Selecting ambient temperature correction factors from Ta

tg2546 · Mar 31, 2005

I want to calculate wire ampacity in a 46C enviroment. I am using 90C wire but all terminals/lugs are rated for 75C. Do I use the 90C column or the 75C column?

rbalex · Mar 31, 2005

Re: Selecting ambient temperature correction factors from Ta

This is an excellent question.

A 75C terminal's thermal rating, is based on the assumption that it will be connected to a conductor with a 75C temperature rating and an ampacity ??based on Table 310.16 as appropriately modified by 310.15(B)(1) through (6).? See 110.14 (C)(1) and 110.14 (C)(1)(b).

tg2546 · Mar 31, 2005

Re: Selecting ambient temperature correction factors from Ta

Bob, thanks for the quick response. I think that your answer is, to use the 75C column correction factor??

However 110.14(C)says " Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both."

So, does this mean, use the 75C correction factor and the ampacity for the 90C wire as long as the resulting calculated ampacity does not exceed the 75C wire ampacity??

rbalex · Mar 31, 2005

Re: Selecting ambient temperature correction factors from Ta

Yes, I did mean to use the 75C based ampacity at the terminal.

I think 110.14 is due for a rewrite as a whole. I'm just not sure I'm up to it.

Section 110.14(C) is basically saying that you are not required to calculate the ampacity of the conductor (in this case a 90C) throughout its entire run based on the terminal limitations; i.e., it is not required to treat it as a 75C conductor in all "conditions of use." [See the definition of Ampacity]

For example the terminations may be in a 30C ambient, but a major run of the circuit is in the 46C ambient. At the terminations a #1/0 would have a 150A rating whether the conductor was 75C or 90C rated. But, "in the field," assuming no other factors in 310.15(B)(1) through (6) needed to be addressed, a 90C conductor would have an ampacity of 170 x 0.82, or ~139A and a 75C conductor would have an ampacity of 150 x .75 or ~ 113A.

Therefore the circuit's ampacity would be 139A with a 90C conductor or 113A with a 75C conductor.

If the ambient of the circuit were uniformly 30C the circuit's ampacity would be 150A for either a 75C or 90C conductor. [See Section 310.15(A)(2)]

Edit Add: I know the circuit's ampacity is based on the OCPD, but lets not go there for the time being.

[ March 31, 2005, 02:29 PM: Message edited by: rbalex ]

charlie b · Mar 31, 2005

Re: Selecting ambient temperature correction factors from Ta

Here is the process that I would use:

Let?s presume you mean that the whole environment is at 46 degrees C.

Let?s presume you have a conductor rated at 90 C, and terminations rated at 75 C.

Start with the 90C column in Table 310.16, and look up the ampacity for the size of your conductor.

Look up the temperature correction factor from the same 90C column. That would be, as Bob used in his example, 0.82.

Multiply the tabulated ampacity by the correction factor. Let?s call that result ?Answer 1.?

Now look up the ampacity for the same conductor in the 75 C, and multiply it by the temperature rating from the 75 C column (that would be 0.75). Let?s call that result ?Answer 2.?

Your final ampacity is the lower of the two answers.

rbalex · Mar 31, 2005

Re: Selecting ambient temperature correction factors from Ta

Charlie's point is well taken - escpecially the bold emphasis, I should have included a uniform 46C set of ampacities too.

BTW the Section 310.15(A)(2), Exception does NOT apply to the terminations. If it did we wouldn't need most of 110.14

[ March 31, 2005, 04:14 PM: Message edited by: rbalex ]

lauraj · Apr 4, 2005

Re: Selecting ambient temperature correction factors from Ta

OK, I'm confused now. This is the way I have always done this:

Step 1: Multiply the ampacity of the 90 degree rated wire by the temperature rating from the 90 degree column and write it down. (Answer 1)

Step 2: Write down the ampacity of the 75 degree rated wire (no derating). (Answer 2)

Step 3: If Answer 1 is greater than Answer 2, you can use that size wire. If Answer 1 is less than Answer 2, you must increase your wire size and calculate again.

Have I been doing this incorrectly all of these years???

