Where to bond Neutral to Ground in Off-grid system

Jaffnike
Jaffnike Registered Users Posts: 3
Hi Everyone!

I have a 1.8kw off grid solar array, charge controller, batteries, and a 3300 watt pure sine inverter/charger as well as a 5000 watt portable generator for charging and backup power. The system is working great off the inverter outlets and I am now wiring up an A/C loadcenter to connect it to the whole house. So Question 1 is Where is the best place to bond Neutral to Ground? The inverter says it has a neutral to ground bond switch that will activate or deactivate depending on if power is coming from the generator or the batteries. This option can be completely deactivated (leaving inverter Neutral to Ground UNbonded). Do I leave the Inverter as is and NOT bond Neutral to Ground in the Panel, OR Do I Disable Neutral to Ground in the Inverter and bond Neutral to Ground in the panel?

Second Question: In my panel  If I want to bond neutral to ground I simply tighten down the green screw, fair enough. If I don't want to bond them, Do I need to buy a separate ground bar? Visually it looks as though the ground and neutral bars attach to each other through a crossbar.

I tried to upload a photo of my loadcenter, but no luck. here is a link to it. This is not an endorsement for anyone or anything.

https://www.homedepot.com/p/Eaton-BR-100-Amp-24-Circuit-Indoor-22-kAIC-Main-Breaker-Plug-On-Neutral-Load-Center-BRP12H100/307648497

Anyway, I hope this isn't too confusing, I'm sure there are some of you much more knowledgeable than myself that can help. Thank you!

Comments

  • MichaelK
    MichaelK Registered Users Posts: 234 ✭✭✭
    edited January 2020 #2
    That is your AC house breaker panel.  Where you should be bonding ground to neutral is in the power distribution panel (PDP) of your inverter before the AC power is lead to the house breaker panel.  Which inverter do you have?  Does it have a power panel that looks something like this? https://ressupply.com/power-panels/midnite-solar-mnxwp6848-2cl150-pre-wired-power-panel


    System 1) 15 Renogy 300w + 4 250W Astronergy panels,  Midnight 200 CC, 8 Trojan L16 bat., Schneider XW6848 NA inverter, AC-Delco 6000w gen.
    System 2) 8 YingLi 250W panels, Midnight 200CC, three 8V Rolls batteries, Schneider Conext 4024 inverter (workshop)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    From my limited experience, smaller AC power sources (North American), say less than ~3 kWatt, tend to have "floating" AC outputs (PSW/TSW inverters, Gensets, etc.)... And larger units (>~3 kWatt) seem to generally have the AC Neutral bonded to Chassis Ground...

    Many (some/most/all?) can have the AC Neutral to Chassis ground lifted, and you can "float" all of your power sources, and have one Neutral to Chassis/Earth ground in one place (typically in the Main AC panel).

    Note--If you have an MSW inverter, these (mostly) have an internal design that will be shorted out if you Ground Bond the AC Neutral and the DC ground together. For MSW type inverters, generally you always have to "float" the AC output.

    And--The reason that folks sometimes need to have a "switched" AC neutral to chassis/earth bond is because not all "power sources" can be modified to floating output. Typically, this has been for RV usage... The "shore power" connection is (north America) always bonded together somewhere at the RV park/panel(s)... So, when RV is on "shore power", you would have the AC neutral/ground bond "lifted". However, when you run from Genset or Inverter, you will need to make a neutral/ground bond connection (and this depends if the genset and inverters can have their neutral bonds lifted or not).

    The typical reasons for not have two or more neutral/ground bonds is that you get parallel current flow between the AC neutral and the green wire safety grounds. 1) you don't want your loads going through safety grounds, and 2) if you have GFI outlets/breakers (say between a N/G bonded genset and a N/G bonded AC panel), you will trip the generator's GFI protection circuit.

    So, as an example, you could wire up your genset with its own Neutral/Ground bond... And the switched G/N bonding in your inverter... When the genset is running, only the Genset N/G bond is present. When the genset is off and the inverter running, the inverter will provide the Neutral/Ground bonding... Generally, this should be a fine solution.

