when to use 90 degrees ampacity conductor?

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skyline77

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Hi,
As per the NEC table Table 310.15(B)(16), a 500 AWG conductor rated has an ampacity of 430 @ 90 degrees C. However, most motors and ALL circuit breakers as far as my limited knowledge are not rated to properly operate at 90 degrees C.
However, most of the conductors we use are based on 90 degrees C ampacities.
My question if most of the equipment aren't rated for this high temperature why have it in the first place?
As per Schneider Electric website "https://www.se.com/uk/en/faqs/FA290363/" it states that 90 degrees cables are not permitted on NSX MCCB.
 
You can use the 90C rating for ampacity adjustment (de-rating) as long as the final overcurrent protective device is not larger than the 75C or 60C whichever is the weakest link.
 
Look at 110.14(C)

(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected and
coordinated so as not to exceed the lowest temperature rating
of any connected termination, conductor, or device. Conductors
with temperature ratings higher than specified for terminations
shall be permitted to be used for ampacity adjustment,
correction, or both.
Click to expand...
 
Dennis Alwon said:
You can use the 90C rating for ampacity adjustment (de-rating) as long as the final overcurrent protective device is not larger than the 75C or 60C whichever is the weakest link.
Click to expand...
Refer to Table 310.15(B)(16), it's Based on Ambient Temperature of 30°C. The 90°C is not related to the ambient temperature, it's the temperature of the copper conductor itself regardless of the ambient temperature.
As copper has very high thermal conductivity, it tends to spread the temperature very fast all over the current-carrying path one of which is the thermal trip in the breakers, that's why breakers may cause nuisance tripping if you use 90°C column ampacity.
For other than 30°C, Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C can be used.

all that being said, I didn't find a circuit breaker that can be used for 90 °C.
My question why are there 90 °C? when can they be used?


chack the below link
Did you get my question?
Or am I missing something?

Thanks
 
I guess I am missing something as I gave you the answer as to why there are 90C conductors. I never mentioned the 30C
 
say I have to de-rate #12 thwn-2 conductors because there are 9 in a conduit. I would have to de-rate at 70% but I can start my de-rating at the 90C rating. For #12 that is 30 amps. 30 x .7 = 21 amps so I can still use #12 wire on a 20 amp breaker. If there wasn't a 90C column then I would have to de-rate from the 75C column and I would need larger conductors
 
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in between the junction boxes

also with doing this you can fall under
210.15(A)(1)Exception 2
Exception No. 2: Where a portion of a feeder is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an allowable ampacity not less than the sum of the continuous load plus the noncontinuous load. No portion of a feeder installed under the provisions of this exception shall extend into an enclosure containing either the feeder supply or the feeder load terminations, as covered in 110.14(C)(1).
Click to expand...

and what Dennis is saying
 
the 30C the table is based on is considered a somewhat normal air temperature. If you run through a boiler room especially a steam boiler) chances are the ambient temp will be higher than 30C - you will need to make adjustments. If you know you will always be less than 30C you actually can adjust the other direction and get more ampacity out of a conductor.

Conductors do give off heat when loaded. Isn't always obvious, but find a situation where there is a pretty continuous load and utilizing a majority of conductor ampacity and they will be warm. Pack multiple such circuits in same raceway and it gets even warmer.
 
david luchini said:
Maximum operating temperature of the conductor.
Click to expand...
Exactly.
If we set the ambient temperature constant at 30°C for all the length of the conductor and we used THHN insulated copper of 1 AWG with a 90°C termination
Based on Table 310.15(B)(16) if the above copper has an ampacity of 110 Amps it would reach a temperature of 60°C
If the same copper carries 130 Amps its temperature would be 75°C and
If the same copper carries 145 Amps its temperature would reach 90°C.
I know that the it has thermal insulation and if I touch it it would never be anywhere close to 90°C.
what this means is that you can use THHN for the three ranges of ampacities, but, if you use it for 90°C, then, we have a problem in the thermal tripping of the circuit breakers.
As some said if the conductor is close to a heat source or under the direct sun we would derate the ampacity based on "Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C" that is given and it's not related to what we are concerned about.
Don't mix the ambient temperature with the copper generated temperature due to the amount of current.
We are working at present on a mega project and one of the designers raised this issue as it started with using the 90°C ampacity, and he said this is wrong as there are no breakers that can tolerate such high temperatures and similarly most motors. Most of the electrical budget is now in these low voltage cables.
This decision is crucial as it would greatly impact the project cost.
 
