PVC Installation at Ambient Temperature

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352.10(I) says higher rated wire insulation above that of the ambient temperature rating of PVC...which is 122F and 50C can be installed in the PVC as long as it isn't used above the temperature rating of the PVC.

So that means that my 90 C degree THWN can only be used at an ampere rating of 50 C degrees...right?

So if #3 THWN copper is good for 100amps at 75C...what amperage is allowed on that #3 copper in PVC at its listed Temp rating of (122F)50C?
 
shortcircuit2 said:
352.10(I) says higher rated wire insulation above that of the ambient temperature rating of PVC...which is 122F and 50C can be installed in the PVC as long as it isn't used above the temperature rating of the PVC.

So that means that my 90 C degree THWN can only be used at an ampere rating of 50 C degrees...right?

So if #3 THWN copper is good for 100amps at 75C...what amperage is allowed on that #3 copper in PVC at its listed Temp rating of (122F)50C?
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Not sure how to apply that section as written, but UL states...
Unless marked for higher temperature, rigid nonmetallic PVC conduit is intended for use with wire rated 75?C or less including where it is encased in concrete within buildings and where ambient temperature is 50?C or less. Where encased in concrete in trenches outside of buildings it is suitable for use with wires rated 90?C or less.
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http://database.ul.com/cgi-bin/XYV/...n=versionless&parent_id=1073985958&sequence=1
 
Would we apply the rule like this...


90 degree C THWN-2 is rated at 115amps...

But installed in PVC with a restriction of 50 degrees C...we have a 45% temperature adjustment to get to 50C...

90Cx45%=40.5...90-40.5=49.5C

So then 115 amps x 45% = 51.75amps?

I don't get it...anyone?
 
shortcircuit2 said:
Would we apply the rule like this...


90 degree C THWN-2 is rated at 115amps...

But installed in PVC with a restriction of 50 degrees C...we have a 45% temperature adjustment to get to 50C...

90Cx45%=40.5...90-40.5=49.5C

So then 115 amps x 45% = 51.75amps?

I don't get it...anyone?
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I going to go with when using 90?C-rated conductors, we still have to size to 75?C ampacity because of terminal temperature limitation imposed by 110.14(C). UL says PVC is rated for 75?C conductors in general. I see no reason to go with any adjustment beyond the norm for any other conduit wiring method. The way I see it, the 50?C limitation is there to allow for temperature rise caused by the conductor and other factors such as exposed to direct sunlight.

But I'm open to other interpretations...
 
According to my 2011 NEC Handbook...352.10(I) was new in 2011...taken from 352.12(E) in the 2008 NEC

In reading the comments in the Handbooks...I gather that "operating temperature" is different than "ambient temperature".

So PVC can NOT be used in a location that has an "Ambient Temperature" above 122F and 50C.

But conductors can be used in PVC at an "Operating Temperature" for which the PVC is listed....(IE 75C from UL)

Still abit confusing in that I don't understand what the difference is between "Operating Temperature" and "Ambient Temperature"...relative to this discussion.
 
shortcircuit2 said:
According to my 2011 NEC Handbook...352.10(I) was new in 2011...taken from 352.12(E) in the 2008 NEC

In reading the comments in the Handbooks...I gather that "operating temperature" is different than "ambient temperature".

So PVC can NOT be used in a location that has an "Ambient Temperature" above 122F and 50C.

But conductors can be used in PVC at an "Operating Temperature" for which the PVC is listed....(IE 75C from UL)

Still abit confusing in that I don't understand what the difference is between "Operating Temperature" and "Ambient Temperature"...relative to this discussion.
Click to expand...
Consider if you were allowed to use PVC in a 60?C environment and installed it in such an environment. You technically could not use 60?C-rated conductors. Current through the conductor would cause the conductor to exceed the 60?C rating.

Compare to most electrical distribution equipment having a temperature rise rating, usually based on an ambient temperature of 40?C...

http://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html

So if you installed such equipment in a 50?C environment, you will have reduced the allowable temperature rise by 10?C. If you search the internet, you will find that you are supposed to derate circuit breaker ratings when installed in an environment above rated ambient temperature.

Now when you do use 75?C-rated conductors at full capacity, that doesn't mean the raceway they are installed in will reach a temperature of 75?C... but the raceway will be warmer than ambient temperature. The 50?C cap on PVC is based on the temperature rise and the threshold temperature (roughly 66?C/150?F) at which PVC starts to deform.
 
A couple of things I learned while researching PVC...

1. You must add 30 degrees F to your calculation for expansion when PVC conduit is in direct sunlight...according to Carlon's instructions.

2. They don't seem to rate PVC j-boxes for temperature or if they are sunlight resistant. They are listed under UL 514C...but not much info on 514C is readily available without buying the standard.
 
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