    But you should verify that the Neutral/Chassis ground is present in the genset so that your AC neutral circuit is always ground bonded (by genset or inverter).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • shawnj72
    shawnj72 Registered Users Posts: 64 ✭✭
    NEC 250.4(A)(1), National Electric Code (NEC) The Neutral is Bonded at the SERVICE Only. At 1 point . Your neutral should not be bonded anywhere else down stream. Where the service Entrance is Very first main breaker. If you have one outside then it is there if that your first main breaker is in your Main Panel the neutral is bonded there. Any thing downstream is a feeder conductor not service conductor. You can have multiple feeder circuits and ground rods but it should all come back to one point and it is at the main Service entrance is where the Neutral is is bonded. If it is a completely off Grid the same rules should apply.. As BB Stated In an off grid a lot of Gen sets the Neutral is bonded there because it is the source..This is where things could get confusing but it isn't. Even if you disconnect the main power at the breaker service your neutral is still bonded at the service. Your neutral is still bonded at the generator but isnt supplying any service when off so it will return back to the service in a ground fault situation.In an off grid it would return at the Generator in an off grid as the service and should be bonded there or at the main first breaker. Simple check with meter will tell you if the neutral is bonded. I ended up taking the panel off me genset and sure enough the ground was bonded to the neutral at the plug but it also has a main beaker at the generator itself.. . .. I would still consult every documentation with your system and visit the NEC Mike Holt.com has good info also.. You can see what they are trying to accomplish no matter what the instillation. https://electrical-engineering-portal.com/bonding-bonded-nec-article-250  
  • Jaffnike
    Jaffnike Registered Users Posts: 3
    Thanks for the replies! Michael K the Inverter is a Spartan 3300 watt 48v inverter/charger, and I have a midnight solar E-panel where the charge controller, batteries, and inverter all are grounded, breakers,etc.
  • Jaffnike
    Jaffnike Registered Users Posts: 3
    Also, reading the genset manual, It is Neutral to ground bonded, so good to go there. Thanks again everyone, very helpful.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    edited January 2020 #7
    I would,  and did, ground at the E-Panel, and un-bond at the service panel you linked. Then the E-Panel is the main and  the inside becomes a sub-panel. Not sure how this will  work  with the generator.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Neutral/Ground bonding at both the genset and main panel:
    1. If there are GFI (ground fault interrupt) breakers on the genset, then you run the likelihood of tripping the genset GFI breakers (and this depends, sometimes the 120 VAC outlets are GFI protected, but the 120/240 VAC twist lock may not be...).
    2. You will have a parallel current path in both the White wire (Neutral) and the Safety Ground/Conduit (green wire) connections. Nothing observable may happen (assuming no GFI breakers on the genset), but it has been reported that this can damage the AC voltage genset regulator (for at least one model of genset).
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    The inverter likely has the neutral ground bond lift to accommodate grid power input rather than generator input so the generator would need it's own ground rod to establish the neutral ground bond, otherwise it would be a floating neutral if isolated from ground. Likewise the inverter itself would need to be grounded to reestablish the bond. The alternative would be to remove all neutral bonding before the main distribution and establish it in the distribution itself, this would eliminate the ambiguity of multiple grounds within a single system.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    Grounding often confuses me.  Anyway, in this case I'm thinking it depends on the setting of the inverter neutral switching. 

    If it is set to switch the neutral, the generator (assuming 120v) and inverter power sources would be separately derived.  In that case, the genny would be like shore power, with its own NG bond.  The inverter is making the bond internally for inverter power, so no extra bond in the epanel (or distribution panel)

    If it's set to not switch, the neutral of the two sources is permanently interconnected and the genny should be unbonded.  A NG bond would likely then be done in the epanel.

    The inverter appears to me as designed for mobile applications.  Having the switched neutral would make sense as a default, but may not be obvious in a fixed applicaion. I'd probably put a sticker on it referencing the bonding scheme, if only to remind myself down the road.

