Connecting off grid inverter to house wiring

[Sverige]

If considering connecting a generic cheap off grid (not grid tie) inverter to house wiring, would it be safe to make a permanent connection between inverter neutral and house (mains) neutral and then just switch the live lines?

In this case, the inverter neutral would always float at the same potential as the house mains wiring and individual house circuits live lines to living room, dining room, etc would be switched via relays such that the mains connection disconnects from a circuit before the inverter live is connected to that circuit. Hardware interlocks would be used to ensure the relays connecting mains and inverter cannot both be on simultaneously.

If strapping the inverter neutral to the house wiring neutral is not going to work, I would need to switch that line via further relays I suppose.

 

[teotwawki.je]

Yes. Why not move all of the circuits you want to switch onto a sub-panel & power that from a single ATS between the grid & inverter?
E.g.My plan is similar: Keeping the heavy loads of cooker & water heater spurs on the original old fuse box and moving the old 1950's socket & lighting rings into a new consumer unit with DIN rail ATS. I did a new set-up like this on a barge using one of these wired appropriately: https://www.cef.co.uk/catalogue/products/4327493-25a-modular-contactor-2-no-2-nc-contacts-230v-coil

 

[Sverige]

Yes. Why not move all of the circuits you want to switch onto a sub-panel & power that from a single ATS between the grid & inverter?
E.g.My plan is similar: Keeping the heavy loads of cooker & water heater spurs on the original old fuse box and moving the old 1950's socket & lighting rings into a new consumer unit with DIN rail ATS. I did a new set-up like this on a barge using one of these wired appropriately: https://www.cef.co.uk/catalogue/products/4327493-25a-modular-contactor-2-no-2-nc-contacts-230v-coil

I guess only because I wanted to keep the cost and complexity down. Moving all the circuits to a new panel will be more work than just interrupting the phase (live) supply within the existing panel and switching it between inverter and mains. Doing it the way you suggest would likely involve extending the cables with wire nuts, to add cable extensions into a new panel location, which seems messy.

The neutral is the real kicker, as I suspect that’s shared across all three phases in the current panel, so I can’t connect inverter to circuits on one phase without the neutral being connected to mains at the same time. But maybe that’s ok?

 

[teotwawki.je]

Ah, you hadn't mentioned the 3 phase aspec on the original post. Ignore what I said & wait for better advice.

 

[Sverige]

Ok, but to be clear I’d only connect the off grid inverter to one or two single phase circuits at a time. I’m not trying to use it to power three phase circuits as clearly it can’t do that.

It’s just that in my house, a three phase board powers a number of single phase circuits (plus some three phase ones on a different board).

So anyhow, the q still relates just to interrupting one phase at any one time.

 

[Supervstech]

There are many inverters that output 3phase power. Several single phase can link to output synced 3phase power.

 

[Sverige]

There are many inverters that output 3phase power. Several single phase can link to output synced 3phase power.

thanks. I don’t need three phase in this application.

 

[Sverige]

So, back to the original question - any reason not to permanently join house mains supply neutral to off grid inverter neutral, then switch individual (single phase) circuits within a house between mains live and inverter live?

 

[Bossrox]

If considering connecting a generic cheap off grid (not grid tie) inverter to house wiring, would it be safe to make a permanent connection between inverter neutral and house (mains) neutral and then just switch the live lines?

In this case, the inverter neutral would always float at the same potential as the house mains wiring and individual house circuits live lines to living room, dining room, etc would be switched via relays such that the mains connection disconnects from a circuit before the inverter live is connected to that circuit. Hardware interlocks would be used to ensure the relays connecting mains and inverter cannot both be on simultaneously.

If strapping the inverter neutral to the house wiring neutral is not going to work, I would need to switch that line via further relays I suppose.

I have my setup connected that way & works fine. I use an 80 amp DPDT relay to switch between grid & solar

 

[Sverige]

I have my setup connected that way & works fine. I use an 80 amp DPDT relay to switch between grid & solar

Thanks for confirming! I knew there must be someone who would have tried this before.

