Inverter earth ground - I’m confused

[Glenn]

This is probably an easy one for most of you…… I have never owned an inverter.

I picked up an 1100W pure sine inverter. It came with a small earth ground cable which I’m guessing is the same as case ground.

I’ve read multiple threads, but find myself still confused. So the easy part- the DC Pos and Neg go to their respective inverter battery posts.

But there is a small earth ground wire it says to hook to chassis. The videos I’ve watched and blogs I’ve read say hook up the DC ground and the AC inverter earth ground to the chassis.

I’m having a hard time getting my mind wrapped around having those shared neutrals oh the chassis especially with the truck having an ECU and other sensitive electronics.

But every source seems to say the same thing, unless I’m misunderstanding. I am simply wanting to make a short pigtail that plugs into my outlet on the inverter and hard wired to a duplex outlet on the camping box to make things look stream lined. No service box or panel for AC, just the inverter and two outlets.

The manual from the manufacture is pretty much useless.

Advice on the grounding issue is very much appreciated! I’ve included pictures of the inverter and circled in red the “earth ground” in question

Thanks everybody

Post Edit: I see in the picture their identifying it as Ground Terminal (red circle) and not earth ground as I had read elsewhere.

 

[Glenn]

Probably going to answer my own question but I found another thread and just talked to one of our EE electricians here at work.

On the latest thread I read it was stated “ with a digital volt meter set to Ohms, check the DC negative post (incoming on inverter) and the AC ground and they are probably already connected”

If this is correct then the connection is already there and the earth ground on the inverter is just another “safety” by hooking it to the chassis.

 

[Jon R]

That wire is not serving as a neutral return - it’s a safety ground. One of its purposes is to ensure that, if the AC hot wire shorts to the likely conductive things in the vicinity, you get a large fault current that is safely routed and quickly blows the circuit protection device to cut off the power rather than having that nearby conductive object stay hot and be a shock hazard. On a vehicle you want to connect it to the chassis.

While cars and trucks use the body as a common return for small loads with only a positive wire routed to those loads, that’s not a good practice in an RV. Your ac and dc loads should have a dedicated neutral or negative wire to carry the current. You then want to tie the neutral and negative to the safety ground at one location only to prevent “ground loops” or unintended circulation of current.

One potentially problematic situation is when you plug into shore power and then have both your inverter safety ground and ac neutral and the camp site source neutral and ground all connected to the chassis. I haven’t thought through the possible failure effects for that arrangement but my initial reaction is not to like it.

Vic has experience with inverter installations, and hopefully he can offer more information.

 

[Dadocut]

This is a great topic. Lots of people working on electricals get grounding issues wrong. My goto source for RV grounding are several tutorials on the DIY Solar Forum. See https://diysolarforum.com/resources/grounding-made-simpler-part-1-ac-houshold-grounding.157/ to download part one of a four part tutorial.

Glenn: Your comments about needing a "NG Plug" (a plug w/ a neutral - ground connection installed on the inverter output) is important. Many inverters don't bond the neutral to the ground. Some do, some do only when the AC input is lost. Unfortunately inverter manufacturers don't typically specify what they do with the inverter neutral output. Most manufactures do however attach the input grounding connector (ie the green wire) to the inverter case. THe DIY solar forum has a document listing the behaviour of certain inverters (see Grounding details for specific make/model of inverters )

Connecting the 120v to shore power affects this also. IF shore power is connected, you don't want the NG Plug connecting shore power neutral to ground. WIth shore power the NG connection is instead made in the distribution panel of the RV park and you don;t want a second NG connection in your truck. I solved this problen by using a multipole transfer switch that connects my plugs to either my inverter or to shore power. THe transfer switch has a center off position. THe transfer switch has multiple poles and I switched both the hot and nuetral. (note the green "grounding connector" is not switched. IT's ok to attach the RV parks grounding connector to the green wire in your RV's 120v electrical.)

Hope this makes sense, it's a broad topic.

 

[Glenn]

Thank you for the Links to grounding.

 

[Dadocut]

Glen, part 4 of the grounding tutorial covers mobile installations like our campers. It's worth reading. It includes detailed schematics that show exactly how to hook up the inverter.

