I posted this question yesterday, I appreciate the response, but I dont think I wrote it out right. Here it is again simplified. I have a 50A receptacle with 60c terminals, and its wired with (THHN) 6 AWG. Take a look at 310.16, under the 60c column its rated for 55A. Now that wire (THHN) is capable of 55A at a operating temperature of 60c at ambient temperature of 86F, right. But what if the ambient temperature was a 100F? I got answers yesterday like, you go ahead and adjust it at the 90c because of the insulation. So do the math, 6 AWG (THHN) in 90c column has an ampacity of 75A, right. Now go to the correction at the bottom of the 90c column and you get .91 as a factor. Math is , 75A x .91 = 68.25A. Okay, that 68.25A is for THHN operating at 90c. What does that have to do with the 60c . Right now everyone thinks thats ok, and its not. See it this way, I have 55A wire, with a 50A receptacle that has 60c terminals, and I increase the ambient temp by 14 degrees to 100F. I only have 5 amps to play with on that wire before I have to change it . But according to the previous math(68.25A), you're way above it. How does that work?
Conductor Sizing
- Thread starter jca0108
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jumper
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For others, here is the earlier post:
http://forums.mikeholt.com/showthread.php?t=123206
http://forums.mikeholt.com/showthread.php?t=123206
jumper
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JC,
Is the question of "why do we derate wires for ambient teperature, but not terminations?", what you are trying to ask?
Is the question of "why do we derate wires for ambient teperature, but not terminations?", what you are trying to ask?
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- Journeyman Electrician
Your conductor, #6 THHN (90 degree C conductor) has a maximum ampacity of 75 amps. After you performed your temperature correction it's maximum ampacity was limited to 68.25 amps. This is the maximum. Now since your terminals are only rated for 60 degrees C you can only use this conductor at the 60 degree C ampacity. This is the so called weakest link in your ampacity.
If your correction adjustment left with an ampacity of 35 amps that would end up being your maximum capacity for that same conductor.
If your correction adjustment left with an ampacity of 35 amps that would end up being your maximum capacity for that same conductor.
Okay picture you holding, a piece of 6 THHN, and terminating it to a 50 receptacle with 60c terminals located in this room with an ambient temp of 100F. I dont feel comfortable hanging my hat on that type of installation when sizing that conductor is unclear to me. I have mike holts calculation library with exam prep book. There are two A and B. A talks about terminal ratings, and B talks about ampacity of a wire, but never mixes both , so im confused. I want to take this with me out to the field, as well as to the master exam in a couple of months, thanks in advance.
can u explain when you said "you can only use this conductor at the 60 degree C ampacity". thanks
jumper
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JC,
Bear with me a second. How would you answer this question?
Why do we derate wires for ambient teperature, but not terminations?
Bear with me a second. How would you answer this question?
Why do we derate wires for ambient teperature, but not terminations?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You need to protect the conductor at it lowest value after any correction factors have been applied or if the terminations are lower than the adjusted value. You would use the lower of the two values. Take a look at this graphic:
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You may use the ampacity for 90C for derating purposes. The final OCPD may not be higher then the 60C rating of the wire because the terminals at 60C are your weakest link.
Thus#6 thhn= 75 amps at 90C
Now Correction factor--- is .91 for 100 degrees and thhn wire.
75 & .91= 68.25
Now we know that with the correction factor we are higher than the 60C amp. of the #6= 55. Therefore we cannot use the 68 amps to determine the OCPD. We know the correction factor really does not affect us in this situation because of the use of 90C wire.
So we can use a 50 amp breaker or possible a 60 amp breaker if the calculated load is less than or equal to 55 amps. (240.4(B)).
Go one step further to T. 210.21 (B)(3) and we see that a 50 amp recep cannot have a branch circuit rated more than 50 amps.
Thus#6 thhn= 75 amps at 90C
Now Correction factor--- is .91 for 100 degrees and thhn wire.
75 & .91= 68.25
Now we know that with the correction factor we are higher than the 60C amp. of the #6= 55. Therefore we cannot use the 68 amps to determine the OCPD. We know the correction factor really does not affect us in this situation because of the use of 90C wire.
So we can use a 50 amp breaker or possible a 60 amp breaker if the calculated load is less than or equal to 55 amps. (240.4(B)).
Go one step further to T. 210.21 (B)(3) and we see that a 50 amp recep cannot have a branch circuit rated more than 50 amps.
jumper
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- 3 Hr 2 Min from Winged Horses
The terminals limit the final derated ampacity to the 60C column. see 110.14(C). Your 90C THHN is limited to 60C max. You can use the 90cC column only for derating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
the only thing i can come up with is heat , causes eddy currents, skin effect, opposes current flow, heats up conductor
That pretty much sums it up.
i think thats the missing piece to my puzzle
jumper
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winnie
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I suspect that this hasn't entered the code because it hasn't been a problem.
Usually we are derating the _conductors_ because of conditions in the middle of the run; eg conduit in a hot attic or lots of conductors bundled together. Usually the conditions at the termination are much more benign.
But imagine a situation where the entire run, including the terminations, is in a very hot environment. The ambient heat plus the self heating of the conductors could easily exceed the termination temperature rating. IMHO this is simply not in the code, and is probably not a big enough problem to be added to the code.
-Jon
Usually we are derating the _conductors_ because of conditions in the middle of the run; eg conduit in a hot attic or lots of conductors bundled together. Usually the conditions at the termination are much more benign.
But imagine a situation where the entire run, including the terminations, is in a very hot environment. The ambient heat plus the self heating of the conductors could easily exceed the termination temperature rating. IMHO this is simply not in the code, and is probably not a big enough problem to be added to the code.
-Jon
your probably rite, thats why i had a hard time because i broke it down so far and i missed the big picture. they should a proposal to clarify this a little further. its hard trying to get your point across when your not face to face , but you understood my dilemma of the bat, i appreciate the comment, thx
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