Derating...is it taken seriously?

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infinity said:
Charlie, although I agree that this is a common sense approach to derating, is it specified anywhere in the NEC that CCC have to being carrying current simultaneously?
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The only clarification I am aware of is that the NEC tells us when to count the neutral as a CCC. Absent any other guidance, I would conclude that a conductor is a "current carrying conductor" only when it is capable of carrying current with all associated loads turned on.

I once did a short math problem to evaluate the impact of current being carried by the neutral in a three phase WYE system. This was with no consideration of harmonics. I only looked at an imbalance in the three phases and a resultant current in the neutral. I was able to show that the total heat generated by four wires could never be higher than the total heat that would have been generated by just the three phase wires, if the same amount of total load had been balanced (i.e., no neutral current). This essentially proved that it was not necessary to derate for a neutral that was carrying unbalanced current. Here again, harmonics is a different game.
 
The only clarification I am aware of is that the NEC tells us when to count the neutral as a CCC. Absent any other guidance, I would conclude that a conductor is a "current carrying conductor" only when it is capable of carrying current with all associated loads turned on.
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So in the case of travelers for three-way switches, only one traveler of the two associated with a three way switch would could as a CCC? IMO only one conductor should count, but I have had asked to back up that opinion with an NEC Article which I can't seem to find.
 
JJWalecka said:
Here in Massachusetts 310.15(B)(2) is revised ..............

........How is it that Massachusetts is contrary to the NEC?.
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The table we use is the table the NEC used to use.

The NEC changed and MA said no.

The table we use is based on 'load diversity' (see if you can find a good definition of that)

The table the NEC now uses is based on all conductors fully loaded. (No diversity)

You can find MA Table 310.15(B)(2) in the back of the NEC as Table B.310.11
 
Iwire,

LOL. I did not know about Table B 310.11 in the back of the NEC. Thank you for pointing that out. Now I have to find the definition of Load diversity. :?

Justin W.
 
JJWalecka said:
Iwire,

LOL. I did not know about Table B 310.11 in the back of the NEC.
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Neither did I until I asked almost the very same question you did.

Someone on the forum pointed Table B 310.11 out to me.

Just passing it on... :)
 
sandsnow said:
Does anyone know of any failures due to NOT derating???
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I've pulled some pretty toasted ancient #14 out of hot attics where the run probably should have been #12 on a 15A because it was up high near the peak. The same stuff in the walls was less crunchy. It attic stuff hadn't quite failed yet, but it was well on its way.
 
paul said:
Don't I get to start in the 90? column when I start my derating for #12 THHN?
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I was told on this forum I believe that if you have to use the column that applies to the particular use, for example, THHN/THHW run in emt outdoors would have to be de rated from the 75c column since you are employing the THHW rating which is in the 75c column. I have one inspector who agrees with this, the rest let us use the 90c column.
 
tonyi said:
sandsnow said:
Does anyone know of any failures due to NOT derating???
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I've pulled some pretty toasted ancient #14 out of hot attics where the run probably should have been #12 on a 15A because it was up high near the peak. The same stuff in the walls was less crunchy. It attic stuff hadn't quite failed yet, but it was well on its way.
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Good example. I've seen the same above old incandescent luminaires.

I wa shoping for examples of numerous conductors in a single conduit though.
 
JJWalecka said:
Now I have to find the definition of Load diversity.
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Good luck with that one. :wink: 8) :lol:
I have a stack of engineering textbooks I can let you peruse, including all the books from my Masters Degree program and all the courses I took after I got the MSEE. But you won't find it there. I don't know what the code authors had in mind when they used that term, and it wouldn't surprise me if they don't either. But for all I have been able to determine, it is not a a term used in electrical power systems engineering.
 
sandsnow said:
Does anyone know of any failures due to NOT derating???
I was shoping for examples of numerous conductors in a single conduit though.
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Larry, do you need to prove a failure, or just code violation?

Wouldn't Max derating (ie) 12A/25A = 0.39, work for Tbl.310-15(b)2a + 310-16 ambient, or
Max #14's for conduit fill, using 1/2" EMT = 12, from NEC Appendix C.
 
ramsy said:
sandsnow said:
Does anyone know of any failures due to NOT derating???
I was shoping for examples of numerous conductors in a single conduit though.
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Larry, do you need to prove a failure, or just code violation?

Wouldn't Max derating (ie) 12A/25A = 0.39, work for Tbl.310-15(b)2a + 310-16 ambient, or
Max #14's for conduit fill, using 1/2" EMT = 12, from NEC Appendix C.
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What I hope to get is a real life example of for example - 26 #12's in a single conduit with melted insulation because the installer did not follow the derating(bundling) rules
 
infinity said:
...only one traveler of the two associated with a three way switch would could as a CCC? IMO only one conductor should count, but I have had asked to back up that opinion with an NEC Article which I can't seem to find.
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It's there in 310.15(B)(2)(a), we just can't see it. :lol:
 
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