Derating

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ceb58

Senior Member
Location
Raeford, NC
I was ask a question today and I am not sure that I gave the right answer:-?.
Here's the hypothetical situation. All #12 wire on 20amp breaker. You leave the panel with 3 separate circ. 1hot 1 net.& grd. each (no MWC) all in same pipe. They go to a j-box and turn and go into a 3 gang switch box. One pipe say 10'long. The switch legs then leave the box in the same pipe back to the J-box and then branch out in different directions to the lights. Would the fact that you now have 12 cond. in one pipe require derating or would the fact that they are still the same circ. with a switch in between them not require derating?
 

augie47

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Tennessee
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State Electrical Inspector
sorry..i'm slow... Is this just 1 circuit (all phase conductors tied to the same breaker) ?
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
If you are asking whether a switch leg is a CCC then I believe it is and must be counted for the derating. Both the hot wire and the switch leg are carrying current so they must be counted.
 

jeremysterling

Senior Member
Location
Austin, TX
IMO, if the traveller is not energized, it is not considered a current carrying conductor. Therefore, I would only count 1 traveller for derating.

The EGC is a conductor, but not a current carrying conductor.
 

augie47

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Tennessee
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State Electrical Inspector
my opinion would be that you would have to derate based on the number of ccc. Even though it's the same circuit, each wire is current carrying thus adding to the heat. The derating tables don't address the load per wire, just the number of cc wires in the pipe.
 

Dennis Alwon

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Chapel Hill, NC
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my opinion would be that you would have to derate based on the number of ccc. Even though it's the same circuit, each wire is current carrying thus adding to the heat. The derating tables don't address the load per wire, just the number of cc wires in the pipe.
Ironic isn't it. I can install a 1/2 emt with one circuit that carries 20 amps and I don't have to derate but if I install a 2" conduit with 10 switch legs from a single circuit with a total connected load of 10 amps then I must derate. :-? The NEC cannot contemplate all the scenarios.
 

charlie b

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Seattle, WA
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Electrical Engineer
Even though it's the same circuit, each wire is current carrying thus adding to the heat.
I agree. If a wire carries current one way, and a different wire carries current the other way, they both contribute heat, and they both must be counted, even if it is the same circuit.
 

charlie b

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. . . but if I install a 2" conduit with 10 switch legs from a single circuit with a total connected load of 10 amps then I must derate.
I am not totally familiar with the various possible design concepts of switch legs. But let me say that if you have a number of wires in a conduit, but because of the wiring configuration and the related switches not all of them can physically carry current at the same time, then I submit that you don't have to count them all. You only count the maximum number that can carry current at the same time.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
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Electrical Contractor
They go to a j-box and turn and go into a 3 gang switch box. One pipe say 10'long. The switch legs then leave the box in the same pipe back to the J-box and then branch out in different directions to the lights. Would the fact that you now have 12 cond. in one pipe require derating or would the fact that they are still the same circ. with a switch in between them not require derating?
I only count six conductors to the switches. The neutral certainly don't have to hit them.

Ckt. 1 hot down and leg up, Ckt. 2 hot down and leg up, and Ckt. 3 hot down and leg up.
 

480sparky

Senior Member
Location
Iowegia
3 cir each with their own neutrals, I believe but I don't know why they each have their own EGC.
EGCs don't count in derating, so it's a moot point. ;)

From what I can gather from the OP, the three neutrals don't go to the switch, or at least they wouldn't need to. If all three neutrals do go to the switch, you've got 12 CCCs and you'll need to change to 15a breakers [30a * 50% = 15a per T310.15(B)(2)(a)]. If they don't, you've only got 6, and you can still use 20a OCDs
 
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infinity

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New Jersey
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From what I can gather from the OP, the three neutrals don't go to the switch, or at least they wouldn't need to. If all three neutrals do go to the switch, you've got 12 CCCs and you'll need to change to 15a breakers [30a * 50% = 15a per T310.15(B)(2)(a)]. If they don't, you've only got 6, and you can still use 20a OCDs
That's the real question. Do we have 6 CCC's or 12? The OP is somewhat unclear on what this installation has.
 

Dennis Alwon

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Chapel Hill, NC
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EGCs don't count in derating, so it's a moot point. ;)
I realize that the EGC's don't count , I was just curious as to why there was an EGC for each circuit.--- but this is a hypothetical which I didn't catch.

Charlie, if you have one fed going to fed a bank of switches and then 10 returns to lights, then why wouldn't they all be current carrying conductors as I stated earlier?
 

charlie b

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Seattle, WA
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Charlie, if you have one fed going to fed a bank of switches and then 10 returns to lights, then why wouldn't they all be current carrying conductors as I stated earlier?
As I said, I am not familiar with switch legs. Wiring methods are not my bag. But if each of the 10 wires is carrying current to a light bulb, then all 10 count, as I also said. Perhaps I was confusing "switch leg" with "traveller." :-?
 

Smart $

Esteemed Member
Location
Ohio

As I said, I am not familiar with switch legs. Wiring methods are not my bag. But if each of the 10 wires is carrying current to a light bulb, then all 10 count, as I also said. Perhaps I was confusing "switch leg" with "traveller." :-?
Yes they would all count under the current (and past) code. But the point is all 10 switch legs cannot carry more total current than the one which supplies the power. If that amount by NEC requirements is a max of 16 amps fed by a 20A breaker, the maxx toal currrent through the section of conduit is 32A... yet we have to derate as if the total max current is 16A x 11 wires, or 176A, which is impossible by the way the circuit is branched out.
 

Dennis Alwon

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Chapel Hill, NC
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Yes they would all count under the current (and past) code. But the point is all 10 switch legs cannot carry more total current than the one which supplies the power. If that amount by NEC requirements is a max of 16 amps fed by a 20A breaker, the maxx toal currrent through the section of conduit is 32A... yet we have to derate as if the total max current is 16A x 11 wires, or 176A, which is impossible by the way the circuit is branched out.
That was my point exactly. Odd isn't it?
 

Smart $

Esteemed Member
Location
Ohio
That was my point exactly. Odd isn't it?
Having been this way for "many" cycles, the oddity of it for me, and I'm sure others as well, has somewhat diminished. Yet that does not make it any more correct ;)

Right now I'm wondering if anyone tried to propose a "countering" stipulation? I just finished viewing the 2001 draft on this matter and nothing is on the table.

One possible reason for keeping it this way is that the wires could be reconfigured at a later date to handle more current... but we often say code can only be applied to what exists, and not future possibilities.
 
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