Where's the ground go?

Hello Everyone,
I have question that is probably simple for most. . .

I am planning out my grid tied pole mount array. I plan on running two #6 THHN wires back on the home-run and a solid #6 ground wire. My question is: Do I run the ground wire along with the other conductors in the same 2" conduit or on the out side?
I am assuming they are run all together in the same conduit; so what happens during a lightning stike? Would'nt the lightinig light up the ground wire and melt the isulation of the other condutors ruining them and lead to a ground fault?

Thanks!

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where's the ground go?

    if you go by the nec you have to run it in the conduit too. i totally disagree with the nec on this. what i would do in your circumstance is to run it down to an approved 8' copper ground rod right there at the pole. from that ground rod i would run another #6 bare copper wire under ground to the ac mains ground rod thus intertying them. doing it the way the nec says will bring lightning into your home needlessly even if you use a rod there at the pole and still run it in the conduit as they suggest. the additional ground rod and buried #6 interconnecting wire will create one very large ground and that is desireable while preventing the ground lead from channeling it into your home. you still have to discharge any lightning potentials from the + and - leads prior to entering the home for better protection regardless of how the ground is handled. search this site for more posts on grounds and lightning.
  • System2System2 Posts: 6,290 admin
    Re: Where's the ground go?

    Thanks for the repsonse Neil. Just to be clear. . . the #6 that you are running back to the main AC ground rod is placed in the conduit per NEC or are you recommending it is buried along side it?
    I forgot to mention that I am planning on unsing SOV lightning arrestors as well.

    Cheers!
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where's the ground go?

    to be very clear, i would not place it in the conduit or even beside it, but rather i would have it go to a new ground rod and then bury it to go to the ac mains ground rod. any grounds from within the house also will still need to be tied to the house ac mains ground as per normal. all is essentially the same except the ground lead is rerouted to, you guessed it, the ground where it's supposed to be going in the first place.
    if it's at all possible, try to use your sov arrestors just prior to the + and - entering the house so as to clamp down on pickup from the wires being in the air. you could do this at the pole too, but it is more effective just before the wires enter the building. the arrestors must discharge straight to ground too and not allow it to enter the building.

    edit to add: don't get the idea of running it both ways, mine and the nec's, for doing so would create a ground loop and that could create problems for you.
  • aj164aj164 Solar Expert Posts: 122 ✭✭✭
    Re: Where's the ground go?

    Related to this question... I have a ground-mount array with the inverter just under the array. From there, AC output goes ~150 feet to the building and service entrance. I think I should have a ground rod at the structure, but we also have a neutral (4ga.) and ground (8ga.) in the conduit.

    There is also a sub-panel on the ground-mount array to allow for multiple inverters. Do I need to bond the neutral and ground in this sub-panel (since there is a grounding electrode)? What happens to ground and neutral at the building end? Obviously, the building's grounding electrode is there, and ground and neutral are bonded in the main breaker panel. I have tried to make sense out of the NEC and Wiles' "PV and the NEC" guide, but I can't come to a conclusion.

    -AJ
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where's the ground go?

    your circumstance is a bit different as you won't get away from the ac grounding/wiring requirements. i do agree to put in a rod, but to send an interconnecting ground wire underground to the household mains ground rod could cause ground loops and that can cause you trouble. generally neutral and ground should only be bonded in one place, at the main panel. bonding elsewhere too could cause the same loop type problems.
  • aj164aj164 Solar Expert Posts: 122 ✭✭✭
    Re: Where's the ground go?

    So the solution is to place a ground rod at the remote structure but not to have a ground wire going back to the building? That is, we'll have the inverter DC, AC, and frame grounded at the remote site. Then, we'll only have L1,L2,and neutral going to the house.

