Ampacity Derating

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texie

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Electrician, Contractor, Inspector
9 switch legs and 1 neutral all in same conduit fed from same branch circuit. Is derating required? This would seem to be the equivalent of 2 current carrying conductors.
 
Unfortunately the code does not make exception for this so yes it must be derated but logically it makes little sense if it is all one circuit.
 
Welcome to the forum. Your thinking is perfect however NEC did not adjust for this situation so yes derate. If your total load is small enough you might get away with #12 on 15 amp breaker. Derate from 90 degree
 
I agree with Dennis. One circuit with all of the current divided amongst 10 CCC's seems like a great exception to the derating rule but as of yet it isn't in the NEC.
 
I won't disagree with the others (not yet, anyway ;) ), but I will say that I do not understand the installation. It is not clear to me what is connected to what, and how, and whether any or all of the switch legs can be carrying current at the same time. I will say that if, by virtue of the manner of connecting wires to switches and to loads, it is not physically possible for (let us say) more than three of the ten wires to be carrying current at the same moment, then IMHO the total count of "current carrying conductors" is three, not ten. In order to make that declaration for any specific circuit, I would have to see the wiring diagram. If I can discern that (let us say) changing the position of one switch will allow current to flow in a new wire, while at the same time stopping current flow in another wire, then I won't count both wires as current-carrying.
 
charlie b said:
I won't disagree with the others (not yet, anyway ;) ), but I will say that I do not understand the installation. It is not clear to me what is connected to what, and how, and whether any or all of the switch legs can be carrying current at the same time. I will say that if, by virtue of the manner of connecting wires to switches and to loads, it is not physically possible for (let us say) more than three of the ten wires to be carrying current at the same moment, then IMHO the total count of "current carrying conductors" is three, not ten. In order to make that declaration for any specific circuit, I would have to see the wiring diagram. If I can discern that (let us say) changing the position of one switch will allow current to flow in a new wire, while at the same time stopping current flow in another wire, then I won't count both wires as current-carrying.
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What he seems to have is 9 lights that are seperately controled from 1 branch circuit so this gives him 1 noodle,1 ungrounded feed, 9 switch legs and likely 1 grounding. While the total is less than the assumed 20 amp circuit no matter what is on the fact remains they all get counted with exception of ground. Is a fault in NEC that simply does not make exception for this situation.
What an inspector might do is apply common since and let it go.
 
Derate, but this should be an easy exception for the code to make; today for the next cycle!
 
So what I am now envisioning is nine switches, each controlling a separate light, such that if all nine switches are ON, then all lights are ON, and all eleven conductors are carrying current. In that case, I do agree that (1) derating is required, (2) derating isn't necessary from a physics perspective, and (3) item 2 is irrelevant.

I will add that this would be a difficult code revision to make, in that the wording of an exception would be tricky. You would need to make it possible to avoid derating, but make it clear when you can and cannot apply the exception. What makes derating for this particular example "unnecessary from a physics perspective" is that the total current on all conductors is less than the rated ampacity of any one of them. How can a new exception be worded such that it is both clear and enforceable? :confused:
 
charlie b said:
So what I am now envisioning is nine switches, each controlling a separate light, such that if all nine switches are ON, then all lights are ON, and all eleven conductors are carrying current. In that case, I do agree that (1) derating is required, (2) derating isn't necessary from a physics perspective, and (3) item 2 is irrelevant.

I will add that this would be a difficult code revision to make, in that the wording of an exception would be tricky. You would need to make it possible to avoid derating, but make it clear when you can and cannot apply the exception. What makes derating for this particular example "unnecessary from a physics perspective" is that the total current on all conductors is less than the rated ampacity of any one of them. How can a new exception be worded such that it is both clear and enforceable? :confused:
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Just a thought;

Exception to derating conductor ampacity due to more than 3 CCC in a raceway:
When the total current on all conductors is less than the rated ampacity of any one of them the ampacity of the conductors shall not be required to be derated.
 
markstg said:
Just a thought;

Exception to derating conductor ampacity due to more than 3 CCC in a raceway:
When the total current on all conductors is less than the rated ampacity of any one of them the ampacity of the conductors shall not be required to be derated.
Click to expand...

Now you know dang well NEC could never write something that easy to understand.
My question is how many inspectors would tag such an install knowing what we all know?
 
texie said:
9 switch legs and 1 neutral all in same conduit fed from same branch circuit. Is derating required? This would seem to be the equivalent of 2 current carrying conductors.
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Why does it matter if you derate ? it seems each switch leg can not have over 2 or 3 amps . The derate would never put the conductor below that . I would go with next larger neutral if the derate puts it below the total load .
 
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Derating

Derating

Regarding my original post... it would seem logic would tell you that you are spreading the available current over 9 conductors with 9 times the surface area to disipate heat as opposed to 1 conductor. Why would this not be addressed in the code?
 
What they say, you'll have to derate. However, 9 switch legs, I assume it's a 20 or 30A circuit. In which case the derating starts at the wire ampacity, not the overcurrent protection. #12 is good for 30A, so even at 70% you still got 21A which is above the max overcurrent protection you can use. Now if the neutral is sharing, than it doesn't count so you're in the 70% derating (6-9). If that's not the case I believe you have to derate to the 60% (too lazy to check the code right now, but I'm pretty sure it's 60%) which only gives you 18A. It's abit beyond my understanding thought why you have 9 switch legs in a conduit, surely you don't have a 9 gang box? do you?

texie said:
9 switch legs and 1 neutral all in same conduit fed from same branch circuit. Is derating required? This would seem to be the equivalent of 2 current carrying conductors.
Click to expand...
 
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