NEC 310.4(C) Parallel Feeder Question

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cflath

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There is a difference in the wording of the 2005 code vs. the 2008 code. The 2005 code states that raceways must have the same physical characteristics. 2008 says they must have the same electrical characteristics.

Does anyone know why they changed the wording?

The reason for my question is this:
Can I run parallel feeders in different size conduits? I have a 3" GRC and a 2 1/2" GRC running in parallel that are already existing in the building feeding two panels side by side. I want to use them to feed a single 600A panel with 350MCM. Everything fits according to conduit fill. The conduits follow the exact same path and terminate in the MDP together.

Thanks for your input!

Chris
 
iwire

iwire

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cflath said:
There is a difference in the wording of the 2005 code vs. the 2008 code. The 2005 code states that raceways must have the same physical characteristics. 2008 says they must have the same electrical characteristics.

Does anyone know why they changed the wording?
Click to expand...

Here is the reason the CMP gave for the change.

Substantiation: The panel has reorganized this section in response to a number of public proposals indicating the need to reorganize the section to make it more usable without changing the intent of the section. A FPN was added to identify the use of sectioned Equipment grounding conductors and the existing 2nd FPN was removed.
Click to expand...


The reason for my question is this:
Can I run parallel feeders in different size conduits? I have a 3" GRC and a 2 1/2" GRC running in parallel that are already existing in the building feeding two panels side by side. I want to use them to feed a single 600A panel with 350MCM. Everything fits according to conduit fill. The conduits follow the exact same path and terminate in the MDP together.
Click to expand...

I definitely see your point about the change but it appears they did not mean to change the requirements.

So while I agree you could argue that the current wording allows you to use different size conduits that is not the intent of the change.

I think it could also be argued that different size steel conduits could create different electrical characteristics because they will likely provide different impedances resulting in unequal current flow.
 
don_resqcapt19

don_resqcapt19

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I didn't find anything in a quick look at the ROP (report on proposals) and the ROC (report on comments) that said anything about that change in wording.
I don't think that the two different size conduits would be permitted in either case as they are not matched physically or electrically.
 
don_resqcapt19

don_resqcapt19

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Illinois
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retired electrician
Bob,
I found that proposal and while it does change the wording from physical to electrical, there is nothing to give any indication of the reason for the wording change.
 
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