Another inverter/solar grounding question for the experts

[BigFly10]

Hello all,

I have just recently installed and wired in my new solar system to my TT which includes the following:
- (4) Deep Cycle / Marine 12V batteries at 120Ah each
- Xantrex Prowatt SW2000 inverter
- GoPower 30A Automatic Transfer Switch
- Sub-panel for breakers to be powered from inverter
- 250W solar panel (plan on adding another 250W panel)
- Tristar MPPT-45 Charge Controller
- Trimetric 2025-RV battery monitor.

I have given it all a fairly thorough testing and it seems to be working pretty well except for one problem. When I run the microwave off the inverter, it "sometimes" trips the GFCI on the Xantrex inverter. When I say sometimes, it seems to be about 50% of the time. I can reset the GFCI and I am away to the races again, but this could be a real pain after awhile.

This got me to thinking about the grounding. I have a decent electrical understanding but not a whole lot of practical experience and am not familiar with the standard wiring practices in an RV. Thinking about grounding gives me a headache. I have done some searching and reading but am still not clear on what is required.

The trailer is basically brand new. Straight from the factory/dealer, the battery had "several" wires connected to the battery - most were from the positive terminal and only one #8 from the battery negative. There is no obvious place I can find where this negative wire is frame grounded I can only "assume" that it runs back to the converter ground terminal strip and that terminal strip is connected to the trailer frame. All these wires "disapper" into the underside of the trailer and it would be very difficult for me to trace exactly where they are going. Rather than changing them, I just left them all connected as they were when wiring in my new components.

When I connected my components, I ran the positive/negative from the battery to the inverter. The inverter has a separate grounding terminal. Then inverter instructions say to connect a cable from the inverter ground terminal to the trailer frame. I did not follow this exactly. From the inverter ground terminal, I joined this ground cable with the ground cable from my Tristar charge controller and from my solar panel, and then ran a bigger ground cable back to my converter which I connected to the grounding strip in the converter panel.

On the A/C side, my inverter comes with two GFCI outlets. I built a cable with a 3-prong plug that I have plugged into the inverter outlet and then ran that cable to my ATS. The other cable to the ATS comes from a 30-amp breaker in the main breaker panel. At the main breaker panel, the cable going to the ATS has positive terminated on the breaker outlet, neutral on the neutral bar, and ground on the ground bar.

The 3 cables at the ATS (2 in and 1 out) are connected to the positive and neutral connections provided in the ATS with the ground cables all connected to the ground terminal strip inside the ATS.

The outlet from the ATS is wired to a small sub-panel with 3 breakers which power my RV outlets and my microwave. The sub-panel has a separate ground and neutral strip which I terminated all of the cables to. This includes the main supply cable from the ATS and the 3 cables supplying the loads in my RV.

So the way I see it, I basically have all of my grounds connected together including my AC and DC grounds. I am assuming they are also connected to the trailer frame somewhere although I am not sure where. I am also wondering if maybe the neutral and ground are bonded together at the inverter although I am not sure how that is wired internally and maybe this is the problem. If the neutral and ground are bonded together at the inverter, could some of the current be returning via the ground instead of the neutral and causing the GFI to sense a fault?

I think I have everything wired okay but, like I said, I am not familiar with RV wiring standards and am wondering if something I have done is causing my GFI problems. Should I be running my inverter ground directly to the trailer frame as per the instructions? Should my AC and DC grounds be kept separate from one another? Is there something else I should be doing?

Thanks for the help everyone.

[shooted]

BigFly10 wrote:


- I opened the A/C breaker for the converter and plugged the trailer shore power cord directly into the inverter. Interestingly, this works great. I can run the microwave over and over again without tripping the inverter GFCI.

In this test running whole house, the only change was exclusion of the feed from the inverter to the ATS, and subsequently excluding ATS operation also.
The main panel and sub panel operated correctly in this test, and this suggest to me that they are not suspect.

[3_tons]

Upon re-read,

"The other cable to the ATS comes from a 30-amp breaker in the main breaker panel. At the main breaker panel, the cable going to the ATS has positive terminated on the breaker outlet, neutral on the neutral bar, and ground on the ground bar."


Since your ATS switch receives it's A/C input from the main panel, your problem may be one of competing 'circular currents' causing the sensitive inverter GFI outlet to trip. Try powering the ATS from ahead of the main panel (where shore power first enters coach), not after (then divide entrance (in parallel) to main panel & ATS), and you should see a difference, problem corrected...