Long story short: you'll need the NG plug if your inverter doesn't ground the neutral internally.

 

[Glenn]

Dadocut, I reviewed Letter 4 of the mobile systems. Above is a partial drawing from the chapter on inverters.

In the red circle I drew, is the “if not bonded inside of the inverter, it must be done externally” as noted in the caption.

While it doesn’t show a direct wire from the AC ground bus to chassis I believe there would be one “and only one” similar to that shown to the left of the diagram, on the DC ground bus. Is this correct?

Then if I understand correctly the purpose of the AC ground bus would be to bring all AC grounds to one bus bar and then only one bond between the AC bus bar and chassis so as not to create ground loops. ?

So essentially my starter batteries’ Negative (diesel engine) along with the engine, frame, body, etc. are all grounded to Chassis in the engine compartment.

The chassis acts as one big ground, though I have a dedicated 6AWG Negative cable (along with the 6AWG Pos.) going from starter battery to the Victron 12-12-30 non isolated smart charger.

My Negatives for my DC loads for the camping box only, in the bed of the truck, all go to the shunt first on the (load /charger side - including my inverter) and then on the battery side of the shunt (Victron BMV - 712 Model) back to my 280A LiFePo4 house battery.

Finally, the AC ground bus would be connected to chassis (as safety ground) somewhere underneath the truck bed to the chassis frame

Does that sound right? Thanks!

 

[Glenn]

On my way to work…..I stopped at shop first:


Post Edit: Removed the post detail and pictures….. I was in a hurry this morning and just realized from the picture that I need to check the AC neutral to case ground……I was on the AC outlet ground to case I’ll check when I get home tonight and repost. Sorry for any confusion if thread was previously viewed.

 

[Glenn]

I was beat down tired, up too late reading up on the inverter wiring.
Unfortunately, it turns out the AC Neutral and Ground is not bonded.

So, for my simple setup of the inverter going to two outlets (no metal), No shorepower, everything attached to wood, could somebody draw up how I externally bond the AC neutral to the inverter case ground?

Later in another thread I read: do not attach the AC neutral to the chassis as it could become live with AC current.

I still have the green inverter ground that has to go to the truck frame, and from what I have read that should be sized to the inverter DC input cables to the inverter by going one size smaller in wire size. I’m running 2 awg to the inverter so if this is true then the inverter ground should be 4 awg.

This whole thing is making my head hurt, I need the Tylenol

 

[Dadocut]

Glen, you're on the right track and are understanding the issues. Thanks for pointing out you are not connecting to shore power and are only using the outputs directly from the inverter. THat keeps it simper.

Like you stated, the truck battery, frame, the neg of the 12v in the camper, the neutral on the inverter output, and the AC ground are all tied together. Note your 12v-12v non-isolated charger is already connecting the truck battery neg to your 12v neg in the camper. Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).

At some place you should attach the inverter cabinet ground to your 12v neg bus bar. The wire making that connection should be a size smaller than what you used to drive the inverter. I suspect most installations use a much smaller ground.

I wouldn't bother to use a special wire to connect the truck frame to your 12v neg bus bar. It's already connected through the heavy guage wire from the battery to your 12v-12v charger.

The inverter output AC neutral-ground bonding can be done with an NG Plug. This is simply a standard plug internally wired such that the neutral plug is attached to the ground plug. Just plug it into your inverter output and it takes care of bonding the neutral to ground. Many people would skip this step but it's safer to have the neutral & ground attached.

That should do it. I don't think you need an additional bonding of the truck frame to your negative bus bar. You mention ground loops. They're an issue for RFI (radio frequency interference). Often times the safety rules are in conflict with RFI best practices. I would suggest you not worry about ground loops unless you have a radio interference problem. Unless you're a ham radio operator or running a nice hifi off the inverter I bet you won't have any ground loop issues.

hopefully I addreseds each of your questions.

 

[Glenn]

Glen, you're on the right track and are understanding the issues. Thanks for pointing out you are not connecting to shore power and are only using the outputs directly from the inverter. THat keeps it simper.

