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Thread: EGC sizing for voltage drop

  1. #1
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    EGC sizing for voltage drop

    I'll try to make this simple. 80A OPD required for protection my PV system. 400' run from inverter main lug panel to disconnect at house. Have to oversize to 350 mcm AL with 2/0 AL EGC to maintain acceptable voltage drop, but seems impractical to tie this size wire in at either end (out of 100A meter socket at array location or out of 100A fused disconnect at house location). If I drop down to #4 CU at either end for my 80A opd rating, am I correct that I can still use a #8 CU egc (per 250.122) since I am not up-sizing for voltage drop. Seems a little counter-intuitive to be able to go from a #8 to a #2/0 and back to a #8.

  2. #2
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    You can use #8 where your ungrounded conductors are #4... not where they are 350kcmil.
    I'll never get there. No matter where I go, I'm always here.

  3. #3
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    Quote Originally Posted by Smart $ View Post
    You can use #8 where your ungrounded conductors are #4... not where they are 350kcmil.
    Agreed. That is what the 2/0 is for, to accompany the 350's in the pvc pipe run.

  4. #4
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    You upsize the EGC for factors due to distance, as opposed to any local factors (such as heating). The #8 EGC is good for all local factors relevant to this circuit, assuming the length is negligible.

    Where the length is not negligible, you need to reduce the total Ohms of the EGC system, and you do so by upsizing the majority of the length of the EGC, from #8 to #2/0. Even if you only upsize 390 ft out of the total 400 ft, you still accomplish this goal. Add up the resistance of 10 ft of #8 and 390ft of #2/0, and observe that it is a lot closer to the resistance of 400 ft of #2/0, than it is to the resistance of 400 ft of #8.

  5. #5
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    Quote Originally Posted by Carultch View Post
    You upsize the EGC for factors due to distance, as opposed to any local factors (such as heating). The #8 EGC is good for all local factors relevant to this circuit, assuming the length is negligible.

    Where the length is not negligible, you need to reduce the total Ohms of the EGC system, and you do so by upsizing the majority of the length of the EGC, from #8 to #2/0. Even if you only upsize 390 ft out of the total 400 ft, you still accomplish this goal. Add up the resistance of 10 ft of #8 and 390ft of #2/0, and observe that it is a lot closer to the resistance of 400 ft of #2/0, than it is to the resistance of 400 ft of #8.
    Thanks for the explanation. Makes more sense now.

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