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Thread: Sizing combined DC/AC GEC with multiple inverters

  1. #1
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    Sizing combined DC/AC GEC with multiple inverters

    I understand that for a single inverter, you would use the combined cm of the ungrounded DC conductors with table 250.66. Yes, I understand that 250.166 is for DC GEC but according to Soare's Book on Grounding, you should use 250.66. That said, if you have multiple inverters feeding a solar A/C combiner panel, do we need to combine all of the ungrounded DC conductors for table 250.66?
    For example (4) #8 ungrounded conductors per inverter x 2 inverters would be 8 x 16,510cm=132,080. This would be equivalent to a 2/0 cable so from 250.66, the DC GEC would be a #4. This would be a commercial installation. It seems to me therefore that since we're using a combined DC/AC (larger of the two), the 125A solar A/C combiner panel will require a #4 Gnd to the main service panel.
    I'm not looking to stir up the previously discussed threads regarding combined AC/DC GEC, just trying to know whether my thought process is correct in adding all of the ungrounded DC conductors. The alternative would be to just consider the conductors to each inverter.
    Thanks,
    J.S.

  2. #2
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    For GEC, assuming you are using a ground rod, I believe 250.66 (A) applies. No need for larger than 6 AWG if going to rod, regardless how many inverters.

  3. #3
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    Ground Rods, etc.

    Since this is a commercial installation, we can assume a combination of ground rods, Ufers, etc. We don't plan to install any additional grounds rods but will connect to the ground bus in the 2000A service.
    J.S.

  4. #4
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    250.166

    In this particular case, ignoring Soare's Grounding Book's recommendation to use 250.66 and instead using 250.166 would allow us to use #8 for the A/C-D/C combined GEC since #8 is the largest ungrounded DC conductor. So now it looks like the right answer could be anywhere between #8 and #4. Any thoughts?
    J.S.

  5. #5
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    Quote Originally Posted by Jeff S View Post
    In this particular case, ignoring Soare's Grounding Book's recommendation to use 250.66 and instead using 250.166 would allow us to use #8 for the A/C-D/C combined GEC since #8 is the largest ungrounded DC conductor. So now it looks like the right answer could be anywhere between #8 and #4. Any thoughts?
    J.S.
    You need to use the larger of the two requirements you mentioned. See 690.47(C).

  6. #6
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    AC & DC Combined GEC

    I agree that you need to use the larger of the AC or DC ground requirements but the question was about the sizing of the DC GEC. I don't believe the you need to use the larger of 250.66 or 250.166 for the DC GEC. We're not required to use 250.66 for DC, it was just mentioned as recommended by Soare's Book of Grounding.
    So what would you use for the size of DC ground in my example?
    Thanks,
    J.S.

  7. #7
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    Sorry, I read this thread a little too quickly this morning.

    Your subject for this thread mentions a combined DC/AC GEC. I take that to mean a combined DC GEC and AC EGC, as described in 690.47(C)(3) of the 2011 code.

    The requirement for that is the larger of 250.166 or 250.122. Typically for small inverters (under 7kW) you do not need anything larger than a #8.

    If you have multiple small inverters feeding a solar A/C combiner panel, and you intend to combine the EGC for that panel with your DC GEC, then your combined grounding conductor run with the panel feeders may need to be larger than a #8. You could use that conductor as a bus and tap multiple #8 conductors to it to serve as GECs for each inverter. Or use a #8 as a bus to the point where it reaches the combiner panel.

    You do not need to upsize any GEC merely because multiple inverters are involved.

  8. #8
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    GEC sizing

    Thanks for your response. Following your recommendation makes sense. The part that has gotten me confused is that in several training courses they site the Soare's statement about using 250.66 instead fo 250.166 for the DC GEC and that you must combine the CM of all of the ungrounded DC conductors to fine the equivalent wire sizing to use in the 250.66 chart. As previously stated, my confusion is from the fact that the two different methods can get you very different GEC sizes on larger systems. As you mentioned on a smaller system it usually comes down to the default #8 minimum size.
    Thanks, again,
    J.S.

  9. #9
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    Quote Originally Posted by Jeff S View Post
    ...using 250.66 instead of 250.166 for the DC GEC and that you must combine the CM of all of the ungrounded DC conductors to fine the equivalent wire sizing to use in the 250.66 chart.
    I never heard that, and don't know who would be teaching that or why. They wouldn't be getting it from the 2008 or 2011 NEC.

  10. #10
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    I would size the GEC for one of the inverters. That would mean sizing it by 4x#8 instead of 8x#8. Then, I agree with what jaggedben said and tap off for the other inverter.

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