Best Regards,

3 tons

[BigFly10]

john&bet wrote:
One point that none have touched on is that you may be overloading your gfi. Is it a 20amp gfi or a 15 amp? There was a time that the school of thought was not to plug a microwave into a gfi even in your home. Not sure if that still holds true. I wonder why your inverter even has GFI output on it for your application?

It is a 20 amp GFI so I don't think it is overloading the GFI. No idea why the inverter would have come with a GFI on its output (I sure wish it didn't) - the instructions clearly state that it is to be mounted in a dry area indoors.

[BigFly10]

westend wrote:
I'm betting that your ground wire for the DC system is not up to the task. Using an 8 AWG ground,with 4 batteries and an inverter trying to pull +1500 watts, the inverter is seeing a low voltage condition. Your use of an extension cord may be just enough to allow some capacitance to overcome the inverter's bad feelings about the situation.
The frame ground from the inverter is just as MrWizard has stated, a protection and interference device, not part of the power profile.

The frame ground and the DC negatives are separated in my system. I apologize if I made it sound as though they are connected together - they are not. I have the frame ground for a few pieces of equipment (solar panel, inverter, charge controller) connected together with #8 cable, but they are totally separate from the power system.

The DC negative is actually a 2/0 cable, not a #8 cable. This is not a low voltage situation. The trimetric shows that the 4 batteries can easily maintain voltage above 12.0V with the microwave running and the inverter does not show a fault when the trip occurs (it would on a low voltage).

[john_bet]

One point that none have touched on is that you may be overloading your gfi. Is it a 20amp gfi or a 15 amp? There was a time that the school of thought was not to plug a microwave into a gfi even in your home. Not sure if that still holds true. I wonder why your inverter even has GFI output on it for your application?

[westend]

I'm betting that your ground wire for the DC system is not up to the task. Using an 8 AWG ground,with 4 batteries and an inverter trying to pull +1500 watts, the inverter is seeing a low voltage condition. Your use of an extension cord may be just enough to allow some capacitance to overcome the inverter's bad feelings about the situation.
The frame ground from the inverter is just as MrWizard has stated, a protection and interference device, not part of the power profile.

[Canadian_Rainbi]

VintageRacer wrote:
Make sure your sub panel does not have neutral bonded to ground. It must separate neutral and ground, the transfer switch must switch both neutral and hot to the inverter, which must bond neutral and ground. Neutral is always bonded to ground at one point only, and that is the source of the power. Remember that GFCI - ground fault circuit interrupter - is a bit of a misnomer. The device does nothing with ground at all and can be installed in a circuit that does not have ground (such is code-allowed for protecting old 2 conductor wiring in old houses). It does trip if there is any difference between the current flowing in the hot conductor and the current flowing in the neutral conductor. Having neutral bonded in two places can create such a difference.

Brian

Actually, Ground Fault is correct terminology. It means that there is a fault to TO ground, ie a fault between one of the current carrying wires and ground, allowing some current to flow directly to ground rather than through the neutral. This ground does not necessarily mean a dedicated ground wire, but some path through something, ie a body, to ground. This creates an imbalance between the current in the hot and the neutral which causes the breaker to trip before the heart does. :E

[BigFly10]

shooted wrote:
Can you provide a wiring diagram of your installation? It appears to be a problem isolated to the sub panel or ATS.

Edit, on second thought, the problem is probably at the ATS, or between the ATS and inverter. I would verify wiring at the ATS paying special attention to proper/correct connection from the inverter. I suspect a wire assembly defect between ATS and inverter.

Agreed, my thoughts too were that the problem must be at the ATS. However, after more testing tonight now I am really stumped.

- Ran an extension cord from the inverter directly to the microwave and plugged the microwave into the extension cord. Everything worked perfect, no GFI trips.

- Ran an extension cord from the microwave to a different outlet in the trailer powered from inverter. It worked perfect again, no GFI trips.

- Now, here is the real mystery. Ran an extension cord from the SAME outlet the microwave was plugged into and then plugged the microwave into the extension cord. Effectively, all I have done is make the microwave power cord longer. CANNOT trip the GFI. I must have tried this 30-40 times because I couldn't believe it. However, plug the microwave into the same outlet without the extension cord and it will trip 1 out of 5.

Any thoughts? I am thinking I may try wiring in a different outlet or just extending the cable to the outlet and see if that fixes it, but I really don't understand what is going on.

