Rating of THNN in Residences

[jjseabra]

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.

 

[pete m.]

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

 

[raider1]

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

 

[jjseabra]

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?

 

[pete m.]

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

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

 

[raider1]

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?

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

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!

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

Chris

 

[lakee911]

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.

So can one transition to a larger sized wire before terminating at a lower temp device?

 

[pete m.]

So can one transition to a larger sized wire before terminating at a lower temp device?

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

 

[benaround]

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,

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.

 

[hardworkingstiff]

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.

Where does the 10' come from? Is that the standard, if not, is there a standard?

Thanks,

 

[Pierre C Belarge]

Take a look at 310.15(A)((2)

 

[benaround]

Where does the 10' come from? Is that the standard, if not, is there a standard?

Thanks,

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.

 

[jim dungar]

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.

The UL test for breakers uses 4' (feet) of conductor.

 

[hardworkingstiff]

The UL test for breakers uses 4" of conductor.

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,

 

[jim dungar]

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,

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.

 

[hardworkingstiff]

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.

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.

 

[jim dungar]

By chance, do you know the link or document that I can present to the AHJ?

This is a link to info on terminations
http://www.schneider-electric.us/su...al-library/?event=detail&oid=090089268011ab63

This link provide you lots of information on breakers and fusible switches, see page 11.
http://static.schneider-electric.us/docs/Circuit Protection/0600DB0301.pdf
Warning this link is biased to breakers.