I thought I knew how to adjust for ambient temp.

[sandsnow]

Ok, we have a 12amp continuous load in a 50'C ambient. What size MN-B to use?

12amp x 1.25 = 15amp, select #14 NM-B, 15amp OCPD and check for temp correction.

#14 in MN-B is THHN, 25amp in 90'C column.
25amp x 0.82 = 20.5amp

So we are OK w/ #14 NM-B protected at 15amp in a 50'C ambient.

Now please look here:
http://ecmweb.com/nec/code_quiz_072506/

He seems to be looking at this statement in 334.80partial code quote)
.....provided the final derated ampacity does not exceed that for a 60'C (194'F) rated conductor.

I always took that to mean you cannot protect above the 60'C rated ampacity, and in the case of #14 we can't exceed 15amp per 240.4(D) anyway.

He seems to be saying you can't use it (#14NM-B) because your calc results in a number higher than the 60'C amps.

Please advise. This is pretty basic stuff. If I'm mistaken, I may need to change careers.

EDIT: I don't know why that frown is up there.

 

[al hildenbrand]

. . .334.80partial code quote)
. . .
EDIT: I don't know why that frown is up there.

LOL

Ain't this smart new BBS software just grand?

 

[jwelectric]

334.80 The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor.

334.80 say that we can use the 90 degree column as long as the ampacity does not exceed that of the 60 degree column.

25 times .82 equals 20.5
20.5 exceeds the 60 degree column therefore a #12 conductor is called for.

 

[sandsnow]

334.80 say that we can use the 90 degree column as long as the ampacity does not exceed that of the 60 degree column.

25 times .82 equals 20.5
20.5 exceeds the 60 degree column therefore a #12 conductor is called for.

How about if we change the ambient temperature to 54'C ?
Then:
25amp x 0.76 = 19amp
19amp is less than 20amp(60'C amps)
So we're good to go, right?

That doesn't make sense to me. At a higher ambient we can get use a smaller conductor. What?

Standing by for enlightenment.

 

[sandsnow]

LOL

Ain't this smart new BBS software just grand?

It makes me feel so stupid. I got enough problems as it is.

 

[charlie b]

Sorry, Mike, but I disagree with you, and with the author of that article. A #14 is adequate for this application.

I always took that to mean you cannot protect above the 60'C rated ampacity. . . .

I believe that is what the author intended, and I submit that that is what the words say. But I will concede the words are not as well chosen as they might have been. It would have been clearer if the words were, ". . . provided that IF the results of the derating calculation are higher than the value shown in the 60C column, THEN the value in the 60C column shall be used as the conductor's ampacity."

But they didn't say that. What did they say? They said something about the "final derated ampacity." But what is that number? I submit that it is not the simple result of multiplying 25 times 0.82. That I will call the "interim derated ampacity." To get the "final derated ampacity," you compare that number to the number in the 60C column, and you use the smaller of the two numbers.

If I'm mistaken, I may need to change careers.

I keep looking to change careers; I want to be a professional lottery winner. But nobody has hired me for that job yet.

 

[infinity]

Sorry, Mike, but I disagree with you, and with the author of that article. A #14 is adequate for this application.



I believe that is what the author intended, and I submit that that is what the words say. But I will concede the words are not as well chosen as they might have been. It would have been clearer if the words were, ". . . provided that IF the results of the derating calculation are higher than the value shown in the 60C column, THEN the value in the 60C column shall be used as the conductor's ampacity."

But they didn't say that. What did they say? They said something about the "final derated ampacity." But what is that number? I submit that it is not the simple result of multiplying 25 times 0.82. That I will call the "interim derated ampacity." To get the "final derated ampacity," you compare that number to the number in the 60C column, and you use the smaller of the two numbers.



I keep looking to change careers; I want to be a professional lottery winner. But nobody has hired me for that job yet.

I agree. It seems that the explanation to the question unnecessarily confusing as well as incorrect. As Charlie said, take the #14 THHN ampacity @ 90 C and multiply by 82%. If your adjusted conductor ampacity is more than 15 amps, then your 15 amp OCPD with the #14 conductors is fine.

 

[jwelectric]

334.80 The 90?C (194?F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60?C (140?F) rated conductor.

334.80 say that we can use the 90 degree column as long as the ampacity does not exceed that of the 60 degree column.

25 times .82 equals 20.5
20.5 exceeds the 60 degree column therefore a #12 conductor is called for.

Thank you for pointing out my typo, (#12) should have been #14 instead.

When the math is done from the 90 degree column then the result is the ampacity of the conductor not the load, ie
25X.82= 20.5 The conductor in the 90 degree column is now good for 20.5 amps and this exceeds the 60 degree column by one half amp. Therefore a #14 conductor is called for.

I might have used the word “exceeds” out of text also.

Edited to add:
After posting this and exiting out I noticed the title of the thread and thought it would be appropriate for me to edit my post and add it to my post as it would seem to describe what I am beginning to think of myself.

“I thought I knew how to adjust for ambient temp.”
.

 

[sandsnow]

Thanks guys, I thought I had been in the Twilight Zone my whole life