N/G Bond Scenario making me dizzy

JAS

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Hey all,

So I have a scenario where I want to connect my MPP LV1012-MS to my 10 Circuit Generator Transfer Switch. This transfer switch does NOT switch Neutral. So it is designed for a generator that does NOT have a N/G bond.

I've determined (another thread and via Ian at Watts247) that my MPP unit does NOT bond N/G when Inverting from battery. That means no problem connecting it to my transfer switch (when running only on battery).

However, the thing that makes me o_O is when I connect the AC Input to the MPP from my main panel.... So, obviously, my main panel has N/G bonded. The MPP will pass through the N/G bond when passing through the AC input to the loads. So now, in this scenario, I feel like I have (2) N/G bonds :unsure: (However, both N/G bonds are in reality the same bonded location (Main Panel))

I ran this by my electrician and he felt it wasn't a problem since it wasn't "actually" two separate bonds. As a test, (to simulate the MPP in AC pass-through mode) I took an extension cord (with an adapter) and plugged right from a GFCI receptacle coming out of my main panel to the input on my transfer switch. Even with all the circuits set to Line mode on the transfer switch, the GFCI immediately tripped when plugging in. :oops:. I assume this is because the GFCI is seeing a downstream N/G bond?

So question is... Obviously, If I wire up the AC input to the MPP from the main panel, I can't go through a GFCI (trips immediately). But, if I wire direct to a breaker in the main panel, will I truly be creating a ground loop? IE: Is it unsafe?

If this won't work with my existing transfer switch, my only other options are:
  1. Pull circuits out to a "critical load" panel and tie the neutrals together on my MPP unit (instructed by Ian to force the N/G bond on battery)
  2. Pull circuits out to a transfer switch that does transfer Neutral (still have to force the N/G bond on the MPP unit)
Thanks for anyone's insight...

Jeff
 

RCinFLA

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MPP LV1012-MS is a 120vac inverter.

Is your generator 10 circuit switch rigged for 120vac only generator input? Many generator switches use DPDT switches so they can be fed from a 240/120vac generator and switch 240vac loads. Their 120 vac outputs are split between the two phases from a 240vac/120vac generator to balance 120vac loads on generator split phase output.

This applies if generator switch is only providing 120vac outputs as different split phase halves of generator may be used for different 120vac outputs.

Most HF inverters internally bond their neutral to ground if AC input is not detected. When AC input is presented, they release neutral to ground internal connection and allow neutral to float, giving neutral to ground bonding responsibility to AC input source (main AC panel in most cases)

If you are using a 120vac only inverter-generator it can be a problem as their neutral side may not be bondable to ground. Check generator manual to see if it says something like 'this generator is not intended to be connected to house wiring'.

Your GFI problem is likely you do not have a ground-bonded neutral source. This can be fixed by externally neutral-ground bonding the inverter AC input. This assumes your 120vac generator input is neutral-ground bondable.
 
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400bird

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May 23, 2020
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750
Hey all,

So I have a scenario where I want to connect my MPP LV1012-MS to my 10 Circuit Generator Transfer Switch. This transfer switch does NOT switch Neutral. So it is designed for a generator that does NOT have a N/G bond.

I've determined (another thread and via Ian at Watts247) that my MPP unit does NOT bond N/G when Inverting from battery. That means no problem connecting it to my transfer switch (when running only on battery).

However, the thing that makes me o_O is when I connect the AC Input to the MPP from my main panel.... So, obviously, my main panel has N/G bonded. The MPP will pass through the N/G bond when passing through the AC input to the loads. So now, in this scenario, I feel like I have (2) N/G bonds :unsure: (However, both N/G bonds are in reality the same bonded location (Main Panel))
I feel like there may be some miss understanding of N/G bond. Bond is a connection, that simple. So, "Neutral/Ground bond" is just the connection between neutral and ground. Your inverter doesn't "pass through the connection" it just passes through the neutral and ground.

I ran this by my electrician and he felt it wasn't a problem since it wasn't "actually" two separate bonds. As a test, (to simulate the MPP in AC pass-through mode) I took an extension cord (with an adapter) and plugged right from a GFCI receptacle coming out of my main panel to the input on my transfer switch. Even with all the circuits set to Line mode on the transfer switch, the GFCI immediately tripped when plugging in. :oops:. I assume this is because the GFCI is seeing a downstream N/G bond?

So question is... Obviously, If I wire up the AC input to the MPP from the main panel, I can't go through a GFCI (trips immediately). But, if I wire direct to a breaker in the main panel, will I truly be creating a ground loop? IE: Is it unsafe?
I'd double check the theory that the inverter doesn't have a bonding relay that activates when there is no AC in power.
 

JAS

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I'd double check the theory that the inverter doesn't have a bonding relay that activates when there is no AC in power.
This has already been confirmed by multiple sources on this forum. (It seems it is hit or miss with MPP models as to which ones bond N/G). I also confirmed using multimeter...
 

400bird

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Yes, you're correct. I understand that.
Ok, you're earlier statement:
I feel like I have (2) N/G bonds
Said otherwise in my reading.

So question is... Obviously, If I wire up the AC input to the MPP from the main panel, I can't go through a GFCI (trips immediately). But, if I wire direct to a breaker in the main panel, will I truly be creating a ground loop? IE: Is it unsafe?
If you're tripping the GFCI breaker, it is because you have current on the ground. So, yes it is "unsafe"

Can you provide a diagram of how you have it wired up?
 

JAS

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MPP LV1012-MS is a 120vac inverter.

