Off grid house bonding sanity check

electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Occupation
Electrician
Lets say I have an off grid house (no utility source). There is an outside generator, and a PV system on the roof of the house. Basically I just treat it the exact same as if the generator were a customer owned outdoor transformer, right? So assuming the generator has a breaker integral to it, I would have a system bonding jumper at the genset, and run 4 wire to the house, right? IF applicable, I could use 250.30(A)(1) exception #2 and run 3 wire to the house, and make another N-G bond at the house. But there would always be N-G bond at the genset, and electrodes there. Is that all correct and are there any other options?
 
The generator is only an auxiliary source. There should be some kind of panel right at the inverter which is the main source. And that is where the bond and ground rods should be. The generator gets wired the same as any home standby generator, floating neutral back to the inverter. I don't think the code really gets into this very deeply.
 
Birken Vogt said:
The generator is only an auxiliary source. There should be some kind of panel right at the inverter which is the main source. And that is where the bond and ground rods should be. The generator gets wired the same as any home standby generator, floating neutral back to the inverter. I don't think the code really gets into this very deeply.
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What determines what is the "main source"? Also a panel is not a source. If a off grid system was designed a certain way, the generator would be running most of the time.

After re-reading the definition of SDS, I agree the generator doesnt have to be an SDS. I thought it said "no direct connection to circuit conductors of a service", but it just says "source".
 
I think if we got into the weeds the code probably permits the option to bond and ground at the PV inverter, generator, or first disconnect from either source (which might be the distribution panel), depending on the configuration of the setup. As long as it’s done in only one spot. Birken's post seems 'right' to me. I think because it's not a service the GEC is supposed to be terminated in the same enclosure as the SBJ.
 
electrofelon said:
What determines what is the "main source"? Also a panel is not a source. If a off grid system was designed a certain way, the generator would be running most of the time.

After re-reading the definition of SDS, I agree the generator doesnt have to be an SDS. I thought it said "no direct connection to circuit conductors of a service", but it just says "source".
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The inverter is the main source because it is running and controlling things all of the time. The generator only runs when called for by the inverter. If it is running all time the customer will be broke in no time.

jaggedben said:
seems 'right'
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That is basically all I got. The inverter and generator are on one side of the main disco, there is usually a distribution panel right "below" the inverter and that is the logical place to bond because it is the closest thing to a main and everybody would expect it there.

Mind you I am talking about old school inverters, mostly Trace and Outback. I have been doing this for over 20 years, that is how it was done. Nowadays with Sol-Ark and EG4 it may be a little different but the concept is the same. The inverter is the heart of everything. You don't even "need" a generator.
 
Birken Vogt said:
The inverter is the main source because it is running and controlling things all of the time. The generator only runs when called for by the inverter. If it is running all time the customer will be broke in no time.



That is basically all I got. The inverter and generator are on one side of the main disco, there is usually a distribution panel right "below" the inverter and that is the logical place to bond because it is the closest thing to a main and everybody would expect it there.

Mind you I am talking about old school inverters, mostly Trace and Outback. I have been doing this for over 20 years, that is how it was done. Nowadays with Sol-Ark and EG4 it may be a little different but the concept is the same. The inverter is the heart of everything. You don't even "need" a generator.
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Do we need to discuss the batteries here? That's the only way to get the solar to work off grid. Or some type of energy storage system.
 
Essencialy your creating a TN-S only system no MGN, I would keep only one G-N bond in the main breaker panel and float the generator or ESS. Keep the GES associated with the structure.
 
gadfly56 said:
Do we need to discuss the batteries here? That's the only way to get the solar to work off grid. Or some type of energy storage system.
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Presuming the batteries are DC and connected to the inverter, that has no bearing on the AC system bonding and grounding. The inverter might be a type that requires DC grounding, but these days it's more likely handled by ground-fault protection in the inverter which would be disrupted by any field grounding of the DC. Same goes for the PV, which may even be the same conductor.
 
gadfly56 said:
Do we need to discuss the batteries here? That's the only way to get the solar to work off grid. Or some type of energy storage system.
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I was just assuming batteries.

Any system that has solar and a generator with no grid connection needs batteries to function.

There is no such thing as solar direct to inverter (there might be something but nothing of real substance).

Off grid people run their batteries to power their lights, fridge, toaster, water, etc. when the sun is not present.
 
What if you just skip the inverter and use a 120 VDC battery and charge controller like the old Edison system? I wonder how much modern stuff (switch mode) would run off 120 DC? Probably lots of smoke LOL
 
Birken Vogt said:
There are a lot of code issues with 120 volt batteries and 120 volt OCPD and stuff.
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Well I was just totally joking :) 120 DC would probably not work for anything, there probably are no 120 DC charge controllers or batteries made. OCPD would probably be screw in fuses lol.
But looking at my coffee grinder it has a universal motor, so I'd be able to grind coffee.
Regarding code issues the old 'war of the currents' did resurface for a second in the last cycle
there was at least one proposal to correct that 100V limit in 706.20(B) to 120V that passed in the 2026 NEC by way of the CMP deleting the entire restriction as 210.6 covers it. The proposal referenced the long history of 120V DC Edison systems in dwelling units.
 
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