#12 in conduit and still use a 20a CB

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Kal-EL

Kal-EL

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This question was posed to me and I have looked it up and submited my answer of 6 and was told I was wrong. Can anyone help me with a code ref?


How many #12 THHN conductors can you put in a conduit and still use a 20 amp Circuit breaker. using the 75 deg. column (without any ambient temp effects).
 
You can use the 90 degree column and it would be 9 current carrying conductors. This would give you 21 Amps for each #12.
 
“Number of current carrying conductors” 7 thru 9 conductors derated by 70%. #12 THHN is good for 30 amps. 30 amps x 70% or 30 x .70 = 21 amps. answer - 9.
 
Kal-EL said:
Can anyone help me with a code ref?
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Welcome to the Forum. Dennis is right.

110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
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Just to be technical, I saw no mention of what type of conductor is to be used; without having the NEC right in front of me, I can say with certainty that this will affect the value.
 
lordofpi said:
Just to be technical, I saw no mention of what type of conductor is to be used; without having the NEC right in front of me, I can say with certainty that this will affect the value.
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#12 THHN in OP...
Kal-EL said:
How many #12 THHN conductors can you put in a conduit and still use a 20 amp Circuit breaker. using the 75 deg. column (without any ambient temp effects).
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2008 NEC Article 310.15 Table Note, wire and insulation is capable of the ampreage of the 90 degree chart in 310.16. The terminations are not normally rated at 90 degrees thus the 90 chart is used for derating primarly. # 12 wire is rated at 30 amps, 7 to 9 wires requires a 70 percent reduction which is 21 amps. The overcurrent device is used to protect the insulation on the wire and the devices or equipment connected. A # 12 wire can carry more current than this but will produce heat and damage the insulation.
 
Since we are on the topic of ampacity: suppose you are running a feeder using THHN through conduit. To run a single piece of wire from breaker to subpanel, lug to lug, requires using the 75C ampacity column since the lugs are typically only rated at 75C. On the other hand, I see many wirenuts rated at 105C.

So it is acceptable to use the 90C column for the THHN feeder if each end is pigtailed with wirenuts to a larger diameter wire meeting the ampacity requirements at 75C? Is this commonly done?

Yours, Wayne
 
wwhitney said:
Since we are on the topic of ampacity: suppose you are running a feeder using THHN through conduit. To run a single piece of wire from breaker to subpanel, lug to lug, requires using the 75C ampacity column since the lugs are typically only rated at 75C. On the other hand, I see many wirenuts rated at 105C.

So it is acceptable to use the 90C column for the THHN feeder if each end is pigtailed with wirenuts to a larger diameter wire meeting the ampacity requirements at 75C? Is this commonly done?

Yours, Wayne
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I would read art 110.14(C)
 
Dennis Alwon said:
I would read art 110.14(C)
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OK, I tried that, and it is still not clear to me if the pigtailing I proposed is NEC-compliant or not. :confused:

"Equipment" includes the wires and wire nuts. As far as 110.14(C)(1) or 110.14(C)(2), are 105C wire nuts "listed and identified" for use with 90C wire? If so, I don't see any problem with 110.14(C)(1) or 110.14(C)(2).

The only problem I see would be with the first sentence of 110.14(C): "Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device." This could be interpreted to mean that each conductor gets a single temperature rating, and once a connection at one end limits the conductor to a lower temperature rating, you can't use a higher temperature rating with regards to a connection at the other end. Is that the common interpretation? If so, then the pigtailing I proposed is not compliant.

Thanks, Wayne
 
I don't know, though Wayne; this could be interesting. The NEC says nothing about that rule applying to that particular circuit, so if we followed that logic, the whole building, and by extension the entire power grid, is limited to some 60-degree conductor somewhere. This cannot be true, therefore maybe from wirenut to wirenut (which, technically, is a completely different conductor anyway since it is terminated), the 90-degree column may be used?

Awaiting responses from those more educated than I.
 
lordofpi said:
I don't know, though Wayne; this could be interesting. The NEC says nothing about that rule applying to that particular circuit, so if we followed that logic, the whole building, and by extension the entire power grid, is limited to some 60-degree conductor somewhere. This cannot be true, therefore maybe from wirenut to wirenut (which, technically, is a completely different conductor anyway since it is terminated), the 90-degree column may be used?

Awaiting responses from those more educated than I.
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Well there are many things that could affect this , from length of the wire run to the type of terminations.
You start derating (in most cases) in the 90 deg. column then you have to use the factors that limit this such as terminations ( rated at 75 deg ) Then there is the Astrix that brings you to the note saying #12 can not be fused over 20 amps. Blah Blah Blah,,,
In a nut shell 9 #12 CCC's at 20 amp . unless you go to UL and get a testing done on what ever it is you want to manufacture, But that may not be cost affective for a MWBC going from place A to place B.:)
 
Kal-EL said:
How many #12 THHN conductors can you put in a conduit and still use a 20 amp Circuit breaker. using the 75 deg. column (without any ambient temp effects).
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It is sort of a trick question just in the way it is phrased. The person asking the question could say you are wrong wven if you give the correct answer.
 
lordofpi said:
maybe from wirenut to wirenut (which, technically, is a completely different conductor anyway since it is terminated), the 90-degree column may be used?
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Yes, this is the conclusion that I reached, that if the wirenuts are rated at least 90C, then from wirenut to wirenut, you can use the 90C ampacity column with a 90C rated conductor such as THHN.

So for example, you could run a 60 amp feeder using #8 Cu THHN by pigtailing a short length of #6 Cu to each end using connectors rated at least 90C. The #6 Cu at 60C (for the terminals) has an ampacity of 55 amps, and the #8 Cu THHN at 90C also has an ampacity of 55 amps. Rounding up to the next standard size breaker, we can use an OCPD of 60 amps (as long as the calculated load is less than 55 amps).

Is that correct? If so, is this ever done in practice? I guess it would be easier to just use #6 Cu for the whole run, the material savings probably wouldn't offset the extra labor involved.

Thanks, Wayne
 
wwhitney said:
So it is acceptable to use the 90C column for the THHN feeder if each end is pigtailed with wirenuts to a larger diameter wire meeting the ampacity requirements at 75C?
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Yes that can be done.
 
chris kennedy said:
WW as well as I would like to know if anyone does this.
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I don't recall seeing it done other then overhead spans.

It seems counter productive as it will only save you money if the run is long and if the run is long you should probably be going bigger for voltage drop reasons.
 
iwire said:
I don't recall seeing it done other then overhead spans.

It seems counter productive as it will only save you money if the run is long and if the run is long you should probably be going bigger for voltage drop reasons.
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Seems a little "hokey doke" , but if you had an existing run and some up-sized equipment it may just work.:rolleyes:
 
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