Inverter to off grid breaker box, no ground - very bad things?

Grounding is a complex issue... I will break it into several parts.

First, if your 2000 Watt PSW inverter has the chassis ground bonded to the AC neutral (green wire ground to white wire neutral)... That is fine. And you should have a cable that goes from the chassis of the AC inverter back to the battery Bank negative bus--Or to a cabin ground rod, which is also wired back to the battery negative bus (negative grounded battery system).

For electrical safety--The system green wire ground (negative battery bank and/or ground rod+negative bus) green wire should also be connected to your electrical panel sheet metal ground. This provides the electrical safety if there is a short from hot to electrical ground anywhere in your system (electrical fixture, metal j-box, etc.). You just carry Hot+Neutral+Green wire ground throughout your electrical system as normal.

Now, here is a tricky part... Normally, you only connect your green wire+neutral bond in one place in your AC power system. That is either inside the AC inverter, OR in your main panel (many panels will have an optional screw or similar to tie or float the neutral/white wire bus).

It does not matter if the ground bonding is done inside the inverter or inside the main panel (you could lift the ground bond and do it in your AC panel for example). The idea is you don't want to do it in two places because you don't want parallel green and white wire connections to carry normal operating current--You want the white wires to carry operational current, and the green wire to carry ground fault current. (same answer if you ever connect an AC generator to your cabin's AC power--One ground+white wire bond in system--In inverter, in main panel, or genset may also have a ground bond).

Next, it also depends on your AC inverter's AC output (similar for some generators too)--Does your AC inverter have a GFI outlet (Ground Fault Interrupter outlet) or did you wire directly to the AC inverter. The GFI Outlet (or a GFI breaker) measures current through the Hot+Neutral wiring. As long as the Hot current equals the White wire return current, all is OK. If, for example, you have a green+white wire bond in the inverter, a GFI outlet, and a green+white wire bond in the main panel--This will usually trip the inverter's GFI outlet (some return current through the white wire, some return current through the green wire because of two green+white wire bonding locations). That is why you want only one green+white wire bonding location "upstream" of any GFI outlet or breaker--To reduce the chances of a false trip.

Assuming you are after safety... Make sure you have circuit breakers or fuses on any power source protecting the wiring (i.e., the battery bank to inverter has a large fuse/breaker. And the 2000 Watts to any 14 AWG wire has a 15 amp fuse/breaker too). This is to prevent wiring fires if there is short circuit (refer to the inverter manual for size of fusing and cable size--Or we can discuss that if you do not have or cannot find the inverter installation manual--Need to know if this is a 12 volt or 24 volt battery bank).

To reduce the chances of shock/electrocution, using GFI outlets at sinks and on outlets that go outside (and other wet areas) will help safety there.

Some AC inverters (and gensets) have GFI outlets directly on their outputs... I don't suggest this--A single Ground Fault in your system will turn off all your AC power from the inverter--Including your lights. Having GFI outlets only, and no GFI on lights (for example) prevents a "blackout" and separate GFI outlets means less hunting around to find the fault (limited to one or two outlets, not looking "everywhere").

Another grounding issue is especially important if you have lightning in your area. A ground rod near your power source (solar, battery bank, AC ground bond) directs lightning energy down to the earth (ground rod should be next to outside wall--You don't want a ground rod in the middle of your cabin--You want to direct lightning to the outside of your cabin). Otherwise, "earth grounding" through a ground rod/cold and hot water pipes/propane gas lines/etc. does not affect the operation of other appliances and lightning (there are a couple of rare cases where ground rod helps things run correctly such as older florescent tube fixtures and reliable starting, and some electronic stove/water heater ignition systems).

If you have chances of lightning in/around your cabin, then ground rod(s) to your AC and DC power systems, solar panel frames/racking, and surge suppressors should be discussed too.

I will stop here--The post is long enough. Your questions and answers?

-Bill

Hi Tim,

Cabin/shed/off grid power--Everyone is different.

***Tim: My battery 'bank' is just four batteries wired parallel to stay at 12v--I don't have a negative bus... nor do I have a "cabin" (120 ft2 shed) ground rod--I actually think that's what I need to install

Personally, don't like to parallel 12 volt batteries for a 12 volt bank... I like to use a voltmeter to measure the voltage across each battery so you can see if all are close together (good thing), or you have a high or low battery (need to investigate why). With 4x 12 volt batteries in series, there is only "one voltage"--so you cannot quickly find a weak battery with a voltmeter.

For example, your next set I would suggest 2x 6 volt @ 200 AH batteries in series (for 12 volts), and 2x strings in parallel (for 400 AH)--Or whatever combination you like/need (2/4/6/etc. volt batteries). You can measure each 6 volt battery and see how they are doing individually.

***Tim: Not sure I follow--I don't have the ground rod, and I'll need to see closer how the panel may or may not be grounded. There is no green "ground bus bar" in the panel, and I can't see any bare grounding wire connected--no copper plumbing, so nothing there either

You want to have any exposed metal (such as electrical panels, AC inverter metal case, metal sink/plumbing if you have any, etc.) so that a short from Hot (120 VAC) to that piece of metal does not become a shock hazard. Tying to green wire safety ground back to the common ground bus/neutral ground bond and pop the fuse/breaker/shutdown the inverter.

