Grounding and Bonding and GFPs Oh My!

As I am in the process of adding a second array to my system, I have been looking over my ePanels wiring more closely and thinking about the thorny issue of grounding, GFP devices, bonding of battery negative to ground, AC neutral to ground, etc.

I realize this is a real can of worms and I've read the most recent threads here on this issue and THIS thread on the Midnite forum. I've done my best to get a grip on the issues described so well by Bill, Neil, Ryan, Bob and others on both forums. My meager understanding of the arguments lead me to believe I should bond my ePanel's battery negative bus bar to ground (and thus disabling the Classic's internal GFP. I'm less sure about whether I should be concerned about bonding the AC neutral bus bar to ground or whether it is already bonded internally in my Outback inverter.

I am grounding my second array as the other one is grounded - with an 8 foot ground rod at the array and no ground wire running into the utility room/house wiring with the PV wires. Both arrays are about 125 feet from the utility room and also about that distance from each other.

So my questions for the experts here are:

1. Is simply connecting a 4 AWG wire from the ePanel's battery negative bus bar the the ground bus bar sufficient? Do I need to worry about the wire gauge handing the full current rating of the main battery positive breaker (175 amps) or more?

2. Should I worry about bonding the ePanel's AC neutral bus bar to ground? While i can see the advantages of a bond between Battery Negative and Ground on the DC side, I'm less sure of the advantages of the AC neutral to ground bonding. Maybe these are already bonded elsewhere?

Thanks for any advice or help and apologies for any headaches I may cause..:blush:

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    mtdoc wrote: »
    As I am in the process of adding a second array to my system, I have been looking over my ePanels wiring more closely and thinking about the thorny issue of grounding, GFP devices, bonding of battery negative to ground, AC neutral to ground, etc.

    I realize this is a real can of worms and I've read the most recent threads here on this issue and THIS thread on the Midnite forum. I've done my best to get a grip on the issues described so well by Bill, Neil, Ryan, Bob and others on both forums. My meager understanding of the arguments lead me to believe I should bond my ePanel's battery negative bus bar to ground (and thus disabling the Classic's internal GFP. I'm less sure about whether I should be concerned about bonding the AC neutral bus bar to ground or whether it is already bonded internally in my Outback inverter.

    I am grounding my second array as the other one is grounded - with an 8 foot ground rod at the array and no ground wire running into the utility room/house wiring with the PV wires. Both arrays are about 125 feet from the utility room and also about that distance from each other.

    So my questions for the experts here are:

    1. Is simply connecting a 4 AWG wire from the ePanel's battery negative bus bar the the ground bus bar sufficient? Do I need to worry about the wire gauge handing the full current rating of the main battery positive breaker (175 amps) or more?

    2. Should I worry about bonding the ePanel's AC neutral bus bar to ground? While i can see the advantages of a bond between Battery Negative and Ground on the DC side, I'm less sure of the advantages of the AC neutral to ground bonding. Maybe these are already bonded elsewhere?

    Thanks for any advice or help and apologies for any headaches I may cause..:blush:

    The main argument against bonding neutral to ground at more than one point is that it allows the grounding wire (the equipment grounding conductor/ EGC) to become a current carrying wire under NORMAL circumstances, since it is now in parallel with the neutral wire. One of the problems with that is ground loops. Another is the mismatch between the current in the hot wire and the neutral wire as they travel, supposed together, between the supply and the load.
    If the neutral wire is accidentally or deliberately interrupted, the equipment will continue to function since there is an alternate path for the current, and exposed metal bonded to the EGC will become "live" by at least the voltage drop along the supposed "neutral" wire.
    If the neutral wire is larger than the EGC, as you indicated might be the case on the DC side in question 1, the new effective neutral wire will not be protected against either normal or fault current from the battery.

    There are lots more potential concerns, most of which you have probably already read.
    I am ambivalent on the question of bonding either panel + or panel - to ground, same with bonding the battery negative (I would not even consider bonding the battery positive, too confusing for others who may encounter the system!)
    On the AC side, I definitely hold for NOT bonding neutral to ground at more than one point, and the discussion can proceed to what is that best point. For on grid, it has to be at the service entrance/main panel, but off grid you have some more freedom. Since the inverter is playing the role of the utility (separately derived system?) there has to be a neutral to ground bond somewhere, and it most should most likely be where the power from the inverter enters the building. If the inverters are already in the same building as the loads, then right at the inverter chassis seems the best.