Laura

bob · Apr 4, 2005

Re: Selecting ambient temperature correction factors from Ta

Charlie and Bob
I agree with lauraj. I see no reason to adjust
the conductor for 90C and 75C. The reason for using the 90C conductor is to take advantage of the higher rating. As an example take #3 thhn rated at 110 amps at 90C and 100 amps at 75C.
Adjusting for temp 36-40 the factor is .91.
0.91 x 110 = 100 amps. This conductor rating does not exceed #3 thhn rated at 100 amps 75C and could be used with a 100 amp breaker.
If you adjust for 90C and 75C the 90C figure is always greater than the 75C in this example.

rbalex · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

The ambient affects the thermal rating of each element of the circuit, including the terminals.

Simply stated, the ampacity of a 75C terminal or connector in a specified ambient is that of the 75C conductor approved to be connected to it and corrected for the ambient.

If multiple ambients apply to a circuit, the conductor's ampacity may be calculated beginning with its own 30C ambient ampacity "...based on Table 310.16 [and] as appropriately modified by 310.15(B)(1) through (6).? Note: "Based on Table 310.16" includes the Ambient Correction Factors at the bottom of the Table.

After all computations are done the lowest computed ampacity applies to the entire circuit per 310.15(A)(2).

lauraj · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

I have to disagree. The derating coefficients from Table 310.16 are for conductors, not terminations.

110.14(C) then states that "Conductors with temperature ratings higher than specified for terminations (90 degree conductor, 75 degress termination) shall be permitted to be used for ampacity adjustment, correction, or both."

This means that we only derate the wire, not the termination.

The March/April 2005 IAEI has a great article concerning this and explains it very clearly.

Laura

rbalex · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Since the article doesn?t seem to be on the IAEI website I can?t comment on it?s clarity.

However, I can submit from the UL 2004 GENERAL INFORMATION DIRECTORY , for Category Code (AALZ) Electrical Equipment for Use in Ordinary Locations :

Appliance and Utilization Equipment Terminations

Except as noted in the general Guide Information for some product categories, most terminals, unless marked otherwise, are for use only with copper wire. If aluminum or copper-clad aluminum wire can be used, marking to indicate this fact is provided. Such marking is required to be independent of any marking on terminal connectors, such as on a wiring diagram or other visible location. The marking may be in an abbreviated form, such as ??AL-CU.??

Except as noted in the general Guide Information for some product categories, the termination provisions are based on the use of 60?C insulated conductors in circuits rated 100 A or less, and the use of 75?C insulated conductors in higher rated circuits as specified in Table 310.16 of the NEC. If the termination provisions on equipment are based on the use of other conductors, the equipment is either marked with both the size and temperature rating of the conductors to be used or with only the temperature rating of the conductors to be used. If the equipment is only marked for use with conductors having a higher (75?C or 90?C) temperature rating (wire size not specified), the 60?C ampacities (for circuits rated 100 A or less) and 75?C ampacities (for circuits rated over 100 A) should be used to determine wire size. Conductors having a temperature rating higher than specified may be used, though not required, if the size of the conductors is determined on the basis of the 60?C ampacity (circuits rated 100 A or less) or 75?C ampacity circuits rated over 100 A)

Category Code AALZ is a general category that, unless specifically modified by another Category Codes, applies to all electrical equipment.

While the Table 310.16 ambient correction factors are applied to conductors, it is those conductors that are connected to the terminals. It is a simple heat-transfer phenomenon. A termination?s temperature rating is based on the temperature rating of the conductor it is connected to at the conductor's rated ampacity; and the rated ampacity is based on the ambient correction factors.

wirenut1980 · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

I agree with lauraj and bob that something does not quite add up with Charlie b?s method shown above. As bob said, you would get a lower ampacity every time for the 75 deg C column, because not only does that column have a lower ampacity to begin with, but the ambient adjustment factor is also a greater adjustment in the 75 deg C column. So what?s the point of the comparison? But in the Code, I cannot find anything else other than what has been said already to back myself up. Another intuitive thought: If one uses lauraj?s method and answer 1 is greater than answer 2, then the heat transferred from the 90 deg C conductor to the 75 deg C termination would be low enough where the termination would not overheat. Right?

charlie b · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

I agree with this statement from Bob:

Originally posted by rbalex: I think 110.14 is due for a rewrite as a whole. I'm just not sure I'm up to it.

