Conductor Ampacity Derating - 90 Degree Column

[shespuzzling]

Can somebody please help explain why we are allowed to use the 90 deg. column when derating conductors? My understanding of Table 310.16 is that for a given wire size (say 6 AWG), the temperature rise associated with a particular amount of current is listed in the 60, 75 and 90 degree columns. So, a 6 AWG wire with 65A flowing through it at ambient temperature will rise to 75 degrees.

I will use one of the examples in the 2008 NEC handbook to explain my question:

Total value of noncontinuous load = 175A
Equipment and equipment terminals rated at 75 degrees
Temperature =112 deg
Correction factor for a temperature of 112 deg for 90 deg rated insulation = .82
Use Aluminum XHHW-2 wire

This is how the NEC example goes:

The required conductor ampacity is calculated to be 175A*.82 = 213A. A 250MCM 90 deg wire size is selected as it has a rating of 230A at 90 deg. At 75 degrees, the 250MCM conductor has a rating of 205A, which is greater than the noncontinuous load of 175A so it is deemed acceptable. But if the adjusted load ampacity is 213A, which is greater than 205A, wouldn't the temperature of the conductor would be over 75 degrees and thus not compatible with the terminal ratings?

Alternatively, if you take a 250MCM 90 deg wire (230A) and multiply it by the derating factor (.82) you get 188A. So now if you have a 188A load then the temperature of the wire will rise to 90 degrees. By the same logic, that same wire will need only 205A*.82=168A before it reaches 75 degrees. Since the load is 175A, the wire temp would be greater than the terminal temperature ratings.

Are both of these methods for solving for derating wire size acceptable? Why is it okay that at full load (175A) the temperature of the wire will be greater than 75 degrees?

 

[david luchini]

Can somebody please help explain why we are allowed to use the 90 deg. column when derating conductors? My understanding of Table 310.16 is that for a given wire size (say 6 AWG), the temperature rise associated with a particular amount of current is listed in the 60, 75 and 90 degree columns. So, a 6 AWG wire with 65A flowing through it at ambient temperature will rise to 75 degrees.

Yes, this is the basic idea:a 6 AWG wire with 65A flowing through it at ambient temperature will rise to 75 degrees. But what if you only put a 35A load on the #6 AWG...the temperature will rise to less than 75 degrees.

I will use one of the examples in the 2008 NEC handbook to explain my question:

Total value of noncontinuous load = 175A
Equipment and equipment terminals rated at 75 degrees
Temperature =112 deg
Correction factor for a temperature of 112 deg for 90 deg rated insulation = .82
Use Aluminum XHHW-2 wire

The correction factor for 112 deg for a 90 deg rated insulation aluminum conductor is 0.87, not 0.82.

This is how the NEC example goes:

The required conductor ampacity is calculated to be 175A/0.87 = 201A. A 250MCM 90 deg wire size is selected as it has a rating of 230A at 90 deg. At 75 degrees, the 250MCM conductor has a rating of 205A, which is greater than the noncontinuous load of 175A so it is deemed acceptable. But if the adjusted load ampacity is 213A, which is greater than 205A, wouldn't the temperature of the conductor would be over 75 degrees and thus not compatible with the terminal ratings?

The adjusted ampacity of the conductor is 200A, so putting 200A through it in a 112 deg ambient would cause the temperature to rise to 90deg. But we are only putting 175A through the conductor. So the temperature will be less than 90deg.

Alternatively, if you take a 250MCM 90 deg wire (230A) and multiply it by the derating factor (.87) you get 200A. So now if you have a 200A load then the temperature of the wire will rise to 90 degrees. By the same logic, that same wire will need only 205A*.82=168A before it reaches 75 degrees. Since the load is 175A, the wire temp would be greater than the terminal temperature ratings.

Are both of these methods for solving for derating wire size acceptable? Why is it okay that at full load (175A) the temperature of the wire will be greater than 75 degrees?

Your logic is a little of here. An XHHW wire (not an XHHW-2) will reach 75 degrees in a 112deg ambient at 168A. Therefore, 250mcm al XHHW would not be allowable on the circuit. You would need 300mcm al XHHW for this circuit.