Like you stated, the truck battery, frame, the neg of the 12v in the camper, the neutral on the inverter output, and the AC ground are all tied together. Note your 12v-12v non-isolated charger is already connecting the truck battery neg to your 12v neg in the camper. Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).

At some place you should attach the inverter cabinet ground to your 12v neg bus bar. The wire making that connection should be a size smaller than what you used to drive the inverter. I suspect most installations use a much smaller ground.

I wouldn't bother to use a special wire to connect the truck frame to your 12v neg bus bar. It's already connected through the heavy guage wire from the battery to your 12v-12v charger.

The inverter output AC neutral-ground bonding can be done with an NG Plug. This is simply a standard plug internally wired such that the neutral plug is attached to the ground plug. Just plug it into your inverter output and it takes care of bonding the neutral to ground. Many people would skip this step but it's safer to have the neutral & ground attached.

That should do it. I don't think you need an additional bonding of the truck frame to your negative bus bar. You mention ground loops. They're an issue for RFI (radio frequency interference). Often times the safety rules are in conflict with RFI best practices. I would suggest you not worry about ground loops unless you have a radio interference problem. Unless you're a ham radio operator or running a nice hifi off the inverter I bet you won't have any ground loop issues.

hopefully I addreseds each of your questions.

First, thank you very much for your help.

QUOTE:
“Furthermore the AC ground is most likely already connected to the inverter chassis ground (internal to the inverter).”

Yes. I checked this yesterday

I also checked and there are two AC 115A outlets on the inverter. With the DVM it confirms both neutrals are tied together and both power feeds are tied together.

It is a small inverter. I only need one AC 115A outlet. But, since there are already two I can (on your suggestion of NG plug) plug this:

Into one of the outlets (which bonds them both) and then run power from the other outlet out two my other two receptacles on the camping box.

I say receptacle(s) on camping box as in two, not to both be used at the same time, (unless very small amp loads) but for convenience of where to plug in at.

I also understand not needing the additional ground cable to frame and can run inverter ground to negative bus bar as I am using the non isolated dc to dc charger.

My last question: On my Inverter AC output, would it be a good idea to run the outlet power out to a GFI receptacle first, and to protect the second outlet (on line side) or isn’t a GFI needed with the NG bonding plug at the inverter?

You’ve been most patient, thank you for your time - Glenn

 

[Dadocut]

Glen, the NG plug cannot be used after a GFCI receptacle. It would trip the GFCI. You could however use an NG plug before the GFCI (ie plug the NG plug into one of your two inverter outputs and plug a GFCI "extension cord" into the other inverter output)

The screenshot of the amazon NG Plug you show says the plug has an internal 15a fuse. That's not what you want. Oddly the other photo doesn't show the fuse. No telling what the thing really does. But you want to get one that is unfused. Consider a fault where the hot wire comes in contact with the inverter case. If the inverter has a 20a circuit breaker and the NG Plug has a 15a fuse, the NG plug's fuse would pop and the case would still be hot! Don't want that. Just find (or better yet buildyour own) a plug that connects the ground and neutral without a fuse. Without a fuse you ensure the high current will trip the 120v output circuit breaker shutting off the unsafe condition.

In a standard good old NEMA 5-15 receptacle/plug the neutral is the wider of the two power carrying conductors. Hot is the narrow one. Looking at the receptacle, and if ground is at "six oclock", the neutral is at "ten o'clock". Just build a plug that shorts the two together.

Different countries handle ground & neutral differently and non UL rated products don't necessarilly do what is considered safe in the United States. Furthermore, these are subtle points and even if you do it wrong you still get power to the outlets!

 

[Glenn]

Glen, the NG plug cannot be used after a GFCI receptacle. It would trip the GFCI. You could however use an NG plug before the GFCI (ie plug the NG plug into one of your two inverter outputs and plug a GFCI "extension cord" into the other inverter output)

yes, I was planning on running from the inverter outlet to a GFCI receptacle on the camping box, and then to the second receptacle, with the bonding plug taking up one of the outlets on the inverter case.