    If this is a hybrid system, the NEC rules applicable to POCO power should be primary. If it is strictly off grid, then again, it is a more open question. If there is an RV powered by the system, you should assume that there is a ground to neutral bond in its AC wiring somewhere unless measurements prove otherwise.

    Now let the fur fly as others wade in.....
    SMA SB 3000, old BP panels.
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    Thank you inetdog.

    What I'm taking from your reply is that since mine is a hybrid system, i should not be concerned about bonding the ePanel's AC neutral bus to ground. i haven't checked but i assume AC neutral and ground are bonded at my home's service entrance and that is sufficient, correct?

    The DC bonding between battery negative and ground and how to do this correctly is a bigger concern. If my reading of the various discussions on this issue is correct, doing so will improve overall system safety - (at the expense of losing the questionable safety benefits of GFP). Equipment manufacturer's are understandably reluctant to give specific advice on this since it is contrary to current NEC guidelines.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    mtdoc wrote: »
    Thank you inetdog.

    What I'm taking from your reply is that since mine is a hybrid system, i should not be concerned about bonding the ePanel's AC neutral bus to ground. i haven't checked but i assume AC neutral and ground are bonded at my home's service entrance and that is sufficient, correct?

    It is not only sufficient, it is the only place that neutral to ground bond is allowed to be located.
    Unless the transfer or pass through switch also transfers the neutral conductor, bonding the neutral to ground at the ePanel will result in two different, widely spaced bonding points with substantial current flowing through the neutral. If the transfer switch transfers the the neutral as well as the hot (phase) wires, then you need to have one bond on the inverter side and one bond on the POCO side.
    But I do not think that is how it is configured.
    SMA SB 3000, old BP panels.
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    OK got it - makes sense. That was my suspicion - I just wasn't sure if there were any significant implications for when my grid connection goes down (i.e. service entrance no longer hot] . Thanks again.

    The DC battery negative to ground bond question remains...
  • BB.BB. Super Moderators, Administrators Posts: 30,641 admin
    Re: Grounding and Bonding and GFPs Oh My!

    You have to check the inverter's manual... But using an Outback TSW (true sine wave) and internal (or external?) transfer switch should mean that 1) the AC/Neutral bond is not inside the inverter but externally made--typically at the main fuse box, and 2) that the DC input is electrically isolated from the output and it is safe to DC ground the battery bank (MSW--modified square wave--Inverters are not usually isolated and you will create a short through the inverter's internal switching circuitry when the battery ground and AC neutral bond is connected).

    Note--always read the manual on grounding. Things change, models come and go, and I am no expert on Outback inverters.

    Second question, should you DC ground the battery bank (typically negative ground). In a home/RV wiring system with lots of metal around (water/gas pipes, electrical equipment, RV frame, etc.), you want to ground the battery bank to the "safety ground" for the same reason we ground the AC Neutral. This allows us to not put a breaker/fuse in the "return"/"Neutral" wiring as they can be "shorted" to ground and virtually no current will flow. Also, if there is a short between "hot" and some grounded metal/equipment, that will cause the breaker/fuse to open--instead of "energizing" a chunk of metal and making an electrical shock hazard.

    Nominally, the AC and DC grounds are all wired separately and then combined at one location (typically at the main ground rod for the home). This helps prevent stray current flow through the electrical system (remember, that the DC current is about 10x the equivalent AC current---I.e,. 12 volts at 100 amps is the same as 120 volts at 10 amps or 1,200 watts). So, you don't want any high DC current going through an inadequate AC safety ground wire system. If you have multiple ties between AC/DC neutrals and returns--it is possible to actually overheat the AC ground/neutral wires because of the much higher DC current in some circuits.

    Regarding the size of the bonding wire--Ideally, you want the DC (or AC) bonding wire to be large enough carry enough current to trip the breaker/fuse (short surge current). For example the fusing current of a #6 awg copper wire is around 668 amps. I don't have an NEC manual--But they should have a list of the minimum awg cable needed for specific maximum circuit fuse/breaker rating (something like #6 awg to 200 amps, #4 to 300 amps, etc.).