NOTE: The ground bonding of the neutral only can be done with (Most) PSW/TSW (pure/true sine wave) AC inverters. If you have an MSW (modified sine/square wave inverter), most (almost all) of those would be shorted by attempting a ground+neutral bond--Do not do that here (as always, check the inverter manual--although, many inverter manuals don't even talk about grounding).

You do not need a ground rod for electrical safety (ground rods are really, mostly, for lightning grounds). If you are concerned about electrical safety (shock, wet location, etc.)--Using a GFI outlet (example) for your 120 VAC outlets work great. They work if the system has a ground rod/system grounding or not. They will trip if there a little current flow (0.025 amps or less) between Hot or Neutral and a puddle. In older homes that don't have three prong plugs (Hot+Neutral+ground), GFI outlets are used for electrically safe three prong outlet conversions.

Sub panels many times do not have a ground bus. You can get ground lugs (and even retrofit ground buses) that bolt into the box as a point to attach (in your case?) the Romex ground to the box sheet metal.

You do have to watch out for bare ground wires--Placing a wire nut does not really make the wire "safe". In your photo, the bare ground wire is very close to the AC bus bar connections in the box (in this case, you could "electrify" the Romex ground wire--issues "down stream"?)

If you do not ground bond the AC Neutral--You would have a floating 120 VAC power system. In theory, that is safe because touching one leg of the AC power will not shock you (no return path). There is the issue if there is a short to ground somewhere in the AC system, it is no longer floating. And now your "previously safe floating AC output" is no longer safe--And you don't know it. That is why using GFI outlets on ground bonded or floating AC outlets in wet areas is the safest. No matter what happens, the GFI will still grip if there is a current flow to a human standing in a puddle.

Your box--I cannot tell all the things that are going on inside it... So I cannot really tell you want is "right or wrong" there with the information provided.

Note that grounding is not "magic"--It is a secondary system that trips breakers if there is a short circuit to bare metal (electrical box, etc.).

***Tim: I attached a picture of my panel--the copper ground is coming from the inverter (which it sounds like is bonded to neutral IN the inverter), and it's just hanging loose. It was originally connect to the right bar (circled in green), but that would keep tripping the GFI built in to the inverter, so I removed it--thinking that was redundant. The romex grounds that lead to the receptacles and lights is attached to that bar on the right--along with the neutrals--is that what you mean by one bond in my AC system? My receptacles show that they are wired/grounded correctly, but I have no idea *where* they are finding a ground--somewhere in the panel?

You have fixed the "duplicate/redundant" ground/neutral bonding. The original double neutral bonding had some current flowing through the white wire and the rest of the current flowing through the green/bare wire. To the GFI on the inverter, that looked like a short from Neutral to Ground (which it is).

When you lifted the bare wire in the electrical panel, you now only have one in the AC inverter (neutral+ground bond). And the GFI on the inverter will no longer trip.

The other option would be to lift the neutral bond in the inverter, and connect the neutral+ground bond in the electrical panel. Either solution works.

In sub panels, typically the Neutral bus bar is insulated from the metal box (although, some have a screw that can tie the neutral bus to the metal box).

Ideally, you should have a ground wire from the chassis of the inverter to the electrical box sheet metal. And run the romex ground wire connect to the "box ground".

If you tie the romex ground to the neutral bus (as you currently do)--That works because you have the neutral+ground bond inside the AC inverter. This is not considered to be good practice (using the neutral from inverter to e-box for both power and grounding)--White and Ground wire are supposed to be parallel circuits but isolated from each other (except that "one spot").

If you ever (for example) decide to add a transfer switch and generator for backup power--This is where it becomes easier to lift both the inverter and Genset (if present) AC Neutral+Ground bonding. And use your electrical panel Neutral+Ground bonding there.

***Tim: This makes me think that I "lifted" the ground bond coming in from the inverter, but I somehow need to make sure the cabin receptacles have a ground fault current path---which seems like maybe it doesn't--even though my tester says I'm okay.

If you floated your AC power system--Your little outlet tester would only show power on Hot-Neutral connections. The ground connection would be floating.

Because your AC inverter has Neutral+Ground bonding, and you are (sort of) carrying both neutral and grounds from the inverter through your system--Your AC outlet tester shows H+N and H+G power ("good").

***Tim: yes, inverter has GFI on the outlets (which I don't use), AND the direct wire (which I do). and yes, I kept flipping the GFI when I had the ground bonded to neutral AGAIN in the AC panel--am I getting closer to understanding now?

Depending on how your attached to the AC inverter's outlet... You may not have anything plugged into the GFI Outlet--But GFI Outlets have a Power In and a Power Out (GFI protected).