    Is this NEC, or is this electrically correct? (The two don't always agree.)
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where's the ground go?

    i'm trying to go by both the nec as well as the common sense to have a remote lightning ground point that will not bring that bad lightning potential toward the main panel. even the ground rod placed remotely could introduce a differential voltage or ground loops. even if we were to intertie the ground rods with an underground bare copper wire the nec still says neutral and ground can only be bonded in 1 place at the main panel. i suppose if that remote ground connection can be made without tying the neutral to that same external ground point then you're go to go and can even extend underground an interconnecting wire to the ground rods.
    i'm not a big nec expert so take my advice with a grain of salt. anybody else have thoughts on this?
  • aj164aj164 Solar Expert Posts: 122 ✭✭✭
    Re: Where's the ground go?

    I discovered something in the installation manual that may help. [ Reference SMA SB3000US ] The instructions on grounding (Pages 37-38 and Figure 3-1 on Page 19) lead me to believe that the AC ground (PE) is isolated from PV ground. (I think "AC input" on Page 37 is a typo and should read "DC input".) If so, then it seems that the PV framework could be tied to the PV ground terminal only. The ground coming from the building would go only to the PE terminal, satisfying "AC Grounding" on Page 38.

    The only question that remains, then, is what to do with grounding of the sub-panel at the array. (I have two inverters going to a sub-panel, and the sub-panel goes 150ft to an AC disconnect on the building. ) NEC 2005 Section 250.32 says the neutral and ground (from the main building) are NOT to be bonded at the remote bulding/structure, and a grounded electrode is required at the remote site. Of course, if I do this, then the PV ground and AC ground do end up being bonded at the grounding electrode.

    Remote array grounding diagram
    GroundPV_wiring.jpg

    Page 37:
    Information
    The AC input and AC output circuits are isolated from the enclosure and system
    grounding, if required by section 250 of the National Electric Code, ANSI/NFPA 70, is
    the responsibility of the installer.
    The Photovoltaic System Grounding shall be installed per the requirements of sections
    690.41 through 690.47 of the National Electric Code, ANSI/NFPA 70, and is the
    responsibility of the installer.

    Page 38:
    AC Grounding
    The Sunny Boy must be connected to the AC ground from the utility via the Ground Terminal (PE)
    (see Figure 3-1: Sunny Boy Internal Components on page 19).
    PV Grounding
    The PV array (frame) ground should be connected to the PV Grounding and DC Grounding
    Electrode Conductor (see Figure 3-1: Sunny Boy Internal Components on page 19).
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where's the ground go?

    aj164,
    they clearly show that the neutral and ground are separated in the remote and only connected in the main by the amber line between the 2 and too ground. i'm not sure of where they connect everything in their many diagrams, but it seems they do want an ac ground wire connection between the inverter and the mains and dc ground be handled separately. not easy to follow as it tends to jump around some. maybe somebody with this type of gt inverter could help you make heads or tails of the instructions. see the 10.2 wiring diagram and see if it helps you out. it has its certifications so i would assume the nec would want you to follow the instructions they provide as deviating from them could pose a hazard.
  • mradtkemradtke Solar Expert Posts: 53 ✭✭✭✭
    Re: Where's the ground go?

    Hello,

    I know almost nothing about the NEC. But, I am an amateur radio operator and concerned with antennas and lightening.


    Weak or indirect lightening strikes:

    Soil has high resistance copared to wire, so the lightening current flows through the grounding wire much better than through the earth. Thus, multiple grounds at different spots don't do any more good than a good ground at a single spot.


    Nearby direct strikes:

    The current from the strike flows through the high resistance earth outward from the strike. Any ground rods that are at different distances from the strike have large voltage differences between them. If equipment shares inputs and/or outputs but use different earth grounds (not connected), the voltage differences will damage the equipment. Thus, having more than one unconnected earth ground location causes damage.

    Direct hit:

    With multiple earth grounds, this is the same result as with a nearby direct strike. With a single earth ground and heavy enough ground wire between the strike and the ground rod, everything may survive.

    One problem with all of this is that it isn't scalable. That is, if this works for you, it would even work better if you went over and disconnected your neighbor's ground. Better yet, if you disconnected everybody's ground on your street.

    Mike
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