May I/should I unbond one of my two main service panels and interlock it for inverter input

[Alvin1047]

I am trying to understand my house wiring to know how, where, what kind of inverter to use, and for example the bonding setting on EG4 6000xp.

I have two 200 amp main panels about 4 ft apart. Each is connected L1 L2 N directly to my customer-owned step-down transformer about 100 ft from my main panels, i.e. six conductors total, no disconnects or junction boxes between the transformer and the two main breakers.

My only meter is one mile away on the other side of my step-up transformer. I.e. I take utility power at metered 240, step it up to 7,500 for a mile, and back down in the yard.

Each main panel has a ground conductor clamped to the single main foundation earth just below. No earth “rods” at the house.

Each main panel is bonded N-G as permitted and inspected in about 2012.

120 loads on either panel cause current in both panels’ ground conductors, as well as a currently empty EMT connection between the panels. The current on the ground wires spikes to about 4 amps when a 120 motor is started and settle to .5 amps or so.

Apparently, the 120v currents returning to panel 1 land on its bonded neutral bus and see a direct path to the transformer in panel 1. But they also see the ground wire leading via the ground system to panel 2’s bond to its service neutral (connected at the transformer to the neutral from panel 1. And vice versa so all neutral currents back to one panel are returned by both panels.
240 loads have no effect and 120 gfci breakers see balanced currents and do not trip, whereas the panel as a whole will always be unbalanced.

Should one of the panels be unbonded? Unbonding would cut the ground flows. It seems to me that a short circuit in any circuit on either panel would find the single bond and trip the associated breaker.

If unbonding one panel is ok, would this allow me to lockout the unbonded panel and serve it from a bonded inverter, or, as I suspect, since the neutrals are unbroken would the inverter neutral returns still see two paths, one at the bonded panel and the other at the inverter? (Meaning unbonded inverter supply required.)

 

[Hedges]

How about putting a disconnect at the transformer, and doing N-G bond there?
You would then need to pull ground wires to each of the panels.

Those of us in residential neighborhoods each bond N-G at our service disconnect, and bond ground to gas and water pipes. I think the gas pipe makes a parallel connection to the overhead neutral wire going to each house.

Yours is a bit different in that you have a short EMT between the two panels, compared to the long gas pipe we have.

You shouldn't just unbond N-G at one panel, because then you don't have a ground wire to where it is bonded. Although a ground wire could be run through that empty EMT, it wouldn't be run together with current-carrying conductors like it is supposed to.

If the neutral to second panel was unbonded, disconnected, run through empty EMT, bonded there, and both neutral and ground brought back through same EMT, maybe that could work. But it would mean two neutral conductors being run in parallel (most of the distance). Maybe that is acceptable for larger gauge wire.

 

[wpns]

Is there a N-G bond at the transformer?

 

[Alvin1047]

Hadn't thought of disconnects at the transformer. My two panels both have no 6 copper ground wires going to brass clamps on the foundation earth about 4 or 5 ft away.

 

[Alvin1047]

Is there a N-G bond at the transformer?View attachment 213858

the 240 neutral in the transformer box is bare and tied to the box with a large copper strap. Can't see it while transformer is live but I recall a ground rod at the transformer. There is not now a ground wire from the transformer to the house ground system.

 

[wpns]

the 240 neutral in the transformer box is bare and tied to the box with a large copper strap. Can't see it while transformer is live but I recall a ground rod at the transformer. There is not now a ground wire from the transformer to the house ground system.

Yeah, sounds like the same as mine, my electrician was explaining how the transformer has to have a separate ground rod, and the service has to have a separate ground, even if the meter box (one foot to the right in the picture above) is where the N-G bond happens.

This all makes my brain hurt, if the transformer has a N-G bond, and it's feeding two houses, and each house has a N-G bond, doesn't that create the same 'problem' as two N-G bonds in one house?

 

[Alvin1047]

I found some nec course materials with diagrams showing bonding adjacent (three foot) main panels as "standard." These also mentioned that a transformer N-G bond is not counted as long as the ground path to the main panel is through the (high resistance) ground. If tied to the house egc by a conductor, then I understand the panels should not be grounded. Unless I learn something new I plan to just leave it the way it was built and just not add any G-N bonds anywhere.