    Does this seem right?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    > The Neutral is Bonded at the SERVICE Only

    Does this actually happen in the bigger picture?  My understand is that neutral is also connected to ground at the utility transformer.    No GFI in this portion of the circuit, so no problem with two ground/neutral bonds.

    I am available for custom hardware/firmware development

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited January 2020 #12
    Yea, this is the "hidden utility" base violation of the Single Point Neutral/Ground bonding rule...

    If you have a single pole transformer serving 5 homes, then you have one Neutral/Ground bond at the pole, and then 5x more bonds at each of the 5x homes.

    Because (as far as I know, which is not much) there is no GFI detection/protection on the pole, and no specific voltage regulation, then there is no "functional" issue for the multi ground bonding to cause problems. And if you add lightning protection, multi-point Neutral/Ground Bonding is a good thing.

    Given that the high end of the ground rod resistance is 25 Ohms, that means even a 120 Volt fault to ground would have a maximum of (I=V/R=120v/25ohm= ) 4.8 Amps of "leakage current"... Not great, but not a huge issue (of course, if you ground to your cold water pipe, and so do your neighbors, the "ground bond" resistance would probably be much lower).

    There have been reports of Hot/Neutral swaps at the meter (ground bonded "hot leg" instead of neutral leg)--And the only way this was noticed was that the snow was melting around the ground rod...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    The case study on improper grounding (about 1/2 way down) might be interesting in terms of the Bad Stuff that can happen with parallel paths.  It's bigger that what we mostly do, but interesting (at least to me).


    https://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/NECArticle250Sections250.6-250.12~20020125.htm
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    There are other issues that can occur with multiple Neutral/Earth grounds and parallel current flow.

    With AC power systems, you want the hot(s) and neutral carrying conductors to all be right next to each other in parallel runs. If you run (for example) the hot lead through one hole in a main panel, and the neutral through a second access hole, the magnetic field between the two current carrying wires can create "circulating" currents in the main panel sheet metal... And can even cause it to over heat (just like an induction cooker).

    Other issues such as parallel neutral/ground bonds, if a neutral connection "fails", then all current will flow through the greenwire return. And you run the risk of somebody working on the power system (changing conduit connections, working on ground) could get electrocuted because of the "energized" ground system.

    Another issue is needing a single ground connection between the DC Battery Bank Return (typically negative grounded) and the AC safety ground system. For example, 1,200 Watts @ 120 VAC is only 10 Amps, but @ 12 VDC, that is 100 Amps.

    So, if you mix up the AC and DC return/neutral/ground/multiple bonding connections, it is very possible to inject the very high current of the DC power system into the lower current AC power system.

    When you have your standard DC to AC inverter--You have both DC and AC power connections, and grounding issues (chassis ground for AC inverter--Should go to DC ground common because of high current... But the same inverter may have an internal 120 VAC Neutral/Ground bond internal--That is also tied to the inverter chassis ground).

    Grounding is not a trivial subject.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    The neutral-to-ground connection for a separately derived system, such as transformers, generators, or UPS systems to be installed at the separately derived system or at the first disconnect after the separately derived system, but not at both locations [250.30(A)(1)]. If a neutral-to-ground connection is made at both the transformer and at the secondary panelboard, then neutral current will return on the neutral conductor and objectionable (neutral) current will flow through the grounding path.

    Sounds like the opposite of what actually happens (as discussed in #11 and #12).

    I also find it interesting that in many cases, hospitals find that no bonding of neutral to ground increases safety.   Marina's are also interesting - very little current flowing through the ground/water can kill swimmers.

    I am available for custom hardware/firmware development

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    It does seem the rules/practise differ depending on which side of the meter you're on.  By definition, a separately derived system would be on my side.  The other side is way above my pay grade, but I assume that different risks make for different rules?

    My marina banned swimming off the docks a few years ago.  Probably wise.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter