Help!!!!!

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Hello everyone,
I installed a new 100 amp electrical panel and on the top I put a trough and joined them by three inch and a half EMT pipes which measure 28 "to eliminate the MC reaching the panel due to the fact that it is mounted on the wall. all tracks reaching the trough are #12. But now the inspector tells me that from the trough to the panel I must go down with cable number 10 since the pipe is longer than 24”. Is this correct, if someone could tell me in which code chapter I could find this rule. thank you
 
Since your raceways are longer than 24" you must derate when you have more that 3 current carrying conductors (CCC's) in the raceway. How many CCC's are in the raceway and how long are the cable runs? Look at Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors.

Welcome to the Forum. :)
 
infinity said:
Since your raceways are longer than 24" you must derate when you have more that 3 current carrying conductors (CCC's) in the raceway. How many CCC's are in the raceway and how long are the cable runs? Look at Table 310.15(B)(3)(a)

Welcome to the Forum. :)
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Thanks.
At least will have 15 in each raceway.
 
oldsparky52 said:
Isn't that for derating for ambient temperature, not conduit fill?
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That seems to be a common comment about the 10' or 10% exception, but I have no idea why.
Exception:
Where different ampacities apply to portions of a circuit, the higher ampacity shall be permitted to be used if the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit.
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The exception was carefully drafted not to use the words "adjustment" or "correction" which would have limited the application of the exception. The CMP chose to use the term "different ampacities" which makes it apply no matter why there is a different ampacity.
 
don_resqcapt19 said:
The exception was carefully drafted not to use the words "adjustment" or "correction" which would have limited the application of the exception. The CMP chose to use the term "different ampacities" which makes it apply no matter why there is a different ampacity.
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How does this interact with 240.4? I mean, the exception doesn't permit me to put a 10' section of #12 Cu in a 100' 200A feeder, does it? The wording could be read to permit that, though.

Cheers, Wayne
 
wwhitney said:
How does this interact with 240.4? I mean, the exception doesn't permit me to put a 10' section of #12 Cu in a 100' 200A feeder, does it? The wording could be read to permit that, though.

Cheers, Wayne
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What part of the tables would allow #12 to carry 200A.
I read the exception as allowing short lengths of conductors rated for the load based on the tables before before other factors have been considered.
 
jim dungar said:
I read the exception as allowing short lengths of conductors rated for the load based on the tables before before other factors have been considered.
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That certainly seems like the intention and seems to be how it is commonly interpreted. But Don caught my attention by pointing out that is not actually what the exception says. It just says "Where different ampacities apply to portions of a circuit." Which on the face of it would also include the case of different ampacities due to different wire sizes.

Now if 210.19(A)(1)(a) and 215.2(A)(1)(a) used the proper terminology to refer directly to the values in Table 310.16 etc, rather than misusing the term ampacity, you could easily say that those sections still provide for a minimum wire size and 310.14(A)(2) Exception doesn't apply to them. But as 210.19(A)(1)(a) and 215.2(A)(1)(a) (mis)use the term ampacity, then it is hard to argue that 310.14(A)(2) Exception as currently worded doesn't apply to those sections.

So I'm just wondering if a PI is needed here for logical and terminological correctness. Since I'm pretty sure we don't want to allow a short section of #12 in the middle of any feeder.

Cheers, Wayne
 
230426-1813 EDt

The fusing current rating for #12 copper wire is about 240 A. In other words about this current level the copper wire will melt. I don't know the time constant on this. But no way would you want to run this much current thru such a wire at room temperature for very long.

.
 
wwhitney said:
That certainly seems like the intention and seems to be how it is commonly interpreted. But Don caught my attention by pointing out that is not actually what the exception says. It just says "Where different ampacities apply to portions of a circuit." Which on the face of it would also include the case of different ampacities due to different wire sizes.
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What about different ampacities of one conductor (or one conductor size) through, say, a nipple?

For example, when a nipple is used to bring several circuits through one knockout into a panel.
 
LarryFine said:
What about different ampacities of one conductor (or one conductor size) through, say, a nipple?
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Right, that is what I've always thought the exception was referring to, one conductor in different conditions.

But Don pointed out that it doesn't specify it's the same conductor, just different ampacities in the same circuit. So I'm asking what if anything in the NEC prevents me from applying the exception to a circuit conductor consisting of two different sizes spliced together at one point?

Cheers, Wayne
 
gar said:
230426-1813 EDt

The fusing current rating for #12 copper wire is about 240 A. In other words about this current level the copper wire will melt. I don't know the time constant on this. But no way would you want to run this much current thru such a wire at room temperature for very long.

.
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looks like 2 seconds:
1682592811896.png
 
wwhitney said:
Right, that is what I've always thought the exception was referring to, one conductor in different conditions.

But Don pointed out that it doesn't specify it's the same conductor, just different ampacities in the same circuit. So I'm asking what if anything in the NEC prevents me from applying the exception to a circuit conductor consisting of two different sizes spliced together at one point?

Cheers, Wayne
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What provision of the tap rule would let you do that? None that I can think of would permit a tap within a conductor run. The load end termination of the tap conductor would be the issue.
 
don_resqcapt19 said:
What provision of the tap rule would let you do that? None that I can think of would permit a tap within a conductor run. The load end termination of the tap conductor would be the issue.
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A straightforward if very naive interpretation of the current wording of 310.14(A)(2) Exception is that if you put 10' of #12 in a circuit next to 90' of 100A conductor, the exception promotes the ampacity of the #12 to 100A. In which case there's no tap, just some #12 with an unreasonably high ampacity.

I think we just need to add the words "of the same size conductor" to the phrase "portions of a circuit" to rule this out.

Cheers, Wayne
 
wwhitney said:
A straightforward if very naive interpretation of the current wording of 310.14(A)(2) Exception is that if you put 10' of #12 in a circuit next to 90' of 100A conductor, the exception promotes the ampacity of the #12 to 100A. In which case there's no tap, just some #12 with an unreasonably high ampacity.

I think we just need to add the words "of the same size conductor" to the phrase "portions of a circuit" to rule this out.

Cheers, Wayne
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I see nothing that actually changes the ampacity when you use the exception. Just like a lot of other rules, the exception simply permits the conductor to be protected with an OCPD rated higher than the actual conductor ampacity.
The small conductor is a tap and must be installed per the tap rules.
 
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