Another (simplistic) way to look at it is this: 250mcm al with a load of 205A will raise the conductor temperature to 75 deg, while a load of 230A (25A more) will raise the temperature to 90deg.

For the 250mcm XHHW-2 adjusted for 112deg ambient, a 200A load will raise the conductor to 90deg. A load that is 25A smaller (175A) will likely raise the temperature to only 75 deg.

 

[charlie b]

Let?s start with some basics. I apologize if some of this is already ?old news? to you, but I need to start somewhere.

The temperature that a wire will reach depends mostly on two things. One is the resistance of the wire. That is a function of the size of the wire. If you put the same current through two wires of different sizes, the larger wire (with its lower resistance) will not get as hot as the smaller wire. The other key factor is the conditions that surround the wire. The ampacity tables are based on one value. If the surrounding temperature is higher, then the same wire with the same current will reach an overall higher temperature. That is why there are derating factors for ambient temperatures above 30C.

With that as a start, I will now say that I think you are looking at the tables from the wrong perspective. The three columns are not about how hot a wire will get at the posted current values. They are saying that a wire with one type of insulation (e.g., TW) can sit in a 30C ambient area, can take the current that is shown for that size wire (e.g., 55 amps for #6 copper), and it won?t get hotter than 60C. The name of the insulation (again, we are using TW for now), and the fact that that name appears at the top of the 60C column, tells us that that wire can handle an overall temperature of 60C, without suffering damage to its insulation. Similarly, sending 65 amps through a #6 copper wire will result in an overall temperature no higher than 75 C, and if the wire has type THW insulation it will be able to handle that current and that temperature, without suffering damage to its insulation. You can say something similar about a #6 copper wire with a current of 75 amps and with THHN insulation: it won?t get hotter than 90C, and the wire can handle that temperature without suffering damage to its insulation. That is what the columns are telling us.

However, if you do put 75 amps through a #6 copper wire with THHN insulation and in an ambient temperature of 30C, the wire?s insulation will be able to handle it, but the point at which you connect that wire to a breaker, a relay contactor, a terminal block, or a switch might not be able to take the full 75 amps without it getting too hot. Not many manufacturers have yet listed their terminations for more than 75C. That is why we generally don?t use ampacities higher than those listed in the 75C column, even if we use wire with 90C insulation.

Now I can (finally) get around to answering your question.

We can use the 90C column for derating (if, of course, we have a wire with one of the insulation systems shown at the top of that column) because the wire itself can handle the current value shown in that column. A 250 MCM wire, aluminum, with XHHW-2 insulation can handle every bit of the 230 amps shown in the table. But with an ambient of 112F, we need to reduce the current going through the wire, to prevent it from getting above the 90C that the insulation can handle. So we derate it by a factor of 0.87. (By the way, your example used 0.82, and that is not right. Look under the 90C column for an ambient of 112, and you will see 0.87.) Multiplying 0.87 times 230 gives you 200 amps. So under this set of ?conditions of use? (please look up the article 100 definition of ampacity), the ampacity of this wire is 200 amps. That is higher than the 175 you need for the load. So you can use this wire size and type. Please note that you are putting 175 amps through a wire that (using the 75C column) would have had an ampacity of 205, but in our case has a more limited ampacity of 200, you will not exceed the 75C rating of the terminations, and you will come nowhere close to the 90C rating of the wire itself.

 

[shespuzzling]

Apologies for the typo, but the actual temperature used in the NEC example is 119 degrees which would correlate to a .82 correction factor, so the rest of the numbers I showed in their examlpe should be correct.

Thank you both for your explanations. There is still one thing that is bothering me about the derating.

David Luchini -

"For the 250mcm XHHW-2 adjusted for 112deg ambient, a 200A load will raise the conductor to 90deg. A load that is 25A smaller (175A) will likely raise the temperature to only 75 deg."

This is where I'm tripped up. If you select the XHHW-2 wire, we know that it will reach 90 degrees with 188A running through it with the correction factor taken into consideration (using .82 correction). Since under normal circumstances with a 30deg ambient temperature, an XHHW-2 wire will reach only 75 degrees with 205A running through it, I would conclude that 205A*.82=168A would make an XHHW-2 wire reach 75 degrees. Since our load is 175A, this would make the wire get hotter than 75 degrees.

