Interesting scenario about T310.15(B)(2)(a)

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Dennis Alwon

Dennis Alwon

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Here are two scenarios to think about.

1) Two 5 gang boxes, are fed with one run of conduit up the wall to a JB. The JB feeds 10 sets of lights with a total load of 1800 watts. There is one 20 amp circuit feed. We have a total of 11 (10 switch legs and one feed) current carrying conductors in a piece of conduit for more than 2 feet. What size wire would I need?

2) We have a one gang box fed with one circuit and one switch leg up to a JB that feeds one set of lights of 1800 watts. In this scenario we have two current carrying conductors in the conduit. What size wire?

Why? Care to discuss....
 
Dennis Alwon said:
Correct answer but a terrible one, IMO. :smile:
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Is that good or bad? :grin:

You could put in a proposal, someone put in a proposal about the travelers used for 3 and 4 way switch loops. The panel said a change was not needed as you only had to count the ones that operate at the same time.

Now you scenario is different but is IMO in need of a loophole of some sort. :smile:
 
iwire said:
Now you scenario is different but is IMO in need of a loophole of some sort. :smile:
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It would be an interesting proposal but I bet it would be rejected based on the difficulty to enforce it.

This was presented to me at a local NC EC meeting. I found it interesting to say the least. If this did get accepted it would really change the concept of bundling in residential where you have multiple gang boxes feeding lights from one circuit.

Would it then change the effect of 334.80 last paragraph. I think it would.
 
The rule about derating for multiple conductors is all about heat. More conductors carrying current can generate more heat. It is true that if you have 11 conductors carrying the current associated with 1800 watts of lighting, they will collectively generate less heat than would a pair of conductors supplying the same light load. So you would think you should be able to use the same wire sizes.

The problem is that this situation cannot be described in the code in such a way that the rule is clear and enforceable. Consider the following proposed exception: ?If you have more than three current carrying conductors in a raceway, but they do not carry much current, then you don?t have to derate.? I cannot think of a way to word such an exception. How and where do you draw the line between requiring, and not requiring, a derate?

I think the rule is what it is because it is easy to count wires in a conduit. It is also easy to say ?don?t count the neutral in this situation,? or to say ?if not all the wires can be carrying current at the same time, you don?t have to count them all.? But it is not easy to say ?let an engineer calculate the amount of heat generated by having more wires, but with each carrying a smaller amount of current, and you don?t have to derate if the answer is lower than XXX.?
 
'Multiple conductors of the same circuit and phase in the same raceway or cable shall be permitted to be counted as one.'

Or something like that. :smile:
 
iwire said:
'Multiple conductors of the same circuit and phase in the same raceway or cable shall be permitted to be counted as one.'

Or something like that. :smile:
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Yes, I like that. I was thinking wires from different circuits shall be counted but I believe yours is closer. The only problem would be that it would have to be counted as 2 if there were a neutral. Get's a little complicated.
 
I've brought this up in some how-many-cables-in-a-hole discussions. To me, there's a big difference between, let's say, six 12-2's (1 HR & 5 loads) on one circuit, and six 12-2's (3 HR's & 3 loads) on three circuits, run through a single hole.
 
iwire said:
'Multiple conductors of the same circuit and phase in the same raceway or cable shall be permitted to be counted as one.'

Or something like that. :smile:
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Well, it depends. In some cases, like 2-wire switch loops, you'd still have two CCC's. Maybe it would be the count should be your rule above +1.
 
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