    There can be good reasons to "float" an AC or DC power system--But for a fixed installation (home, cabin, permanently mounted in RV, etc.), I would probably go with the ground referenced type design (AC Neutral to ground bonding--if TSW inverter and allowed, and DC Grounding of the negative terminal of the battery bank/bus).

    It can get tricky--AC power for an RV is grounded by the RV power box (home, camp ground) and may be floating when not plugged in. Generators below ~3.5 kW are typically floating AC outputs, and generators over ~5kW are shipped with AC White wire bonded inside the genset.

    Sometimes you have to use an AC Transfer switch to lift/make grounds (i.e., parked with shore power vs generator power), or if powered with a larger genset (lift the Generator Neutral bond and make the bond at the Main panel of the cabin/home--if off grid or is an emergency backup genset).

    It is a complex problem and one that you need to research and understand the system you may currently have installed and how your changes/upgrades will affect everything. Here are some more threads/discussions:
    BB. wrote: »
    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidated FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    Towards the end of this thread is a very nice discussion of proper generator grounding.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    Outback's do not have internal neutral-ground bonding.
    Hard-wire inverters should not as they are meant to be connected to an existing AC distribution system where the bond would be found in the service panel.

    The only inverters that would have an N-G bond internally are true sine with outlets built-in.

    If there are any that do not follow this logic then they are stupid and deserve to go up in a puff of smoke! :p
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    Thank you Bill and 'coot. I really value your input.

    I have read the Outback manual. This is what it says :

    "The unit must be connected to a grounded, permanent wiring system. If a bond is made between neutral and ground make sure only one bond is present in the AC system at any time. Some codes require the bond to be made at the main panel only."

    and:

    "For all installations, the negative battery conductor should be bonded to the grounding system at only one point. If the OutBack GFDI is present, it can provide the bond"

    I have a prewired Midnite ePanel with a Classic CC. The ePanel's AC neutral bus bar is not bonded to ground. As you all have pointed out there is a bond at the main service panel and what I am hearing is that this is the only one there should be. Makes sense when the grid is up and the GVFX is in "pass through" mode. I guess when the grid is down and I'm on Inverter/Battery power that bond at the now dead main service panel is still adequate (this is the part that confuses me):confused:

    As far as the DC side - I don't have an Outback GFDI but I believe the Classic CC has one internally. Based on my reading of the above referenced thread on the Midnite forum (and Bill's paper submitted to Wiles) it seems that bonding battery negative to ground (and thus disabling the Classics GFI) is a good idea and what I am planning on doing. I plan to use a short length of #4 AWG between the battery negative and ground bus bars on the ePanel- unless someone talks me out of it !!;)

    Thanks again for the input.
  • boBboB Solar Expert Posts: 975 ✭✭✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    One way to help understand AC neutral to ground bonding is to remember that the ground/neutral bond is supposed to be located close to the AC source. In a home connected to the grid, for instance, the incoming power panel is the closest to the "source" and ground rod of course.

    In an RV or boat inverter/charger connected to the grid, (shore power), the ground to neutral bond is on shore when it is the active AC source and plugged in to the boat/RV.

    The RV inverter/charger has a relay contact that, when the battery/inverter is supplying AC to the RV, it connects its AC neutral to GND terminal and chassis. When the RV
    is plugged into shore power, the relay will engage and pass shore power AC through to the inverters' output and its charger tap (to charge the battery),
    and at the same time, the ground to Neutral relay contact will reconnect the outgoing ground connection to the shore powers' ground because it is the source at
    that time. When shore power is disconnected from the RV, the inverter is the AC source and it connects the ground and neutral together at the inverter.
    This ground switching is part of UL458 for RVs and boat inverters. In the off grid or battery backup market, UL1741 normally rules.

    When you have an AC generator instead of grid present (usually for charging batteries), the the generator is the source.

    But for an off-grid inverter that does not have the ground to neutral bond internally, the e-panel or Balance Of Systems box will have that bond connection.

