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Derating Cable

Posted by hinman (My Page) on
Thu, Sep 15, 11 at 20:51

Hi, I am in the process of installing a generator transfer switch. I have two sub panels that are seperated by a concrete slab (25 feet apart).

I have three unused 1 1/4 pvc conduits that are underneath the slab connecting both subpanels. (They are within ten feet of both panels).

I need to pull (2) 12 amp circuits and (2) 14 amp circuits through with thhn and properly box on both sides and continue with romex to isolate the circuits I need for the transfer switch. (I have 6 x 6 x 4 boxes to successfully pigtail the circuits).

I have read the NEC code book and would like to clarify the derating issue. I am staying with dedicated neutrals for each circuit and decided not to run any multi wired circuits. I just want to isolate the circuits for the transfer switch. So I have (8) current carrying conductors in 1 1/4 pipe (I want to keep them in one pipe so I have the other pipes for later use). The NEC states with 90 Degree I should only be worried about derating if the are more than 9 current carrying devices in the pipe.

I am correct with this interpretation or do I have to derate with 70 percent for conductors of 7-9. Do the nuetrals count as a current carrying conductor? Should I run 10 guage and 12 guage wire respectively in the conduit to help with derating? Or am I ok? any help is greatly appreciated.


Follow-Up Postings:

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RE: Derating Cable

"I need to pull (2) 12 amp circuits and (2) 14 amp circuits through with thhn and properly box on both sides and continue with romex to isolate the circuits I need for the transfer switch."

I think you mean two 20-amp circuits (#12 wire) and two 15-amp circuits (#14) wire, so I'm operating on those assumptions.

"The NEC states with 90 Degree I should only be worried about derating if the are more than 9 current carrying devices in the pipe.

I am correct with this interpretation or do I have to derate with 70 percent for conductors of 7-9. Do the nuetrals count as a current carrying conductor?"

You are correct in concluding that derating requirements won't force you to increase conductor sizes until you exceed 9 conductors.

What sometimes leads to confusion is that when you begin derating, you start with higher potential ampacities for the wire sizes. In the case of #12 AWG copper, for example, the full ampacity is 30 amps. Although you do, in theory, derate to 70% when there are 7-9 conductors in the raceway, the ampacity allowed still doesn't drop below 20 amps (for #12) or 15 amps (for #14). Therefore, there's no practical effect of the derating when you have up to 9 wires.

Yes, with some exceptions, neutrals count as current carrying conductors. (The neutral in a multiwire branch circuit is excepted, since it only carries the imbalance of the two related hots.)

Your specs are fine and there's no need to increase wire size to #10/#12 because of derating.


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RE: Derating Cable

Yeah. After I read it again. I realized I could have been more specific. Sort of bulleted it out without utilizing the proper terms. I appreciate your time and help. Many thanks.


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