Charlie b -

"Multiplying 0.87 times 230 gives you 200 amps. So under this set of ?conditions of use? (please look up the article 100 definition of ampacity), the ampacity of this wire is 200 amps. That is higher than the 175 you need for the load. So you can use this wire size and type. Please note that you are putting 175 amps through a wire that (using the 75C column) would have had an ampacity of 205, but in our case has a more limited ampacity of 200, you will not exceed the 75C rating of the terminations, and you will come nowhere close to the 90C rating of the wire itself."

If I replace .87 with .82 in your above explanation, the ampacity of the XHHW-2 wire would be 188A, which is still greater than the 175A full load that is flowing through the wire. So if lower ampacity of 188A can make the wire temperature increase to 90 degrees, shouldn't a lower ampacity (lower than 205A) make that wire increase to 75 degrees? I tried to find what this magic number is by multiplying the correction factor by the 75 degree ampacity so 205A*.82=168A and concluded this wire size wouldn't be sufficient.

In summary, if we use the 90 degree column for derating and conclude that a smaller amount of current will cause a wire to increase to 90 degrees than is listed in the table, why are we still using the printed nubmers in the 75 degree column? Shouldn't those have to be derated as well?

Thanks again for your help.

 

[david luchini]

Since our load is 175A, this would make the wire get hotter than 75 degrees.

Yes, the conductor may get hotter than 75 degrees, but the conductor is rated for 90 degrees, so the additional heat will not damage the insulation.

In addition, the 250mcm terminations (rated 75deg) can carry 205 amps without overheating, but they will only be carrying 175A. So neither the conductors nor the terminations will get "overheated."

 

[shespuzzling]

Ah-ha! I think that was the missing link. Can you please confirm if the following statements are accurate:

1. Equipment temperature ratings and termination ratings DO NOT need to be derated when exposed to higher ambient temperatures.

2. You can have a much hotter wire (say 85 degrees) come into contact with a 75 degree rated termination without having a problem, as long as the wire is insulated to handle at least 85 degrees, and as long as the current through the termination is low enough that it won't heat the termination up past 75 degrees. I assumed that just the mere fact that the 85 degree wire was touching the 75 degree rated termination, that the termination would overheat, but your response tells me that this is not in fact true.

 

[shespuzzling]

bump.

 

[Dennis Alwon]

Ah-ha! I think that was the missing link. Can you please confirm if the following statements are accurate:

1. Equipment temperature ratings and termination ratings DO NOT need to be derated when exposed to higher ambient temperatures.

Not that I am aware of but I certainly would consider it if temperatures were extremely high. Not sure how you would deal with it.

2. You can have a much hotter wire (say 85 degrees) come into contact with a 75 degree rated termination without having a problem, as long as the wire is insulated to handle at least 85 degrees, and as long as the current through the termination is low enough that it won't heat the termination up past 75 degrees. I assumed that just the mere fact that the 85 degree wire was touching the 75 degree rated termination, that the termination would overheat, but your response tells me that this is not in fact true.

That's is the way I see it also

 

[david luchini]

Ah-ha! I think that was the missing link. Can you please confirm if the following statements are accurate:

1. Equipment temperature ratings and termination ratings DO NOT need to be derated when exposed to higher ambient temperatures.

Not that I am aware of but I certainly would consider it if temperatures were extremely high. Not sure how you would deal with it.

In the original example, at temperatures higher than 113degF, the terminations take care of themselves when the conductor is derated.

Although the original example uses 250mcm Al XHHW-2, #4/0awg Al XHHW-2 would be acceptable. So if you used #4/0 conductors and #4/0 terminations, the terminations should be able to carry 180A, the actual load would be 175A, so the terminations should be OK, and the conductors have a derated ampacity of 178 (at 105-113 degrees), so they are also OK.

At 114-131 deg, you would need 250mcm Al XHHW-2 for the load. 250mcm, 75deg terminations should carry 205 Amps, but the load is only 175A, so the terminations are effectively derated. At 132-140 deg, you would need 300mcm Al XHHW-2...the terminations should carry 230A, but the load is only 175A. At 141-158deg, you would need 400mcm Al XHHW-2, and at 159-176deg, you would need 750mcm Al XHHW-2.