The screenshot of the amazon NG Plug you show says the plug has an internal 15a fuse. That's not what you want. Oddly the other photo doesn't show the fuse. No telling what the thing really does. But you want to get one that is unfused. Consider a fault where the hot wire comes in contact with the inverter case. If the inverter has a 20a circuit breaker and the NG Plug has a 15a fuse, the NG plug's fuse would pop and the case would still be hot! Don't want that. Just find (or better yet buildyour own) a plug that connects the ground and neutral without a fuse. Without a fuse you ensure the high current will trip the 120v output circuit breaker shutting off the unsafe condition.

I hadn't considered the fuse being a bad thing, I’m glad you pointed that out. Yes, I don’t have a problem building my own. I’m not sure the inverter has breakers on the case. I know it came with two fuses but haven’t had time to look at them closely yet. I will look closer at it tomorrow. I hope to get a lot done this weekend. Hunting season is just around the corner but we’re waiting for this insane heat to break.

Thank you!

 

[Glenn]

Dadocut, the inverter package came with two 30A blade fuses, yet there is no mention where they go?

Also, could you look at the two photos and give me your thoughts about protection:

In the one photo it says there is ground fault circuit interrupter protection (under Key Features)

In the second photo it says it has short circuit protection (under Protection Description)

The manual seems vague to me. And just to confirm I checked again and there is no bonding between AC neutral and ground. To be sure I was making a good connection for the digital volt meter I took apart a 115V plug, inserted it into the inverter outlet, and probed the plug contacts where the appliance power cord would fasten to make sure I was making a solid contact with the meter probes.

Thank you

 

[Dadocut]

Hi Glen,

The documentation on your inverter is ambiguous. On one page it says it has a GFCI, but then in the detailed section it leaves off any discussion of GFCI. So one of two things is true:

Try the NG plug and a small load. If the output works fine and all is good. Success. You've followed the grounding strategy set forth in the DIY Solar forum tutorial.

If the inverter output immediately trips off, we can assume the inverter output has a GFCI and you can't use an NG plug. In this case, continue to ground the inverter case (to the 12v neg bus bar) but let neutral float relative to the camper battery 12v neg bus bar. Assume the internal GFCI will cover any issues of the hot wire touching a grounded piece of metal. It's not necessarily the best approach but it would be good enough and certainly the way the inverter was designed to operate. I suspect most people operate with their small inverters like this (no bonded neutral). And all too many don't even tie the case ground to the camper's 12v neg bus bar. Arguabley not the best, but it would still work. I wish the industry did a better job with this topic.

Good luck on your hunting trip. I'm sure the electrical will work great. If you're out having fun in a camper you should certainly have easy/safe power.

 

[Glenn]

Thank you! Well I lost half the day working on the inverter ground. I went to attach the ground cable and the bolt that goes through the inverter case just spun …… so it was time to open the case. The aluminum is anodized, not sure if that is conductive or not. Tried tightening Phillips head bolt to no avail.

So, I drilled hole slightly larger. Used my dremel and a polishing brush to take it down to bare aluminum inside and out. Used a larger hex head bolt, and tightened a much larger wire and lug to the case end, and then reassembled. I’m much happier with the rework. Not the first time I’ve had to repair cheap China stuff

There won’t be a next time, I’ll spend more money for a name brand inverter


This was the small ground wire (internal) that was bolted to the case ground bolt

Removed anodized coating on both sides

Inside the case:
Could not get the cheap ground machine screw to tighten, ground always felt loose

I replaced it with a hex head bolt and much larger ground, lock washer, nut, and tightened it before assembling the case back together. The ground wire provided by the manufacturer wasn’t any larger than the yellow/green striped wire on the inside (visible through vent) you can see how much larger the new ground is

You’ve been a big help, thank you Sir

Glenn

 

[Vic Harder]

Not suggesting any improvements or corrections here, this was informative.

FYI, my 3000W pure sine AIMS brand inverter has two GFCI protected outlets. I ran short (heavy duty) extension cords to my outlets in the camper. I did not ground the case. I used 0/2AWG wiring for the inputs.... adding a 4AWG ground to the ground bus, which is itself only grounded to the camper frame with a 10AWG wire would seem silly. Likewise, on the input for my shore power connection, I have a GFCI breaker so all AC inputs are protected.

