310.16 correction factors

stew · Apr 29, 2004

Need some direction using the correction factors for temp in 310.16. I can see where if you had a boiler room for instance where the ambient was say 90 degrees then you would adjust downward for temp. On the other hand lets say you have feeders from an outside service panel running underground to a feed thru lug at a remote panel in Seattle where the average temp is say 65 degreees. Could you then use the 1.05 factor and rate a 250 mcm for 215?

websparky · Apr 29, 2004

Re: 310.16 correction factors

Stew,

I would use the seach feature here. You will find several lengthy post on this subject!

As far as your Seattle example, not likely. The figures are based for the mean average in most of the US. If you were to try and use something different, I think an EE would need to be involved!

[ April 29, 2004, 11:07 PM: Message edited by: websparky ]

stew · Apr 30, 2004

Re: 310.16 correction factors

Why would you use the mean usa average? The mean average has nothing at all to dp with the ambient temp.No more than using an average temp in a boiler room. Thats why I am interested in what the code really inteneded for those temperature figures to mean.

iwire · Apr 30, 2004

Re: 310.16 correction factors

Stew I think the code allows this, I also think it would be a hard sell to the AHJ.

I do not think we should use average temps though.

Take my area, Boston, MA / Providence RI.

Average Maximum Temperature

(The monthly mean of the maximum daily temperature)

Average temp for Aug. 80.6 F

Average temp for year 59.5 F

It sounds a lot cooler than it is, we have many 90 to 95 F days along with a few in the high 90s.

The Source of the temps I posted

Check the Temps in your area

As Dave said this is certainly a topic of discussion and there is no consensus on it.

IMO it should be figured for worst case, the hottest days are typicaly the days with the highest sustained current levels as the HVAC loads are running long or continuous.

Perhaps we should be directed to use the same temperature that the HVAC has been calculated on.

Any air conditioning design must start with an expected high temp to cool from.

JMO, no facts to support it.

[ April 30, 2004, 05:05 AM: Message edited by: iwire ]

mweaver · Apr 30, 2004

Re: 310.16 correction factors

Stew,

If I could suggest.... The values listed in Table 310.16 are to be based on the statement at the top of the table and have nothing to do with your location on the planet and everything to do with exactly where these conductors reside for their lifetime.

The top of Table 310.16 says "Not more than three current carrying conductors in a raceway, cable, or earth...... Based Uopn an Ambient temperature of 86?F. What that means precisely is the amperage values of 310.16 are based on 3 current carrying conductors whose ambient temperature NEVER exceeds 86?F. A boiler room is a good example, but a better example is your own attic in July at 4:00PM. Most people do not want to hear this .... but this is how this table was designed and written.

Very few real world commercial application exist where the values in 310.16 can be utilized without deration fortemperature.

This is the most misunderstood area of the NEC.

hope this helps.

mweaver

charlie b · Apr 30, 2004

Re: 310.16 correction factors

Originally posted by mweaver: What that means precisely is the amperage values of 310.16 are based on 3 current carrying conductors whose ambient temperature NEVER exceeds 86?F.

With respect, mweaver, I very STRONGLY disagree with this statement. I submit that the intended meaning is ?year-round average.? I concede that the NEC does not define or explain the intended meaning of ?ambient temperature,? in the sense used by Table 310.16. I would like to see an NEC revision that makes this point clear. However, I take some comfort in the fact that the laws of Physics are on my side.

I have made this point before, and as websparky had noted, you can look up the earlier discussions. Here is a brief summary of the basis for my interpretation:

The hazard that higher temperatures impose on a conductor is an increase in the rate of deterioration in the insulation system.

For every 10 degree F increase in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will decrease by about 50%.

For every 10 degree F drop in ambient temperature, held for the entire life of the cable, the expected lifetime of the insulation system will double.

The relationship between temperature and the rate of degradation is not linear. If a cable spends one hour at a temperature of 90F (i.e., 4 degrees above the de-rating threshold), it may have to spend 3 hours, or even 5, at a temperature of 82F (i.e., 4 degrees below the de-rating threshold), in order to break even (i.e., on the overall average rate of degradation)

The concept is similar to buying a car that is intended to last you 10 years, but that has a gas tank that cannot be refilled. You start with 10 years worth of gas in the tank, but that assumes you burn gas at the rate of a car moving at 30 miles per hour. Once you run out of gas, you must buy a new car (similar to replacing a cable, once its insulation system has degraded too far). If you drive faster than 30 mph for a short while (say for 2 hours), you burn gas faster than the design rate, and you might run out of gas before the car?s 10 year expected life. If you drive slower than 30 mph, you save gas. But you might have to drive at 20 mph for 6 hours, before you can save enough gas to restore the car?s 10 year expected life. Finally, if you constantly drive below 30 mph, you could, in fact, extend the useful life of the car beyond its expected 10 years.