[shooted]

Can you provide a wiring diagram of your installation? It appears to be a problem isolated to the sub panel or ATS.

Edit, on second thought, the problem is probably at the ATS, or between the ATS and inverter. I would verify wiring at the ATS paying special attention to proper/correct connection from the inverter. I suspect a wire assembly defect between ATS and inverter.

[MrWizard]

the inverter 'grd' is a safety and interference grounding

Ground/connect Directly to TT frame, it is Not to be connected to ANY negative 'power' carrying wire

[BigFly10]

Just thought I would post an update on this problem that I still have not resolved......

- My OP says that the microwave trips the inverter GFCI about 50% of the time. This is not true - it is actually more like about 20% of the time, 80% of the time it works fine.

- I tried disconnecting the inverter ground cable AND connecting the inverter ground cable directly to the trailer frame. No change - still trip the GFCI about 20% of the time.

- I opened the A/C breaker for the converter and plugged the trailer shore power cord directly into the inverter. Interestingly, this works great. I can run the microwave over and over again without tripping the inverter GFCI.

- I started to think that maybe it was the male plug that I built myself that plugs into the inverter for the cable that runs to the transfer switch, so I removed it and tried a different plug but that did not help.

- I have a 120V fridge in the back of my TT that plugs into an outlet. I turned this fridge on (pulls about 6-7 amps from my batteries) and tried the microwave with the fridge running. The GFCI would not trip. It would seem that with another A/C load on the inverter in addition to the microwave, it somehow helps the issue.

It may also be noteworthy to mention that IF the GFCI is going to trip, it will do so IMMEDIATELY when I hit the start button on the microwave. As long as the microwave starts, it will run to completion without tripping.

I am going to check the wiring inside my transfer switch to see if there could be some problem in there. I am also going to try plugging my microwave into a different outlet to see if that has any affect. However, if neither of these straws give me any new information, then I have to say that I am completely baffled........

[BFL13]

The "main grounding point at the converter" is for both 12v and 120v ground to frame. This can make for some strange things.

I have an inverter with non-GFCI receptacles and I use the shore power cord in that for "whole house." (converter off) This works great, but now the neg wire battery disconnect I have has no effect, because the shore power cord neg wire is making a 12v neg path from battery to DC dist panel.

Another inverter I have does not do that, with its "floating neg" GFCI receptacles.

Of course if the battery disconnect were on the pos wire, like many are, that would not be an issue.

On a deck-mount converter, which has a chassis ground lug same as an inverter does, when you look inside, you see the 120v power cord's neg wire goes to the converter's chassis (not connected to the DC output) The chassis ground then takes that to the frame. An inverter is arranged differently inside, but the point is that funny things are possible.

I would still think an intermittent problem would indicate it is not a standing problem with the set-up, but a loose connection.

[BigFly10]

I have the chassis ground running to a main grounding point at my converter, so I am not connected to inverter negative input but I am also not connecting straight to the frame as suggested in the manual.

I am starting to think this might have something to do with the problem. I am going to try going straight to the frame and see if it makes any difference.

BFL13 wrote:
My inverter's instructions make a big point to NEVER run the chassis ground lug wire over to the inverter's own neg input terminal (which goes back to the neg grounded battery) but run it straight to the frame.

Even though there is a path through the frame back through the battery (also frame grounded) the direct inverter frame ground seems to be ok under the rules.

I got lost in the OP 🙂 but there may be a variation on this no-no there.

The purpose of the inverter's chassis to frame wire grounding is to reduce radio and TV interference. The inverter will run just fine without the chassis ground being connected.

An intermittent GFCI issue may be due to a loose connection somewhere?

[BFL13]

My inverter's instructions make a big point to NEVER run the chassis ground lug wire over to the inverter's own neg input terminal (which goes back to the neg grounded battery) but run it straight to the frame.

Even though there is a path through the frame back through the battery (also frame grounded) the direct inverter frame ground seems to be ok under the rules.

I got lost in the OP 🙂 but there may be a variation on this no-no there.

The purpose of the inverter's chassis to frame wire grounding is to reduce radio and TV interference. The inverter will run just fine without the chassis ground being connected.

An intermittent GFCI issue may be due to a loose connection somewhere?

[Rbertalotto]

I believe he is calling Ground from the inverter which is 120V.

I would ground the inverter to the chassis as the instructions call. I would bet dollars to donuts you have two different ground potentials that the GFI is confused by.