Using the 90 Degree Amp Rating

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lloydm

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If I need to use the 90 Degree C rating of THHN conductors to fit the size I need into the conduit I have, to what extent must the terminals be rated 90 degrees? Is it just the circuit breaker or must the whole panel be rated at 90 degrees? Could I come out of a panel or wireway with larger conductors that meet my amperage at 75 degrees to an enclosed CB rated 90 degree C before going underground?
 
I suggest you find a different way, you are going to find it nearly impossible to meet the 90 degree rating (imsho)
 
lloydm said:
could I come out of a panel or wireway with larger conductors that meet my amperage at 75 degrees to an enclosed CB rated 90 degree C before going underground?
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In my opinion yes you can do that. You will have to do this at both ends and the wire connectors used to splice the large to the small will have to be rated at least 90C, along with the raceway. This could rule out PVC ( 352.12(E) )

Voltage drop may well become an issue with this method.
 
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lloydm said:
Could I come out of a panel or wireway with larger conductors that meet my amperage at 75 degrees to an enclosed CB rated 90 degree C before going underground?
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I hope you ment to say "an enclosed breaker rated at 75C".
I don't know about that Iwire. If the amperage was enough to heat the
smaller conductor to 90C, the point of connection for the 90C and the 75C conductor would heat up to 90C. If the length 75C conductor was small
the 75C breaker might reach a temperature of over 75C.
 
There are no UL Listed overcurrent protective device rated for use with conductors sized using the 90?C columns (except for some drawout circuit breakers).

Your only solution is to install a junction box and "splice" to conductors sized per the 75? columns.
 
bob said:
. If the amperage was enough to heat the
smaller conductor to 90C, the point of connection for the 90C and the 75C conductor would heat up to 90C. If the length 75C conductor was small
the 75C breaker might reach a temperature of over 75C.
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I agree, that could happen but there is no code language requiring a certain distance. Hopefully the OP can get some distance away from the 75 C terminals at each end.
 
iwire said:
I agree, that could happen but there is no code language requiring a certain distance. Hopefully the OP can get some distance away from the 75 C terminals at each end.
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The UL test for breaker termination heat rise uses 4ft of conductor, so that is probably the minimum that should be used
 
per SqD... and more

per SqD... and more

From a Square D white paper?

110.14(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.

You can use the 90?C conductor value for derating and correction, typically through conduit or a raceway. If the resulting derated value is more than that of the 75? rating of that conductor (haven?t seen it happen for what I do) you must use the 75? conductor rating for terminations. If the calculation result is less, you must use the lower value.

Example: A dimming rack with a 300A service

Calculate using a 75?C conductor:
500kcmil copper THWN has a 75?C ampacity of 380A (NEC 310.16)
A dimming rack has a current carrying neutral so we have 4 current carrying conductors in a conduit, derated to 80%, 380A x 80% = 304A.

Calculate using a 90?C conductor
400kcmil copper THHN has a 90?C ampacity of 380A
We have 4 current carrying conductors in a conduit derated to 80%, 380A x 80% = 304A.
400kcmil THHN has a 75?C ampacity of 335A, but the derated value of the 90?C wire is 304A, so the lower value is used. Had the result been higher than the 75?C rating, the 75?C rating would be used for the terminations.

This is a bit confusing as an example because the 380A amp value shows up for both conductors in 310.16, but it is correct.

Also, there are a lot of Al9Cu terminals out there that lead you to believe the connection is rated for 90?C. This is generally NOT true; check the manufacturers UL listed rating for the device for the correct value.

Lastly, I?m not sure a junction box to transition from 90?C to 75?C solves the problem because as I understand it connections are limited to the lowest rating in the chain? I would love to know this wasn?t true :)

kb
 
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