HOWEVER, the fact that you drove over 30 mph for a single hour would not, by itself, drain the gas tank, and render the car useless. Similarly, if you allow a cable to experience more than an 86F ambient for a limited time, without reducing the current to the de-rated value shown in 301.16, that will not instantly destroy the cable. What it will do is to reduce the cable?s useful life by some small amount. You can make this up by keeping it below 86F for an extended period.

ANOTHER HOWEVER: Websparky is right about needing an EE (and a PE, at that), if you want to take advantage of the lower ambient temperatures. The EE would have to be familiar with the Arrhenius equation and the methodology for applying it. The Arrhenius equation is k=A*exp(-Ea/R*T), where k is the rate coefficient, A is a constant, Ea is the activation energy, R is the universal gas constant, and T is the temperature (in degrees Kelvin). It would be no easy task to apply this to a cable?s insulation system. You would also need an AHJ willing to go along with the EE?s calculated results. You are not likely to get both.

This is the most misunderstood area of the NEC.

That I agree with. Or at least, I would put it in the top ten.

mweaver · Apr 30, 2004

Re: 310.16 correction factors

Charlie b,

When you install conductors the allowable ampacity of those conductors are determined in part by the ambient temperature which will surround those conductors for their lifetime. Just as 310.16 says, if a conductor of a given insulation type is to be installed in an ambient environment of 86?F and carry an ampacity of it?s listed table value, then that given insulation type will effectively live out its lifetime without insulation degradation due to temperature. Exceeding the temperature of 86?F or increasing the amperage in the conductor (which also causes the temperature to increase) will exceed the temperature limitations of the insulation causing insulation degradation. This has nothing to do with Boston or Seattle. It has to do with conductor environment.

Example: If conductors feed lighting in conduit in a warehouse in the proximity of the roof deck where the ambient temperature could easily routinely be 110?F, then you would use THIS information in determining the proper conductor size for a given insulation type and amperage. When you are sizing conductors in December, you must take July into consideration on those conductors. Average, or Boston or Seattle is not a factor in 310.16 calculations.

I mean no disrespect. I realize you strongly disagree, I would suggest you ask Mr. Holt.

mweaver

stew · Apr 30, 2004

Re: 310.16 correction factors

And mr weaver the enviorionment that those conductors are exposed to for there entire liftime is in Seattle so how could you say that the coductor envioronment is what counts and disregard the fact that they are installed in a cool envioronment for there lifetime? Your staement contradicts itlself.. Sorry

mweaver · Apr 30, 2004

Re: 310.16 correction factors

Stew,

Example: If conductors feed lighting in conduit in a warehouse in Seattle or Boston or Arizona or Texas in the proximity of the roof deck where the ambient temperature could easily routinely be 110?F, then you would use THIS information in determining the proper conductor size for a given insulation type and amperage. When you are sizing conductors in December, you must take July into consideration on those conductors.

Average, or Boston or Seattle or Arizona or Texas is not a factor in 310.16 calculations.

I mean no disrespect. I hope you would agree that a consesus on this board has little effect in the proper application of Table 310.16

I realize you also strongly disagree. I would suggest you ask Mr. Holt

mweaver

iwire · Apr 30, 2004

Re: 310.16 correction factors

Originally posted by charlie b:
With respect, mweaver, I very STRONGLY disagree with this statement. I submit that the intended meaning is ?year-round average.? I concede that the NEC does not define or explain the intended meaning of ?ambient temperature,? in the sense used by Table 310.16. I would like to see an NEC revision that makes this point clear. However, I take some comfort in the fact that the laws of Physics are on my side.

Charlie I very much respect your opinion and knowledge.

That said I have no idea how you can say we can use an average even if I agree physics is on your side.

All the book says is ambient no more no less, until a revision comes and says year round average ambient I will stick to my view and take it at face value.

With respect, Bob

bphgravity · Apr 30, 2004

Re: 310.16 correction factors

I believe the 1971 NEC had a table (310-20(c)) that provided "Typical Ambient Temperatures". The table provide locations, the temperature at those locations, and the minimum rating of insulation required. The location column related to the installation location and not location on Earth. The temperature column was a maximum possible temperature listing.

I assume that at one point, the code either allowed or required you to use this table to figure correction factors. It also appears that the correction factor was based on maximum exposed temperature and not an average.

charlie b · Apr 30, 2004

Re: 310.16 correction factors

To mweaver:

The only thing that I had strongly disagreed with is your use of the word ?never.? If I can reasonably predict that a cable will spend no more than one hour at 87F during a 40 year lifetime, and that it will spend the other 350,000 + hours below 80F, then I would not choose to de-rate for ambient temperature.

If conductors . . . in a warehouse . . . could easily routinely be 110?F

That is a different story, and I would agree that de-rating is needed here.