skyline77 said:
Exactly.
If we set the ambient temperature constant at 30°C for all the length of the conductor and we used THHN insulated copper of 1 AWG with a 90°C termination
Based on Table 310.15(B)(16) if the above copper has an ampacity of 110 Amps it would reach a temperature of 60°C
If the same copper carries 130 Amps its temperature would be 75°C and
If the same copper carries 145 Amps its temperature would reach 90°C.
I know that the it has thermal insulation and if I touch it it would never be anywhere close to 90°C.
what this means is that you can use THHN for the three ranges of ampacities, but, if you use it for 90°C, then, we have a problem in the thermal tripping of the circuit breakers.
As some said if the conductor is close to a heat source or under the direct sun we would derate the ampacity based on "Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C" that is given and it's not related to what we are concerned about.
Don't mix the ambient temperature with the copper generated temperature due to the amount of current.
We are working at present on a mega project and one of the designers raised this issue as it started with using the 90°C ampacity, and he said this is wrong as there are no breakers that can tolerate such high temperatures and similarly most motors. Most of the electrical budget is now in these low voltage cables.
This decision is crucial as it would greatly impact the project cost.
Click to expand...

In general, you need to use the 75C rating for your terminations (no ambient or bundling factors apply), and the 90C rating as the starting point for your calculations to correct for bundling and ambient temperature. It is rare that equipment is rated for 90C terminations. It could exist, but it has to be part of a listed assembly for everything in the enclosure. You might have equipment on one side that is 90C rated, but equipment on the other side still requires 75C terminations. Often times you see lugs on equipment stamped with AL9CU, but you usually don't get to take credit for the "9" meaning 90C in its rating, because the entire assembly of equipment has to be listed for 90C.

There is the aformentioned work-around, by splicing your conductors to a slightly smaller conductor in separate otherwise-empty enclosures on both ends, and then using that smaller conductor (sized with 90C terminations) to complete the majority of the run. By "otherwise-empty", I mean that it isn't part of a panelboard or disconnect, but is just a box for wiring and splicing. A lot of separately installed splice connectors are 90C rated (Polaris blocks, split bolts, insulation piercers), and most individual conductors manufactured today carry this rating as well. This could be a value-engineering solution, or it could be a way to recover from a mistake. I anticipate that if length is significant that the benefit justifies the extra complexity, voltage drop would also be a significant factor governing design. You may not be able to use the desired 90C sized circuit, if it is insufficient for voltage drop anyway.
 
I'm still not sure what your concern is.

skyline77 said:
Exactly.
If we set the ambient temperature constant at 30°C for all the length of the conductor and we used THHN insulated copper of 1 AWG with a 90°C termination
Based on Table 310.15(B)(16) if the above copper has an ampacity of 110 Amps it would reach a temperature of 60°C
If the same copper carries 130 Amps its temperature would be 75°C and
If the same copper carries 145 Amps its temperature would reach 90°C.
Click to expand...

As has already been pointed out, you cannot use the temperature rating associated with the conductor ampacity that exceeds the lowest rating of any connected termination or device. If the breaker, terminations, or devices are rated at 75°C, you are limited to the 130A for #1 AWG.

skyline77 said:
We are working at present on a mega project and one of the designers raised this issue as it started with using the 90°C ampacity, and he said this is wrong as there are no breakers that can tolerate such high temperatures and similarly most motors. Most of the electrical budget is now in these low voltage cables.
This decision is crucial as it would greatly impact the project cost.
Click to expand...