I don?t think I am up to it either.

I also agree that a close look at 310.16 reveals that correcting a 90C wire for temperature, using the 90C correction factor, will always give you a higher number than correcting the same size 75C wire for temperature, using the 75C correction factor. True, but irrelevant. Keep in mind that you might be under a 30C ambient, but have to correct the 90C wire for more than three current-carrying conductors. This might bring the total ampacity below that of a 75C wire.

Try looking at it this way:

An excessively high temperature could destroy the terminations.

The terminations are rated for 75C.

That means that in a 30C environment, using a 75C rated conductor (e.g., THW), if you operate the system at the current levels given in the Table (e.g., 50 amps for a #8), then we can be sure that the temperature rise from the I2-R losses in the wire will be no higher than 45C. That is, the 30C ambient plus the 45C temperature rise will not exceed the 75C rating of the terminations. Also, the 30C ambient plus the 45C temperature rise will not exceed the 75C rating of the conductor.

Now let?s consider using the same THW conductor, but make the ambient 40C.

Suppose that we do not correct for temperature, and that we apply the same current as above (e.g., 50 amps for a #8).

The conductor will experience the same I2-R losses, since that is based on the resistance of the metal, and not the insulation system. (For this discussion, we can safely ignore the second-order effect of a higher temperature on the resistance of the wire.)

Therefore, the 40C ambient plus the 45C temperature rise will exceed the 75C rating of the terminations (and the conductor), and something is likely to become damaged.

Now let?s consider using a type THHN conductor. It is rated for 90C, but the terminations are still rated for 75C. Let?s consider a 40C ambient. Let?s stay with the #8.

The 90C rating of a #8 is 55 amps. Correcting for 40C (from the 90C column) is a factor of 0.91. Thus, 55 amps times 0.91 is 50 amps. That is the same as the 75C column for a #8 (with no correction for ambient). That is, if we use the method proposed by lauraj and endorsed by ?the other Bob? and by wirenut1980, we would see that "Answer 1" and "Answer 2" are the same, and that the "lower of the two answers" is still 50 amps.

So let?s run the 50 amps through the #8 THHN in this 40C ambient.

It?s the same 50 amps we talked about earlier, so we can expect the same 45C temperature rise.

The conductors will see the 40C ambient plus the 45C rise from I2-R losses. That puts the conductor at an overall temperature of 85C. That is within the 90C rating of the conductor?s insulation system, so it will be safe.

However, the terminations will see the 40C ambient plus the 45C rise from I2-R losses. That puts the terminations at an overall temperature of 85C. That is beyond the 75C rating of the terminations, and they will be subject to failure.

The way to avoid the risk of failure is to derate the 75C value of ampacity for the temperature of the terminations.

The ampacity of a #8 THW at 40C ambient is 50 amps times 0.88, or 44 amps.

For this scenario, the ?Answer 1? is 50 amps, and the ?Answer 2? is 44 amps.

Conclusion: If we keep the current below 44 amps, then neither the conductor?s insulation nor the terminations would be above their temperature ratings.

bob · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

I am not correct in my post. Here is why.
Using the example I posted, take #3 thhn rated at 110 amps at 90C and 100 amps at 75C.
Adjusting for temp 36-40 the factor is .91.
0.91 x 110 = 100 amps. At the new temperature of
36-40C, a continous 100 amps flow in the #3 thhn will raise the temp to 90C. This not exceptable
for 75C equipment. If the equipment is a 100 amp breaker 75C, we must recaculate a 75C conductor rating so that it will not cause the conductor temp to be higher that 75C. The adjustment factor for 36-40C at 75C is .88. WE need a rating of 100 amps to match the breaker.
Therefore 100amps/0.88 = 114 amps. #2 thhn has a rating of 115 amps and when adjusted for the new temp we get 115 amps x 0.88 = 101 amps. At the new temperature, 101 continous amps will raise the conductor temp to 75C.
Charlie was posting at the same time.

[ April 05, 2005, 02:11 PM: Message edited by: bob ]

rbalex · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Charlie?s method is just fine.

