Ground Bond for Off Grid PV with Generator as Emergency Backup

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Hello all. This site has been a great help to me but finally I've found an issue that hasn't been addressed yet (at least, not that i could find). As the subject suggests, I have a 3.6kw off-grid PV system with a 48v battery bank that I'd like to add a backup generator to, via a manual transfer switch, for the occasional long rainy weeks. The two systems are currently standalone. The generator is used only when the welder is required and it does share my PV grounding electrodes (there are 4, 10' long, 10' apart). With two independent systems, naturally I have two ground bonds (one for each system) but with the addition of the manual transfer switch, this will need to be changed to one ground bond. In a typical 'Service with Generator Backup' system, the ground bond would be made at the single panelboard. But in my situation, I will have two panelboards (one for the building and one for the generator to provide emergency power to the building with circuits for the welder). In this situation, I believe that the neutral to ground bond should be made at the manual transfer switch, as shown in the attached diagram, as i will not have a separately derived system from the generator. Any input would be greatly appreciated. Thank you all.

PV with Gen.jpg
 

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George Stolz

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Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Welcome to the forum! This thread remains open with the understanding that you are an engineer seeking advice on this system that you have designed, and your question is limited to design in nature and does not concern wiring methods. Please do not stray from this presumption as we cannot give DIY advice on this forum.

That said, is it possible to connect the grounded conductor of your inverter to the grounded conductor of the generator? That would relieve the need to rework the grounding, aside from removing the N-G bond in one or the other. Your two SDSs would become one.

As an aside, why so small on the run feeding the emergency panel board? Seems pretty limited at 20A.
 
Thank you for the reply. I am indeed an engineer and my question is about proper design and the safety that it provides.

Electrically speaking, the generators ground conductor and the inverters ground conductor are connected at the grounding electrodes. If given the choice on a g-n bond to remove, I would removed it at the building panel since it is a relatively low power panel.

True, 20A is relatively low but this is a weekend cabin and there aren't any high power consumption devices in it, such as a dryer or dishwasher. The heat and hot water are propane and the PV system makes enough power in the summer to run the air conditioner with ease. All lighting is LED.

Thank you for your reply. It's much appreciated.
 

jaggedben

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Location
Northern California
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Solar and Energy Storage Installer
As with most transfer switch situations, I think there are two options...

Option 1) Don't bond N to G at either source. Bond N to G at the transfer switch, and don't switch the neutral.

Option 2) Bond N to G at both sources and switch the neutral at the transfer switch. The neutral is grounded at whichever source is connected.

Your diagram shows Option 1 but doesn't specify that there are no N-G bonds at either the generator or the inverter. As long as you can confirm that neither of those pieces of equipment have an internal N-G bond -- or that it's optional to remove it per the manufacturer -- then I think your diagram is okay, at least conceptually.

With that said I think Option 2 would be the better option as far as ground-fault current paths. For example, I'm concerned in your diagram about what happens if there is a fault to ground in Panelboard 2 and the fault path back to the generator relies on the small grounding conductor in that 100ft of 12/2 romex. If I were doing Option 1 as in your diagram, I would probably install a #10 grounding conductor alongside that romex so that the circuit breakers in Panelboard 2 are more sure to trip in such a fault. More likely I'd go with option 2. I'd bond neutral to ground at the generator or at Panelboard2, and at the inverter, and I'd switch the neutral with the other pole of that transfer switch.

Fundamentally I believe either option is okay if you do it right.
 

George Stolz

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Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
250.30(A)(1) would require a n-g bonding connection at the source or disconnecting means for the separately derived systems. I don't think the transfer switch is an option in this case.

By connecting the neutrals in the transfer switch, there is only a single SDS.

If it proves difficult to remove either existing n-g bond, it would be necessary to switch the neutral, which looks possible using existing equipment because the second pole of the TS is not used in this case.
 
Thank you for the replies.
I don't think switching the neutral and making the N-G bond at both panelboards is correct for this particular situation since I'd like to have the option of having both panelboard independently powered or both panelboards powered by the generator. With the MTS switched to solar power, all is well. But when switched to generator backup, you will then have (2) N-G bonds (see revised drawing "PV wGen Rev1").
PV wGen rev1.jpg

I believe what George has stated previously ((1) N-G bond at either panelboard, do not switch N) is correct. Would you agree? See revised drawing "PV wGen Rev2".
PV wGen rev2.jpg

To set all minds at ease, this work will be performed by a certified electrician. I'm just looking for a good concept for when I sit down with him. I would hope that he would know these answers but its always good to have a good plan rather than no plan. Thanks again.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Keep it simple: is there a disconnecting means at the generator? Then the bonding jumper can't be any further downstream.

Is there a disconnecting means at the inverter? Ditto.

I think bonding jumpers at the panels run afoul of this.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Keep it simple: is there a disconnecting means at the generator? Then the bonding jumper can't be any further downstream.

Is there a disconnecting means at the inverter? Ditto.

I think bonding jumpers at the panels run afoul of this.

I would basically agree with this, except that if the generator is close to Panelboard2 then I think the main breaker there can be the disconnecting means mentioned and you can put the N-G bond there. (Again assuming that the generator doesn't have an N-G bond itself.) The same can not apply to Panelboard1 because the transfer switch is in between.
 
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