductors.
As I said before, I think the issue here is greater, as you are talking some very high ambient temps, and maybe THHN is not suitable for these temps over a period of time. Hence call the manufacturer.

You should contact a manufacturer with this question and see what they have to say.

 

[macmikeman]

I might be wrong, but I kinda doubt they get to 185 deg F. Water boils at 212 deg F. If it was 185 and you went up there you would probably croak.

 

[iwire]

So, let's imagine this inspector is a good one, operating under the assumption that the attic in question is going to reach 185?F.

How do you proceed? How do you find "the number"?

If the space in question will reach 185 F than you can not run conductors in that space based on 310.16 it's that simple.

To run conductors in a space that reaches 185 F would be ignoring (violating) 310.16.

It would be no different than ignoring the ampacities listed in 310.16 at 'normal' temps.

How do we tell if that space reaches 185 F?

The same way we tell a damp location from a wet location....a judgment call by the inspector.

 

[George Stolz]

How do we tell if that space reaches 185 F?

The same way we tell a damp location from a wet location....a judgment call by the inspector.

So the enforcement of 310.10 falls back to 90.4?

 

[iwire]

How do we tell if that space reaches 185 F?

The same way we tell a damp location from a wet location....a judgment call by the inspector.

So the enforcement of 310.10 falls back to 90.4?

No.

There is no option, we must always follow 310.10 each and every time.

Determining what the ambient temperature will be is a judgment call just as deciding if a receptacle located outside under an overhang in a damp or wet location.

Or

Deciding if electrical equipment is subject to physical damage.

Or

Deciding if electrical equipment is 'securely fastened'.

You know as well as I do there are many times where the NEC leaves a lot to those on the scene. 8)

So the only thing up in the air here is determining the temp in an attic. which will be different in Death Valley than Northern VT.

 

[George Stolz]

So it sounds as though we have consensus: A proposal is in order.

Pierre, who was doing that study, do you remember?

 

[iwire]

So it sounds as though we have consensus: A proposal is in order.

Not shooting you down I just don't know what your proposing. :?

Forget ambient temperatures all together?

Require a temperature history to be done?

Predict the rate of global warming?

Pick a number out of a hat?

How about a temp sensor that dumps the main if the temp raises past our best guess. :wink:

 

[George Stolz]

Either:

1. Ambient temperature is the maximum temperature to be expected in a space at any moment in time.

or:

2. Ambient temperature is the average temperature to be expected in a space over an extended period of time.

FPN: An extended period of time may be determined by the AHJ. For most pediatricians, three months from June through August is preferred, but some folks do a year. One curious little hamlet in North Dakota uses the temperature in the five minutes between the bark of the ardemute and the cry of the oozack.

I don't care. But it's a start.

I do like the temp-dumping main idea, I'll workshop that.

 

[pierre]

I know that NFPA is part of the study. In one of the past IAEI and NEC Digest magizines I read about it.

I believe that when the study is complete, they are going to try and incorporate the results into some kind of code.

 

[iwire]

George I firmly believe it will never be the average temperature.

Averages can mean very little.

You can have a very low average with spikes that are very high.

Again I point out if we could use average temperatures we could use average ampacities.

The primary reason any of our conductors are limited in ampacity is to protect the insulation from heat damage.

I am baffled why anyone thinks the NEC would or should allow the average ambient to be used. :? :?

Next time I get pulled over for speeding I am going to tell the cop my average speed was far below the posted speed limit and he should let me off. :lol:

 

[George Stolz]

Watch this patented move: I'm gonna throw a definition in the air, and while everyone is looking up, I'm going to blaze outta here...

The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.

Bob, I agree with what you're saying. Let me think on it.

 

[charlie b]

I hadn't been watching this one, and you guys let it get out of hand. Actually, to me the issue is simple.

You can have a very low average with spikes that are very high.

True, but irrelevant. Unless the spike is so high that it causes an immediate melting of the insulation system, then the spike has a limited, and well defined, impact.

I've talked about this issue before, and am too lazy to look up my earlier theses. So I'll just summarize.

It's not the ambient temperature that matters. It's not the temperature rise (due to current flow and I^2R heating) that matters. It's the combination of the two ? the overall temperature of the insulation ? and the time spent at that temperature, that matters. The thing that it matters to is the conductor's insulation system. Higher temperatures degrade the insulation faster, and reduce its useful lifetime. Lower temperatures degrade the insulation slower, and extend its useful lifetime.

The impact can be modeled with the Arrhenius Equation. It's an "exponential equation," a term that means the part about "higher temperatures degrade the insulation faster" has a more dramatic effect than the part about "lower temperatures degrade the insulation slower." For example, if a cable spends one hour at a temperature 10 degrees above normal, it would have to spend two or more hours at a temperature 10 degrees below normal, in order to break even.

