Conductor Derating

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Coreyd1@optonline.net

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Is there another code section to reference besides 310.15 b 2 a for derating my current carrying conductors? I was told under certain applications I can have 8 number 12's in a 3/4 conduit before I have to start derating.
 
To be more specific, if you are installing conduit and #12 conductors, the magic number is 9 before you have to worry about derating being a problem, barring any temperature factors.

This calc does not work for all sizes, but is used a lot because #12 THHN is so common for receptacles and lights in commercial.
 
There are other derating locations such as Annex B for "engineered installations", but, for everyday use 310.15(B)(2) and the ambient listings are the norm.
It is important to remember that in derating you use the conductor insulation rating, normally 90 deg.
When applying derating to #12s as in your example, as jumper shows, the end result is normally a mute point until you get to your 10th current carrying conductor. With THHN and Romex having a 90 degree insulation you start off with a 30 amp rating. Until you get to the 10th ccc, 70% is your highest rating (ignoring ambient) so you have an end ampacity of 21 amps which is higher than the 240.4 allowed OCP of 20 amps.
Obviously in 240(G) applications such as motors the result may be different.
 
jumper said:
Assuming THHN, 30 x .7 = 21.

If you have a nipple 24" or less I think that you do not have to derate.
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You don't have to derate on the nipple, but you do have to watch your fill

Chapter 9 Notes to tables (4)
 
Let me clarify, and add to, what the others have said.
Coreyd1@optonline.net said:
I was told under certain applications I can have 8 number 12's in a 3/4 conduit before I have to start derating.
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This is not a true statement. You do have to derate, if you have more than three current-carrying conductors in the same conduit. But under the circumstances described by the others, the derating does not cause you to have to change wire sizes, or to use a smaller rating for the overcurrent device.

Welcome to the forum.
 
jumper said:
To be more specific, if you are installing conduit and #12 conductors, the magic number is 9 before you have to worry about derating being a problem, barring any temperature factors.
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And to add it also applies to NM through fire caulked holes.
I was recently red tagged for more than 2 14-2 through a caulked hole. When I very nicely ask the inspector why and for code reference his reply was " we want it that way". I then ask what about 334.80. It tells me I must derate. Now I have 4 14-2 which would be 8ccc. 14 is rated for 25amp at 90 deg. 25 * .70 = 17.5. My OCP is 15amp so what is the problem? He then laughed and said most of the electricians we deal with cannot figure that out. He then signed off on it and went on his merry way.
 
ceb58 said:
And to add it also applies to NM through fire caulked holes.
I was recently red tagged for more than 2 14-2 through a caulked hole. When I very nicely ask the inspector why and for code reference his reply was " we want it that way". I then ask what about 334.80. It tells me I must derate. Now I have 4 14-2 which would be 8ccc. 14 is rated for 25amp at 90 deg. 25 * .70 = 17.5. My OCP is 15amp so what is the problem? He then laughed and said most of the electricians we deal with cannot figure that out. He then signed off on it and went on his merry way.
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In case some haven't noticed this is a pet peeve of mine. :)

Inspectors love to apply arbitrary amounts to the number cables that they will allow through the hole. Maybe it's not only the electricians who can't figure it out. :roll:
 
ceb58 said:
And to add it also applies to NM through fire caulked holes.
I was recently red tagged for more than 2 14-2 through a caulked hole. When I very nicely ask the inspector why and for code reference his reply was " we want it that way". I then ask what about 334.80. It tells me I must derate. Now I have 4 14-2 which would be 8ccc. 14 is rated for 25amp at 90 deg. 25 * .70 = 17.5. My OCP is 15amp so what is the problem? He then laughed and said most of the electricians we deal with cannot figure that out. He then signed off on it and went on his merry way.
Click to expand...

Nice job. :)
 
augie47 said:
... Until you get to the 10th ccc, 70% is your highest rating (ignoring ambient) so you have an end ampacity of 21 amps which is higher than the 240.4 allowed OCP of 20 amps...
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Not ignoring ambient, one could drop down one range or more in ambient temperature and compliantly go with up to 20 #12 90?C ccc's on 20A breakers in a single conduit. With an ambient factor of 1.04 or higher, #12 90?C conductors would have, before-derating, an ampacity of 30A ? 1.04 = 30.12A (or greater). The derating factor for 10-20 ccc's is 50%. So 30.12A ? 50% = 15.06A. Though very close to 15A, this amount is still over 15A, and permitted under 240.4(B) to be protected with a 20A OCPD. However, each circuit's loading is limited to the derated ampacity.
 
I have 6 current carring conductors in a 3/4" conduit onto 75 deg terminattion on either end. The load on each conductor max amps is 22 amps. They have a 30 amp breaker to protect the wire. Am I allowed to use a #10 thhn wire for these circuits??
 
atorres said:
I have 6 current carring conductors in a 3/4" conduit onto 75 deg terminattion on either end. The load on each conductor max amps is 22 amps. They have a 30 amp breaker to protect the wire. Am I allowed to use a #10 thhn wire for these circuits??
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Your load is less than the circuit ampacity at 30A (even if continuous 22A ? 125% = 27.5A) and the circuit ampacity is less than the max current at the #10 at 75?C value of 35A. Also, the adjusted ampacity of #10 THHN at 80% for 6 ccc's is 32A, and greater than your circuit ampacity.

I say yes, you are good to go ;)
 
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