Am I missing something besides the inverter case ground?

 

[Glenn]

Dadocut and I had discussed the “what ifs” and in the case of a dead short, and a power lead shorted inside the case.

I did find an answer to my question “the anodized coating is non-conductive, which I suspected.

I still don’t know if the inverter has the neutral and ground bonded together, but every reference I found said the AC Ground wire which goes to chassis should be one size smaller than the DC negative input, As my dc to dc charger negative goes from my neg fuse block back to starter battery (non isolated charged) I connected inverter ground there.

in my case I ran 2AWG DC cables to the inverter lugs for powering up the inverter. Probably way more than was needed.

I should be powering it up next weekend. I’m waiting on my Anderson plugs for my battery disconnect… I thought I had a pair but apparently was down to one. I’m running 2/0 from the LiFePo4 with a blue seas 150A terminal fuse……the 2awg wires run from the fuse blocks to the inverter with the positive to a 140A ANL fuse and fron there to the inverter.

I decided to add the ANL 140A fuse so that if a fuse would happen to blow it would be the 140A ANL and not my main 150A fuse on the battery.

I’m running the large wire and fuse to take into account the 2200W surge rating of the 1100W inverter, although I expect most of my loads to be small (800W or less)

Checking the neutral and ground on the AC outlets with a DVM seems to suggest it is not bonded. Though the manual makes all kind of claims. I made a bonding plug. It was suggested to try it with a small load until I know for sure. I did pick up a tester today to plug into the AC outlet that tests for the outlet being wired correctly or for faults.

Even if the ground is much larger than needed it doesn’t hurt a thing, except maybe the pocket book.

Not sure if I answered anything or if I misunderstood your question.

On the DC Neutral, I have only the one 2/0 from the LiFePo4 to the “Batt. Only” side of my shunt. On the “Load Side” of the shunt I have one 2AWG going to my 300A Neg fuse block to which all other negatives/grounds are connected (Smart Charger, Inverter, Fuse distribution panel, and of course the 6AWG going back to the starter battery(s).

The Pos. 6AWG for my dc to dc charger is both protected at the starter battery with a 60A breaker and a 60A breaker on the charger output before going to the 300A pos. fuse block.

I hope to have some pictures posted soon.

 

[Glenn]

Also, I’m setting up an isolated 15A AC for when I’m at a State Park using one of their panels.
Not Shore Power…. Just power plugged in, routed through the rear truck fender. with its own dedicated outlet(s) 15A duplex receptacle with USB charger that won’t be live when extension cord isn’t plugged in and no way for the systems to cross.

Below:
Marinco Plug mounted flush with bottom of bumper. The yellow 12-3 extension cord is pure copper strand. I looked at Harbor Freight cords (much cheaper) but with no specs. I figured it was copper clad aluminum.
(Not shown in the picture but I left enough slack to pull down and make a drip leg- I also have a piece of marine shrink tubing slid up the cord to bring down over the cord and weather proof bonnet on the Marinco plug.

Below: the insulation is very thick on the heavy duty cord.

Below:
Grommets in all sheet metal passages due to AC voltage,
I could not fit my big meat hooks down inside the fender,,,,barely got the grommet installed

Below:
Picture from Behind tail light housing (removed) : Drilled a hole for the grommet and four holes for the 8-32 stainless inserts

Below:
Put a grommet in the bottom of the steel box also and sandwiched the two grommets together forming a cut proof passage.

The outlet inside the corner of truck bed will have a weatherproof cover for a single gang outlet. From there to a GFCI outlet on camping box and to the duplex outlet with the built in USB ports.

Technically i could run the parks AC power and my Lithium house battery loads and they would be completely seperate, though I shouldn’t ever need to.
The AC power inside the topper would be for resting / sleeping / CPAP, and other extension cords from the parks panel would be strung out to the camping area (light, fillet knife, radio, refridge/freezer) electric skillet.

 

[Vic Harder]

looks like good work.... precautions are always a good idea.