But in the vast majority of real-life applications, it will break even. OK, so it gets hot in the attic during summer days. But it cools down at night, and it doesn't get as hot the other three seasons. Keep in mind that in most of the country 30C is very hot, compared to a spring or fall evening, not even to mention winter. If you were to calculate the net impact (using the Arrhenius Equation), I am convinced that the rate of degradation is lower than the table's "base case" (i.e., holding the ambient at a constant 30C forever, and running the maximum allowable current forever).

I firmly believe it will never be the average temperature.

I firmly hope that your prediction does not come true.

 

[iwire]

I hadn't been watching this one, and you guys let it get out of hand. Actually, to me the issue is simple.

You can have a very low average with spikes that are very high.

True, but irrelevant. Unless the spike is so high that it causes an immediate melting of the insulation system, then the spike has a limited, and well defined, impact.

Charlie your right it is a simple issue there is nothing in the NEC that allows a conductor to be overloaded. (other than how we ignore start up currents)

You can not ignore 310.16s ratings

Not for 1 minute not for 10-12 hours of daylight heating the attic.

I have no issue with your thoughts that short term overheating may not be damaging but that is the issue that is irrelevant.

The relevant issue is it is a violation to use the conductor outsides its rating.

I am not going to agree to disagree on this one.

Bob

 

[iwire]

Charlie please consider this that I posted earlier.

Lets keep this simple.

I have a circuit running through a room with a constant ambient temperature of 160 F the load on the circuit is 100 amps non-continuous.

What size conductor do I need?

4/0 copper 90 C.

(260 * .41 = 106 amps)


Now things change, 1/2 the day the temp drops to 40 F or an average temp of 100 F each day.

Can I now reduce the conductor to 3 AWG due to the 100 F average temp.

(110 * .91 = 100 amps

No.

Because each time the temp goes up to 160 F and the 100 amp load is applied the conductor is overloaded per table 310.16

This rise in temp causes all the same violations of article 240 as not derating when more than 3 CCCS are in a raceway or simply ignoring 310.16 in general.

It is that simple.

 

[pierre]

I am going to agree with Bob here. I have seen what damage to insulation does occur when there are elevated temperatures. Attics may cool down, but not much in the summer. Also attics are still quite warm on sunny fall and spring days, just ask the guys who work in them. I have even been in an attic on cold winter sunny days where it is actually quite a pleasent temperature, not freezing like outside.

Lets not forget that when the conductor is under load, that the temperature of the conductor from the resistance is elevated, and in conjunction with the ambient temperature, we are sure to have a situation that is above the operating temperature.

Lets also remember we are talking over the life of the installation, which today will hopefully be more than 50 years - I see cracked insulation...

 

[charlie b]

It is that simple.

It's really not very simple.

The relevant issue is it is a violation to use the conductor outsides its rating. . . . I am not going to agree to disagree on this one.

We can at least agree to agree that it is a violation to use a conductor outside its rating. Secondly, perhaps we can also agree to agree that the NEC neither defines nor explains what is meant by "ambient temperature." Finally, perhaps we can also agree to agree that the essence of our disagreement relates to what is, and what is not, a conductor's temperature rating.

My view is that "ambient temperature" is "year round average," and that if you determine that average and use that number to enter Table 310.16 to discover the rated ampacity, then you will not have exceeded the conductor's rating, and you will not have violated the NEC.

I infer that your view is that "ambient temperature" is "peak temperature seen, for any duration, during the conductor's lifetime." Put another way, I infer that your view is that if a conductor exceeds 30C at any time, then you must derate.

I concede that your view is more conservative, and gives more assurances that the conductor's insulation system will be protected. I further concede that it is unlikely that my view will ever be incorporated into the NEC. I do not know the intent of the authors. I do not know whether there is any experimental evidence that backs up one point of view over another. But I can say that if the NEC were to add a definition of "ambient temperature," and if the authors were to choose as that definition "year round average," then I believe the conductors will not be at risk.

 

[A/A Fuel GTX]

WOW......Now that's what I call answering a question....

OP

 

[iwire]

Hi Charlie I have been busy so I could not get back to you.

I agree with much of your opinion.

For what it is worth I agree with you in that a short term over temperature is not going to damage the insulation significantly.

I still can not buy into the year round average method when I think of my attic which spends the winter months below freezing and the summer days easily into 120 F - 130 F and sometimes higher.

How do I know that?

I have a remote reading temp sensor in my attic that controls the attic fan. (Left over control equipment....$500 worth of controls running a $40 fan :lol: )

So in case like that I do not agree the average is going to mean much, as for 8 to 10 hours each sunny day of summer the temp is above the 86 F of 310.16. Of course luckily most circuits in a residential attic are 30 amps and below and because of 240.4(D) are essentially already derated.

As Pierre has stated we electricians do find brittle heat damaged insulation in areas of high temps.

your view is more conservative

Boy that is not something I hear often.....I must be getting soft. :shock:

 

[pierre]

I don't know if I would consider his position as conservative, I would call it logical and realistic. Sometimes being logical and realistic do not play well in the NEC :wink: