Rating of THNN in Residences

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jjseabra

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I need some guidance on an issue im having. When I would do installs in residences, I would always use the 60c column for sizing my conductors. However im now being told that thhn used in residences does not have to abide by that same rule as NM. Is this true or am I misinterpreting the code?

Sorry my title should have read thhn, not thnn....lol my goof.
 
The wiring method employed, namely NM cable, is where the limitation to 60 degrees is found. THHN conductors installed in say EMT would be allowed to be calculated for ampacity based upon 90 degrees.

The limitation that you would find for an installation of THHN conductors in EMT would not be the wire itself but the termination of the wire. The breakers that I have seen are limited to 60 or 75 degrees. That being said you may have a wire that has an ampacity based upon the 90 degrees but the terminations of that wire will limit it to the temperature rating of the device.

You can use the 90 degree ampacity for derating purposes for NM cable as long as the adjusted ampacity does not exceed the 60 degree ampacity of the cable.

Pete
 
334.80 limits the ampacity of NM cable to that of the 60 degree column of 310.16.

334.80 does not limit THHN conductors that are part of another wiring method such as individual conductors installed in a raceway to the 60 degree column.

110.14(C) contains the requirements for temperature limitations for conductors.

Chris
 
What about when that thhn is connected to NM as it relates to installing a spa in a home? Meaning that NM is run to the spa disconnect box and then from there in THHN to the spa? Does that not get governed by the 60c column?
 
raider1 said:
334.80 limits the ampacity of NM cable to that of the 60 degree column of 310.16.

334.80 does not limit THHN conductors that are part of another wiring method such as individual conductors installed in a raceway to the 60 degree column.

110.14(C) contains the requirements for temperature limitations for conductors.

Chris
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Thank you Chris. I should have backed up what I was saying with the applicable code sections. I'll try to do better in the future!:)

Pete
 
jjseabra said:
What about when that thhn is connected to NM as it relates to installing a spa in a home? Meaning that NM is run to the spa disconnect box and then from there in THHN to the spa? Does that not get governed by the 60c column?
Click to expand...

Depends on the temperature rating of the terminals that the THHN is connected to. Again take a look at 110.14(C).

pete m. said:
Thank you Chris. I should have backed up what I was saying with the applicable code sections. I'll try to do better in the future!
Click to expand...

Not a problem, I posted at the same time you did.:)

Chris
 
pete m. said:
The breakers that I have seen are limited to 60 or 75 degrees. That being said you may have a wire that has an ampacity based upon the 90 degrees but the terminations of that wire will limit it to the temperature rating of the device.
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So can one transition to a larger sized wire before terminating at a lower temp device?
 
lakee911 said:
So can one transition to a larger sized wire before terminating at a lower temp device?
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Good question.

I would think that as long as the ampacity of the smaller wire, after any adjustment factors were taken into consideration, was not exceeded that this would be acceptable. I'm not sure under what application this would gain an advantage but I believe it would be acceptable.

Pete
 
pete m. said:
Good question.

I would think that as long as the ampacity of the smaller wire, after any adjustment factors were taken into consideration, was not exceeded that this would be acceptable. I'm not sure under what application this would gain an advantage but I believe it would be acceptable.

Pete
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Pete,

An example given here before is similar to that of a long run of large wire, terminations are

75C, 10' past termination put a j-box and reduce size of conductor due to 90C rating, do

the same at the other end.
 
benaround said:
Lou,

No, that is just a made up number, I don't know if there is a standard or not, but, there

has to be enough copper between connections ( heat sink ) to stay in the rating.
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The UL test for breakers uses 4' (feet) of conductor.
 
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hardworkingstiff said:
Jim,

That might become a very useful piece of information for me in the future, do you have a UL document or something like that I can use with the AHJ in the future?

Thanks,
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Use the edited value of 4' (feet).

The testing for breakers, and fusible switches, requires that there be 4' of conductor on the device. Among other things this conductor is part of the test to make sure the cable insulation is not damaged and that the cable does not 'pull out' of the lugs during a short circuit test.
 
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jim dungar said:
Use the edited value of 4' (feet).

The testing for breakers, and fusible switches, requires that there be 4' of conductor on the device. Among other things this conductor is part of the test to make sure the cable insulation is not damaged and that the cable does not 'pull out' of the lugs during a short circuit test.
Click to expand...

I understand the 4' (really didn't think you meant 4"). By chance, do you know the link or document that I can present to the AHJ?

Thanks Jim.
 
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