Is your generator 10 circuit switch rigged for 120vac only generator input? Many generator switches use DPDT switches so they can be fed from a 240/120vac generator and switch 240vac loads. Their 120 vac outputs are split between the two phases from a 240vac/120vac generator to balance 120vac loads on generator split phase output.

This applies if generator switch is only providing 120vac outputs as different split phase halves of generator may be used for different 120vac outputs.

Most HF inverters internally bond their neutral to ground if AC input is not detected. When AC input is presented, they release neutral to ground internal connection and allow neutral to float, giving neutral to ground bonding responsibility to AC input source (main AC panel in most cases)

If you are using a 120vac only inverter-generator it can be a problem as their neutral side may not be bondable to ground. Check generator manual to see if it says something like 'this generator is not intended to be connected to house wiring'.

Your GFI problem is likely you do not have a ground-bonded neutral source. This can be fixed by externally neutral-ground bonding the inverter AC input. This assumes your 120vac generator input is neutral-ground bondable.
Ugh... I think we're even more confused now ;)

Yes, my transfer switch can operate either 240 or 120. My electrician set it up as 120v on both sides (same phase). Not sure what difference that makes in regards to the N/G bond?

As stated, it has been confirmed that my MPP model does NOT bond N/G. At any rate, the transfer switch does NOT switch Neutral, which means it can NOT accept an input that has a N/G bond (actually should NOT would be more appropriate). For example, if attempting to run a Honda Generator with GFCI (and N/G bond internally), the GFCI on the generator will trip. Therefore, the N/G bond has to be removed on the generator.

Your GFI problem is likely you do not have a ground-bonded neutral source
Not following here... My GFCI receptacle that is hanging off my main panel would have a ground-bonded neutral source via the main panel, correct?

So back to my initial problem....

What happens when my MPP is "passing" through AC from my main panel (and thus the N/G bond) into my transfer switch (which shouldn't have an input with a N/G bond)?
 

JAS

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If you're tripping the GFCI breaker, it is because you have current on the ground. So, yes it is "unsafe"
That's what is weird... I hadn't even flipped any of the breakers on the transfer switch to the "GEN" position yet. So no current flowing anywhere? It just tripped as soon as I plugged the cord in.

Can you provide a diagram of how you have it wired up?
Yes, let me work on that. That may help explain things better.
 

RCinFLA

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So is your 120vac generator have a groundable neutral
So no current flowing anywhere? It just tripped as soon as I plugged the cord in.
If there is no current flowing anywhere the GFI would not be tripping.

You did not say what type of 120vac generator you have. A 120vac only inverter-generator has a balanced H-bridge output. Some models do not allow their neutral to be bonded to ground, If you take an AC voltmeter you will read about 60vac from N-G and about 60vac from L-G on sockets,. Ground on sockets connect to any metal surfaces in generator.
 

400bird

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If there is no current flowing anywhere the GFI would not be tripping.
I guess I missed that statement from the OP. I agree 100%

I'd note that adding a second connection between neutral and ground will allow current to flow on the ground and trip the GFCI (edit: even when there is no current on the related hot/live wires)

Edit to clarify: current flowing on the ground means it's not flowing on the neutral and trips the GFCI. As it was pointed out by Zil, GFCI compares current on line and neutral.
 
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JAS

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So is your 120vac generator have a groundable neutral
I'm not even using a generator for this anymore. (Since rewired the main panel with a interlock and run the entire panel now)
But, the generator I used in the past with the transfer switch was a Champion 2000W Inverter Generator.

That being said... I don't want to get too wrapped up on generators as I'm more concerned about the Inverter :)

If there is no current flowing anywhere the GFI would not be tripping.
I guess technically there is still current coming from the main panel to the GFCI receptacle (IE: the receptacle was still live). But, nothing down stream (on the transfer switch) was pulling any current.

I'd note that adding a second connection between neutral and ground will allow current to flow on the ground and trip the GFCI (edit: even when there is no current on the related hot/live wires)
I bet this is what the GFCI is seeing when plugging into the transfer switch. (IE: It's seeing the N/G bond in the main panel from the shared neutral in the transfer switch. This is what I was getting at when I said it was like there are (2) N/G bonds (even if it is physically only in one place)

So in summary, it sounds like it would not be safe to hardwire the AC Input of the MPP directly into my main panel. (At least not with this existing transfer switch)
 

Zil

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GFCI measure the difference in milliamps between the neutral and hot legs of an AC circuit. The earth ground is of no consequence to the GFCI.
 

JAS

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GFCI measure the difference in milliamps between the neutral and hot legs of an AC circuit. The earth ground is of no consequence to the GFCI.
OK, I believe the earth ground is of no consequence. But, the N/G bond could be, correct?
 

400bird

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GFCI measure the difference in milliamps between the neutral and hot legs of an AC circuit. The earth ground is of no consequence to the GFCI.
Doh! Good point, I'll go back and edit my original post to remove incorrect information.

A second neutral/ground bond still trips the GFCI, even when there's no load in the hot/line.

To the OP, it's probably worth double checking everything to ensure there isn't an accidental swap or connection on the neutral and ground connection .
 

WYtreasure

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Try the same checks with a new GFCI. You may ruin the new GFCI, get the same result, or learn that the old GFCI has you chasing your tail.
 

JAS

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Try the same checks with a new GFCI. You may ruin the new GFCI, get the same result, or learn that the old GFCI has you chasing your tail.
I'm not following you? I didn't say anything about an old or new GFCI???
 
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