If you have wired your barn/cabin power to the Power Out on the GFI outlet--Then the GFI is "protecting" your downstream AC electrical box. And when you have the "Neutral+Ground bonding" in the AC sub panel--It would trip the GFI outlet (even though you don't have an anything plugged into the GFI outlet).

If you connected on the "input side" of the GFI outlet (or even just removed the GFI outlet), then your AC box power would not be GFI protected.

Make sense?

***Tim: Yes, I have a 15a fuse between inverter and AC panel. I have fuses to install between batteries and inverter as well as pv panels and controller (ran out of time last weekend). 12v bank--max of 20a or so from panels

Good... Note that a 12 volt 2,000 Watt AC inverter can draw, under full load:

• 2,000 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage = 224 Amps max draw (2x that for a few second surge)

More or less, roughly 1,200 to 1,800 Watts is about the maximum you can cost effectively and reliably run from a 12 volt battery bank (maybe push to 2,000 Watts with very heavy and short wiring, and an 800 AH @ 12 volt flooded cell battery bank).

So, a ~220 amp to (1.25x NEC derating for fuses/breaker) 280 Amp breaker/rated wiring to get optimum power from your 2,000 Watt inverter.

**Tim: Yes, I have one installed! (GFI outlets near sinks, etc.)

That is very good. And why I suggest that the GFI on the AC inverter is at best redundant, and at worse--A source of frustration).

***Tim: someone else gave a great suggestion of a ground plate (my soil is crazy rocky) and I plan to install one by my pvs, and maybe I have to do another one near the AC panel as I still can't figure out how my receptacles are showing correctly grounded.

Generally, you don't want to send DC power very far (need heavy/short power cables for DC input to AC inverter--For solar charge controllers, put the controller near the battery bank and run longer wiring to panels--Sized for current and voltage drop).

For larger solar power systems, an MPPT type solar charge controller and higher voltage solar array (from ~30-100 VDC Vmp-array typical--depending on panels and charge controller selected--Higher Vmp-array, smaller diameter wire needed/longer wire runs possible).

Note on ground bus--That can be (for example) just where all the battery negative cables come together, or it could be a copper or brass bar with cables running to it. Just depends on the size of your system and your design.

Just to give you an idea of how one person's DIY solar system evolved over the years (1/2 way down the page, links to lots of pictures and narriations):

http://2manytoyz.com/

And your grounding is (as I understand) your Neutral+Ground (chassis ground) bond in the inverter. And tying the Neutral+Romex ground inside the AC sub panel

-Bill

I am using "Romex" which is a name for a set of cables (B+W+Bare copper, etc.) in a plastic sleeve instead of wires in conduit--or whatever.

Yes, taking the ground wire from a ground in the inverter (chassis ground, ground bus connection, etc.) to a screw inside the sub panel is great.... The idea is that you want to ensure that a short to the sub panel metal does not "energize" the box. Insted, it sends current back to the inverter and short circuits the output (popping a fuse/breaker/inverter protection).

In a system with a ground bonded neutral--In a working system, the neutral and the white wire are pretty much electrically the same. The parallel connection of the ground wire is for safety (if there is a break in the Hot/White/other wiring). It is a redundant connection, and with standard electrical test gear, you really cannot tell the "differences" between a ground bonded white wire (return) and the green/bare ground wire. And that is why you have color codes--So the various wires are not mixed up.

For an FYI on parallel connections of batteries (so they better share charging/discharging current), this website is nice:

http://www.smartgauge.co.uk/batt_con.html

They also have a lot of white papers on battery power systems--If you ever want to get deep into the details.

We use a lot of "rules of thumbs" here that are known to work well... And avoid getting the very complex/confusing details when 90% of the time, the details don't really matter unless you choose to ignore the basic rules of thumbs (sort of like the NEC national electric code and other codes and their standard practices, and not always explaining the "why" behind them). If you follow standard practices, it is easier to add on/repair/have other support the installations. Rather than each person having to "re-invent" the previous installers' thought processes.

Eventually, you probably will want to revisit your battery connections. It appears that you are using the battery bolts to hold upwards of 4+ sets of wires in some cases... Using Bus Bars can make "common connections" easier (and safer). Also using crimp connectors for physically secure connections of wire to ring lug to battery terminals/bus bars.

Again--Lots of stuff out there (including various lugs, crmp tools, crimping vs soldering, parallel connections of smaller cables vs 1 large cable, etc.). More reading/research to do.

https://www.solar-electric.com/search/?q=crimp

-Bill

I am not sure it makes any difference which "ground" you attach the wiring too... Use the screw that gives you a reliable "ground" connection (chassis of the inverter is ground--And there should be an external chassis ground screw that goes back to the battery bank "negative" terminal/bus/common connection. That way, any internal inverter short (transistors, wiring, etc.) will short circuit from Battery + to chassis ground to Battery - connection.

It is not a huge deal--But you are after a good basic install so when you send your 120 VAC wiring elsewhere in the building the the 3 wires are correctly polarized and connected back to the source(s) of energy. If you ever add anything later (like a genset+transfer switch), all the down stream wiring was done correctly and you should have no issues.

-Bill