 

[mrzed001]

Why don't you switch the phase with the neutral ?
Like with this:
https://shop.osd-schenck.de/epages/...h=/Shops/es549793/Products/ABB2CCA703046R0001

The main reason you want to do this way is because you need RCDs ( or RCBOs).
In theory if you have a 3 phase RCD for the house and after that you put an inverter with its own phase and neutral, that should not trip the main RCD. But I am not sure of it (since the phases if used together zero themselfs out in the neutral and you put in a total other 4th phase ...).
So I plan to have no main RCD, but 6 changeover switch (main phase + main neutral <-> inverter phase + inverter neutral ) and after them 1-1 RCBO (or RCD + breaker) to power 1-1 circuits of my house.

 

[rfbuehler]

I want to use a 200A safety switch to switch between the grid and my solar generation. The grid is a 200A service necessitating the 200A switch. The solar can supply 100A surging higher with instant loads. I can get a 2P 200A Siemens switch for around $850-900. So now to my ultimate question can I tie the neutral from the grid to the neutral from the solar generation. This ultimately comes down to $, if I use a 3P switch it will cost me an additional $2K.

So can I tie the neutrals together?

 

[mrzed001]

I want to use a 200A safety switch to switch between the grid and my solar generation. The grid is a 200A service necessitating the 200A switch. The solar can supply 100A surging higher with instant loads. I can get a 2P 200A Siemens switch for around $850-900. So now to my ultimate question can I tie the neutral from the grid to the neutral from the solar generation. This ultimately comes down to $, if I use a 3P switch it will cost me an additional $2K.

So can I tie the neutrals together?

To have true galvanic separation you need to switch the neutral too

But why don't you let the inverter do it automatically ?
If it is an OGWGS (off-grid with grid support) inverter then connect (L1, L2, N) AC_in to grid, AC_out to the distribution board (and nothing else).
And you are ready. Your house has an UPS.

 

[Bossrox]

My system is set up with both neutrals tied together, it's safe from anything backfeeding into the grid & zapping a lineman.

 

[gnoeld]

I am planning on putting two 15 or 20 amp breakers in my panel (one for each side on the buss bars) I will connect my inverter to one of the breakers and then jump to the other breaker to tie in both bars.
When the main is turned off I can flip the two inverter breakers to "ON" an only use the 120 volt circuits I choose
Your thoughts?

 

[SparkyJJO]

Do not ever do something that you could even accidentally cause a backfeed to the grid, ever.

 

[Supervstech]

I am planning on putting two 15 or 20 amp breakers in my panel (one for each side on the buss bars) I will connect my inverter to one of the breakers and then jump to the other breaker to tie in both bars.
When the main is turned off I can flip the two inverter breakers to "ON" an only use the 120 volt circuits I choose
Your thoughts?

Wait… so is the jumper only put in place when you turn off the main? Because otherwise, that jumper will vaporize…

 

[WindWizard]

I have a 30 amp relay inside my solar power panel which is automatically switched between the Grid and the Inverter depending on the battery voltage level. I am switching both hot and neutral so it is a double pole double throw relay. The coil is a 12volt so I use the 12v from my Charge controller to power it. It works very well. I would not advise connecting the neutral from the grid to the neutral from the inverter. Connecting the grounds together is okay. If your neutral is ground then you should still seperate them and treat them as such.

 

[WindWizard]

I am planning on putting two 15 or 20 amp breakers in my panel (one for each side on the buss bars) I will connect my inverter to one of the breakers and then jump to the other breaker to tie in both bars.
When the main is turned off I can flip the two inverter breakers to "ON" an only use the 120 volt circuits I choose
Your thoughts?

It does not sound safe. I would recommend using a relay that is automatic rather than relying on flipping breakers and such.

 

[gnoeld]

I agree with being safe. I have a Generator setup with an interlock system. I was going to do the same for the Inverter. What I need to know is if I use the 2 breakers and jump from one to the other will it work? Is neutral going to be a problem?