The designer is correct. You cannot start with using the 90°C ampacity.
 
Dennis Alwon said:
David, what are you saying here? I know that you know we can start derating at 90C if the conductor is 90C
Click to expand...
If I understand the OP correctly, he wants to use the 90C ampacity of THHN, and his designer is telling him he can't. The ampacity of THHN is limited to the 75C column per 110.14(C)...the designer is correct.
 
You can not use 90 C rating as minimum size that attaches to a 75C breaker.

You can use that 90C rating as a starting point in places where you are going to make adjustments to the conductor for either ambient temperature or adjustment for number of conductors in raceway, but regardless the results of that adjustment you still must use conductor based on 75C ampacity to land on the 75C breaker.

100 amp breaker, 3 AWG is good for 100 amps @ 75C. run that conductor in raceway with more than 3 CCC's and/or in a higher ambient temp, you are allowed to use the 90C ampacity as the starting point to make adjustments for that portion of the run, but regardless of the adjustment you still must have at least the 3 AWG conductor because of termination rating, or as some have mentioned a changeover method like using a 90C splice method then change conductor size.
 
Dennis Alwon said:
I saw that he said they started with 90C so I assumed he meant for derating. Hopefully the op understands
Click to expand...

I don't think he means for derating...

skyline77 said:
we would derate the ampacity based on "Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C" that is given and it's not related to what we are concerned about.
Click to expand...

This part leads me to believe he is not talking about starting from the 90C ampacity for derating, but starting from the 90C for conductor ampacity.
 
skyline77 said:
Exactly.
If we set the ambient temperature constant at 30°C for all the length of the conductor and we used THHN insulated copper of 1 AWG with a 90°C termination
Based on Table 310.15(B)(16) if the above copper has an ampacity of 110 Amps it would reach a temperature of 60°C
If the same copper carries 130 Amps its temperature would be 75°C and
If the same copper carries 145 Amps its temperature would reach 90°C.
I know that the it has thermal insulation and if I touch it it would never be anywhere close to 90°C.
what this means is that you can use THHN for the three ranges of ampacities, but, if you use it for 90°C, then, we have a problem in the thermal tripping of the circuit breakers.
As some said if the conductor is close to a heat source or under the direct sun we would derate the ampacity based on "Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C" that is given and it's not related to what we are concerned about.
Don't mix the ambient temperature with the copper generated temperature due to the amount of current.
We are working at present on a mega project and one of the designers raised this issue as it started with using the 90°C ampacity, and he said this is wrong as there are no breakers that can tolerate such high temperatures and similarly most motors. Most of the electrical budget is now in these low voltage cables.
This decision is crucial as it would greatly impact the project cost.
Click to expand...
When you derate the ampacity, there eventually comes a point where you will need to increase conductor size. If you are derating for insulation rating only (which is where you typically can use the 90C rating) you are usually only derating for a portion of conductor that is subject to higher temp conditions, but the terminations that are only 60 or 75C often are still in a low or moderate ambient. In fact most miniature breakers have 40C marked on them. They still have 75C terminal rating, this just means trip setting was set based on 40C ambient.

Mix ambient with copper generated temps? Both are additive in the conductor, that is why we have to derate for ambient temp as well as for number of conductors in raceway/cable.
 
Carultch said:
There is the aformentioned work-around, by splicing your conductors to a slightly smaller conductor in separate otherwise-empty enclosures on both ends, and then using that smaller conductor (sized with 90C terminations) to complete the majority of the run
Click to expand...
Brilliant idea. What if we use LONG BARREL TWO HOLE Compression Connector?
I found BURNDY TWO HOLE HYLUG™ CODE CONDUCTOR LONG BARREL
Catalog Number "YA452N"
So, say I have the cable run "long-run" using the ampacity of 90 C for a 1250 Kcmil (ampacity = 665 A) will be crimped with two 400 Kcmil at an ampacity of 75 C "335 A each". Is this OK?
 
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