Bob?s statement that a 90C conductor would always have a higher ampacity is only strictly true if the entire circuit ampacities/temperature ratings are computed/determined based on the same ambient throughout.

See the example in my second post. If the termination were in a 30C ambient and the bulk of the circuit in a 46C ambient, the 75C termination would be temperature rated for a #1/0, 75C conductor at 150A; however a 90C conductor would only have a 139A ampacity at its temperature rating "in the field." Under the conditions of use, the circuit?s maximum ampacity would be 139A ? which is less than the 75C termination?s rating.

Section 110.14(C) says clearly the termination?s temperature rating is associated with the ampacity of the conductor. The rest of the section says that conductors with higher temperature ratings may be used for ampacity adjustment, correction, or both. It may be awkwardly stated, but it means you don?t have to treat a 90C like a 75C conductor throughout the rest of the circuit. That was the common misunderstanding and misapplication of 110.14(C)?s text when it was first introduced in the 1993 NEC :

110.14(c) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be so selected and coordinated as to not exceed the lowest temperature rating of any connected termination, conductor, or device.

It is not uncommon in my industry that a circuit's source termination is in an air-conditioned substation, it then runs through various combinations of cable trays, raceways and/or underground duct banks to a final termination. If the final termination is at a motor it is always 75C or better rated. (That was absent from the original 110.14(C) too). The temperature rating / ampacity must be determined in each condition of use.

Edit Add: I was responding to David Z; it looks like we were in a race to post and Charlie and [the other] Bob beat me

Charlie also did a good job explaining the heat transfer issue.

[ April 05, 2005, 01:31 PM: Message edited by: rbalex ]

wirenut1980 · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Rbalex, and bob, and charlie B - Well said! It is much clearer for me now. I hope one day to achieve that level of knowledge of electricity and the Code.

rbalex · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Originally posted by charlie b:
I agree with this statement from Bob:

Originally posted by rbalex: I think 110.14 is due for a rewrite as a whole. I'm just not sure I'm up to it.

Click to expand...

I don?t think I am up to it either.

...

I think I've decided to take a shot at it. There are enough good folks in the forum to help make sure I can develop a strong substantiation - Charlie B - you're one of the keys.

lauraj · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Charlie b and rbalex,

I agree with your statements about heat transfer. What I need to know is this: Where in the code does it say that I need to use the temperature adjustment factor based off the rating of the terminal and not the conductor?

I don't see anything in the UL reference that rbalex posted that deals with temperature adjustment factors either.

I don't mean to nitpick, but I teach this and I need a solid code reference to base this on. I suppose if there is not one, that should definitely be addressed in the next code change cycle.

Laura

charlie b · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Originally posted by lauraj:What I need to know is this: Where in the code does it say that I need to use the temperature adjustment factor based off the rating of the terminal and not the conductor?

110.14(C), first sentence. What we have been discussing so far is based on the second sentence.

What the ?first sentence? tells me is that when I look up the ampacity of a conductor, I have to use the 75C column (presuming, as is most often the case, that I have 75C terminations). What the ?second sentence? tells me is that I can start with the 90C column (presuming I have a 90C conductor), apply any appropriate de-rating factors (e.g., for temperature, more than three current-carrying conductors, burial depth, etc.), and use that number (what we have called ?Answer 1? in the discussion above). However, that doesn?t free me from still applying the ?first sentence.? I still can?t assign an ampacity higher than that obtained from the 75C table (what we have called ?Answer 2? in the discussion above).

Did I answer your question, or did I perhaps misunderstand your question?

lauraj · Apr 5, 2005

Re: Selecting ambient temperature correction factors from Ta

Charlie,

Sentence 1 still seems to me that it is just dealing with conductor ampacity and terminal ratings if there are no correction factors applied. Example - if I have a 90 degree conductor, I am still limited to use the 75 degree ampacity column if my terminals are rated 75.

However, sentence 2 says that if I need to derate - I can use the 90 degree column, as discussed before.

I don't see anything in sentence 1 or 2 that tells me if I derate, I must derate according to the temperature ratings of my terminations. (75 degree C in the example we've been using)

Selecting ambient temperature correction factors from Ta

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