Let me offer the following distinction. Suppose a facility required constant air conditioning for some reason. Suppose that the temperature is maintained between 70F and 80F year round. Now suppose that an HVAC failure one summer day caused the temperature to climb to 90 for 2 hours, and then the HVAC repair brought the temperature back to its normal range. Finally, suppose that you were brought in to inspect the facility, and advice the owner as to whether the electrical system had suffered any damage. You note during your inspection that one specific conductor would have been undersized for a 90F ambient, if that temperature had to be taken into account during the initial construction of the facility.

Question: Would you tell the owner that the conductor has to be replaced, because it had spend two hours at 90F, that forever thereafter it would require a de-rating, and that it is now undersized?

I would not.

To Bob (iwire):

Originally posted by iwire: All the book says is ambient no more no less, until a revision comes and says year round average ambient I will stick to my view and take it at face value.

I have to agree. That is why I said that to take advantage of a lower ambient temperature (i.e., to ?up-rate?) would be no easy task, and would require the services of an EE / PE. To that statement, I will now add two others.

First, even though I am an EE and a PE, this is one calculation that I would decline to perform. I understand the basic physics, but I am no expert in the application of the Arrhenius methodology.

Secondly, let me state that Seattle temperatures exceed 85F perhaps five days a year, and that they almost never exceed 95. Nevertheless, if an AHJ insisted on my applying the 310.16 de-rating factors in a project that I felt did not need them, I will concede here and now that I would likely lose the argument. I wish the NEC were more clear, but it is not.

brentp · Apr 30, 2004

Re: 310.16 correction factors

I realize I am out of my league on this thread, BUT I am going to post anyway.

Where does it state, in the 2002 code, that conductors should be derated based on the 'maximum' OR 'average' ambient temp?

Why can't ambient temps be based on the 'average' temp where conductors 'live'? I thought Charlie B gave an excellent example of why one would average the ambient temps (he is an EE & PE afterall). Why NOT average the 110? high to the 70? low at a roof deck? Geographical outdoor ambients should have nothing to do with nothing.

And lastly, if it's AHJ vs EE/PE, I'll 'do' what the AHJ says, but will 'believe' the EE/PE is correct.

BTW, I apologize if I have offended anyone, as I have respect for everyone on this forum.

stew · Apr 30, 2004

Re: 310.16 correction factors

brent . the reason I was indicating outdoor temperatures is because this is where these conductors do live . They are all installed outside and none of the conductor is inside. Therfore the envioronment to which they are exposed is considerably below the 86 degree ambient listed onthe top of the chart. This particular installation is in a boatyard and falls within 555. The theoreticall load calc using the tables in 555.12 for 20 each 30 amp receptacles is 210 amps. The feed thru lugs of the quad outlet boxes will accept a maximum of 250 mcm wire which is good for 205. So the dllemma is a 5 amp difference. Using the correction factor we get 215 amps which then allows the use of 250 mcm and not having to go to 300.Now that being said the actual load on this feeder is about 85 to 100 amps peak well within the conductor ampacity. Its only when the laod calc is done that the 5 amps makes any difference.I want to put 5 quads on each service panel so the 5 amps make a big difference.The only other thing to do is blank off one out let to get below the 205 amp ampacity.

brentp · Apr 30, 2004

Re: 310.16 correction factors

Stew,

Like I said, I am out of my league on this thread.

All I can say is to use the lower ambient temperature to 'uprate' your conductor ampacity.

With that being said, I would hope to uderstand the rest of your calc problem after others, smarter than myself, reply.

Brent

stew · Apr 30, 2004

Re: 310.16 correction factors

brent the 555.12 table requires that you multipy the number of receptacles in amps. The math for this is 20 receptacles @30 amps =600ax120 v=720000va divide this by 240 volts and the resultant is 300 amps. Then apply the demand factor in the table 555.16 of .70 for 20 receptacles and your result is a theoretical load of 210 which is 5 amps above the conductor rating.I am also trying to see whether the lugs might be rated for 90 deg and then use a 225 amp breaker rated 90 degrees and the problem would be eaqsily solved.A load study on this service has been done and the peak load on the existing service is only around 210 amps on the 2 existing panels.Because we are upgrading the receptacles from 15 and 20 amps to 30 the resultant load calcs require a slightly larger service and also we are upgrading for future use. The actual connected load has not changed just the load calc due to the new receptacles.

websparky · May 1, 2004

Re: 310.16 correction factors

Here is an informative link to the methods employed for conductor ratings.

http://floasis.siemens-asp.com/OASIS/FPL/ratmethod.doc

stew · May 1, 2004

Re: 310.16 correction factors

thanks for the link web but I cant get it to open.

tonyi · May 1, 2004

Re: 310.16 correction factors

The link appears to be a Microsoft Word document. If you search around with Google a bit, there's some Word document viewers out there.

stew · May 1, 2004

Re: 310.16 correction factors

I did not find the document to be that informative as it pertains to a specific metodology for hi voltage power transmission lines and not 240 volt buried cable, It was interesting in that it shows how thier engineers do it but ZI dont think it is germaine to this discussion. Thanks anyway Joe

310.16 correction factors

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