90 C conductor ampacity rating.

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mikehughes8

Senior Member
Location
NC
There are no UL listed 600V max. overcurrent protective devices that can be used with conductors sized per the 90?C column


I wonder why that is? Is it to much of an engineering challange to design protective devices with that temp. rating. If it is not, I don't understand why they are not readily available.
 
I wonder why that is? Is it to much of an engineering challange to design protective devices with that temp. rating. If it is not, I don't understand why they are not readily available.



Circuit breakers are designed with the use of 90C conductors as a "heat sink".
The conductors will help to take some of the heat away from the circuit breaker itself onto the conductor.


NOTE:
More than 80% of the termination failures is in human error. because of this, the general train of thought within the manufacturing portion (and others as well) is to keep the terminations at a 75C level.
 

philly

Senior Member
So if I understand what everyone is saying:

If using the full ampacity value for cable without any derating, the ampacity value has to be selected for the temperature rating which the cable is terminating on which most of the time is the 75deg rating.

If derating a cable for any reason you can use the 90deg ampacity to apply the derating multipliers to (assuming cable is rated for 90 deg) as long as the final derated value does not exceed the 75deg ampacity rating when terminaing on a 75deg rated lug.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
So if I understand what everyone is saying:

If using the full ampacity value for cable without any derating, the ampacity value has to be selected for the temperature rating which the cable is terminating on which most of the time is the 75deg rating.

If derating a cable for any reason you can use the 90deg ampacity to apply the derating multipliers to (assuming cable is rated for 90 deg) as long as the final derated value does not exceed the 75deg ampacity rating when terminaing on a 75deg rated lug.

Yep, I think you have it right.:)

Chris
 

Smart $

Esteemed Member
Location
Ohio
...as long as the final derated value does not exceed the 75deg ampacity rating when terminaing on a 75deg rated lug.
Yep, I think you have it right.:)

Chris
I know how philly stated it is how it is usually stated but to be more precise...

The final derated ampacity cannot be less than the required minimum circuit ampacity while the circuit ampacity cannot exceed the 75?C rating of the same size wire.

The only reason I put it this way is, I have a feeling that some may look at the stipulation as the final derated value of the 90?C-rated wire cannot be less than its same-size, 75?C-rated counterpart—which is not correct.

Take for instance a #4 wire connected to a 75A breaker. If you use 90?C-rated wire, you can go as low as 75A for the final derated ampacity (if circuit ampacity is based solely on the OCPD rating, i.e. not a motor feeder or such where the OCPD can be rated higher than the circuit ampacity), rather than the 85A rating of 75?C rated #4. A sustained current of 75A on a 95A, 90?C wire will have a temperature below 75?C at its termination.
 
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glene77is

Senior Member
Location
Memphis, TN
Circuit breakers are designed
with the use of 90C conductors as a "heat sink".
The conductors will help to take some of the heat away from the circuit breaker itself onto the conductor.

Pierre,
I am respectfully curious now.

(1) Does UL test circuit breakers for thermal trip levels
with the internal heat being dissipated somewhat by the conductors?
or
(2) Does the conductor dissipate its own heat (outside of the test) ?

I have worked with Load Centers which were heated by ambient temp
such that the breakers tripped at 15A, instead of 20A load. Understandable.
The internal bi-metalic heater does not know where the temp comes from.

In any case,
I don't see how the temp rating of the insulation enters into the picture.
The CB terminals are rated at 75 degrees which limits the wire temp rating.

Another question, does the 60 degree insulation (old TW) dissapate more heat
than the 90 degree insulation (new THHN) ?

:smile: I can't always answer my own questions.
 
Glen I have not seen the standard(s) they use for testing CBers.

I have spoken with a couple of UL representatives and others during meetings and classes, and this is what I have been told.

I am sure if you 'google' the topic something will come up.
 
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