    In reality this is not always perfectly black and white a and may be slight variations, but thinking about these scenarios helps me to remember where AC neutral to ground bonding is supposed to be located at.

    boB
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    boB wrote: »
    One way to help understand AC neutral to ground bonding is to remember that the ground/neutral bond is supposed to be located close to the AC source. In a home connected to the grid, for instance, the incoming power panel is the closest to the "source" and ground rod of course.

    Thanks boB - that really helps my understanding.

    The RV inverter/charger has a relay contact that, when the battery/inverter is supplying AC to the RV, it connects its AC neutral to GND terminal and chassis. When the RV
    is plugged into shore power, the relay will engage and pass shore power AC through to the inverters' output and its charger tap (to charge the battery),
    and at the same time, the ground to Neutral relay contact will reconnect the outgoing ground connection to the shore powers' ground because it is the source at
    that time. When shore power is disconnected from the RV, the inverter is the AC source and it connects the ground and neutral together at the inverter.
    This ground switching is part of UL458 for RVs and boat inverters. In the off grid or battery backup market, UL1741 normally rules.

    So does a "grid interactive" inverter like the Outback GVFX 3648 (which is UL1741 listed) have a similar ground switching relay to deal with grid outages?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    mtdoc wrote: »
    Thanks boB - that really helps my understanding.



    So does a "grid interactive" inverter like the Outback GVFX 3648 (which is UL1741 listed) have a similar ground switching relay to deal with grid outages?

    No, it's not necessary. With an RV there is no Earth ground unless you have shore power, so it is safer to keep the neutral 'floating'. With a GT inverter the neutral & ground are connected to the household system all the time as it is designed to back-feed the grid with surplus power. The only thing that changes in the event of a power outage is that the loads on the inverter's output are disconnected from the grid and connected to the inverter (it is a 'pass-through' when the grid is up).
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    No, it's not necessary. With an RV there is no Earth ground unless you have shore power, so it is safer to keep the neutral 'floating'. With a GT inverter the neutral & ground are connected to the household system all the time as it is designed to back-feed the grid with surplus power. The only thing that changes in the event of a power outages is that the loads on the inverter's ouput are disconnected from the grid and connected to the inverter (it is a 'pass-through' when the grid is up).

    Ok but then with the grid down - the AC to neutral bonding is quite a distance from the AC source - which is not the ideal as per Bob's post.

    Also - what happens if I need to disconnect from my service entrance panel (not inconcievable since it is located in a detached garage some distance from my ePanel/inverter room) ?. Should I then temporarily bond neutral to ground at the ePanel until the connection is restored? Sounds like the answer would be yes.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!
    mtdoc wrote: »
    Ok but then with the grid down - the AC to neutral bonding is quite a distance from the AC source - which is not the ideal as per Bob's post.

    Also - what happens if I need to disconnect from my service entrance panel (not inconcievable since it is located in a detached garage some distance from my ePanel/inverter room) ?. Should I then temporarily bond neutral to ground at the ePanel until the connection is restored? Sounds like the answer would be yes.

    The distance between the power source and the N-G bond may not be ideal, but it's way better than having two bonds.

    I can't imagine why you'd need to disconnect from your main service panel. You would have to sever AC Hot, Neutral, and Ground on both AC IN and any return coonections (which you may not know are there) on AC Out to do this. If you do totally isolate the inverter from the main AC distribution then yes you will want a new N-G bond near the inverter output, providing you have a good Earth ground connection as well. If the grounding is not available do not make the N-G connection as there will be no actual ground to sink the Voltage potential of the neutral line to zero. This will result in energizing all ground wiring and anything connected to it via the neutral output of the inverter. A common problem in RV installs, btw.
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    OK thanks. Yes - unlikely that I would need to physically disconnect from the main panel but good to know what I would need to consider if doing so. I would need to place a new ground since the ground rod is at that panel.

    The idea came to mind because during a big windstorm a few years ago my neighbors garage was completely crushed by a large fir tree. Our area was without power for about 1 week from that same storm. The only other scenario I could see happening would be - god forbid - a fire that destroyed my garage.

    Thanks again!
  • boBboB Solar Expert Posts: 975 ✭✭✭✭
    Re: Grounding and Bonding and GFPs Oh My!

    I believe this comes under the heading of "slight variations" ...

    boB
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