MPP LVX6048 with 2 pole manual transfer switch

BrianH

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Mar 28, 2022
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Looks like the best answer to my question is to retrofit a transfer switch that switches the neutral as well as both lines but there's still a couple of things I'm not sure I've got my head wrapped around and maybe a couple of options I haven't thought of.
I'm planning to use an MPP LVX6048 with batteries and PV primarily for back up power. The plan was to feed the house sub panel through the existing generator transfer switch. In my application, that switch is placed immediately after the main panel which contains a 200A breaker and the neutral-ground bond for the house. The xfer switch operates on only L1 and L2, neutral passes through and the bare ground bonds to the switch case before going back to the main panel.
FWIW the generator is just a 5500W portable - suitable for a day if the power is out but not much more than that.

As backup power through a manual switch, I had not intended to connect the AC mains to the inverter at all. Operating off of DC inputs only, the inverter will bond N-G and as the transfer switch does not disconnect the neutral, I've got a loop. I had also not intended to have a separate critical loads panel. With the generator, I just kill the breakers in the sub panel for the loads I'm not powering with the generator prior to connecting with the transfer switch.

Option 1 is to get a new transfer switch which switches neutral as well as the lines. This seems to be the correct solution but likely the most expensive.

Option 2 is to disable the N-G bond in the inverter. Easier and cheaper, this seems like it would work for a back up power scenario but I'm not sure if this causes a real safety issue as:
1, The neutral would still be connected to the utility even with the lines disconnected. A danger external to the house???
2, I'm uncertain whether or not there's an issue with a breaker being able to clear a ground fault that occurred in the inverter. With no AC input to the inverter (and the internal N-G bond disabled) I'd have the N and Grnd from the inverter both connected to the point where the N-G bond in the main panel occurs. Wouldn't that provide a sufficient pathway back to the source?

Option 3, abandon the back up power only idea, run AC to the inverter and add a separate sub-panel that the inverter is always powering some of the loads in the house. This seems like the most common way of using these things but in going with this option, It dawns on me that if the utility power goes out, I'm kind of right back to square one.

I'm going to get a price on changing out my transfer switch. Thoughts on options 2 and 3?
 

BuffaloTrace

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Dec 3, 2020
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This has been an on going topic of concern for many. I am in a similar situation and have considered many options. That being said, there are nice inverters out there setup to feed a main panel with NG bond. I hesitate to say there are cheaper and easier solutions, but in the instance of most every MPP solar inverter it seems they are designed to be used to feed a sub panel with separate neutral and ground bus bars.

There are other threads that have been discussed on this in great detail with a lot of various wiring diagrams and suggestions.
Unfortunately the MPP solar units are just not designed to simply backfeed a main panel using a 2 pole transfer switch or even a simple lockout breaker in conjunction with the main panel disconnect as many household backup generators are setup for.
 

Zwy

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Start here about the LV6048. https://diysolarforum.com/threads/lv6048-output-neutral-ground-bonding.33719/post-442290

If you use something like the Reliance 310C which does not switch N, then you will have the same problem.

You have 2 choices, a 3 pole transfer switch where N is switched or to install a subpanel/critical loads panel; wire it according to the diagrams shown by Filter Guy depending on which choice you make. https://diysolarforum.com/threads/lv6048-output-neutral-ground-bonding.33719/post-442499

The discussion is quite lengthy, we discussed this in detail both in that thread and others.
 

BrianH

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I read through that whole thread yesterday. That one is why I concluded that a transfer switch that switches neutral is likely the best solution.
In the second linked post though, FilterGuy confirms that using the 2 pole transfer switch and "screw removal method" of defeating the internal N-G bond would remain safe so long as the inverter does not have an AC input.

Sort of separate issue but I guess I'm just having a hard time wrapping my head around why there is internal N-G bonding in the LVX6048 at all. It's a dedicated split phase unit that seems to aimed squarely at the American market. If we bond N-G at the main panel as a mater of code, then the inverters internal N-G bond is not only unneeded but unwanted. I can see some convenience in the feature if I were building an entirely off-grid system but otherwise, not so much...
 

BuffaloTrace

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My only suggestion, if the screw removal method is something one must do as in your situation, is to take great care to label, document, warn, etc. that this modification was made to the unit so anyone would know what is going on with the setup if something were to go wrong in the future and you were not around. I believe that would work and considered doing something similar as well.
 

Zwy

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I read through that whole thread yesterday. That one is why I concluded that a transfer switch that switches neutral is likely the best solution.
In the second linked post though, FilterGuy confirms that using the 2 pole transfer switch and "screw removal method" of defeating the internal N-G bond would remain safe so long as the inverter does not have an AC input.

Sort of separate issue but I guess I'm just having a hard time wrapping my head around why there is internal N-G bonding in the LVX6048 at all. It's a dedicated split phase unit that seems to aimed squarely at the American market. If we bond N-G at the main panel as a mater of code, then the inverters internal N-G bond is not only unneeded but unwanted. I can see some convenience in the feature if I were building an entirely off-grid system but otherwise, not so much...
Two things you need to remember, all current returns to the source and will take any path it can find. Second, the reason why N-G is bonded at the inverter under inverter power is the same reason N-G are bonded at a main service panel and earthed, to create a zero voltage reference point for ground fault detection.
 

BrianH

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Mar 28, 2022
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Thanks, that's helpful - I need to read up on ground fault detection. I can see how I could wind up with a voltage potential on the ground conductor.
 

kanelr

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Mar 1, 2022
Messages
163
Looks like the best answer to my question is to retrofit a transfer switch that switches the neutral as well as both lines but there's still a couple of things I'm not sure I've got my head wrapped around and maybe a couple of options I haven't thought of.
I'm planning to use an MPP LVX6048 with batteries and PV primarily for back up power. The plan was to feed the house sub panel through the existing generator transfer switch. In my application, that switch is placed immediately after the main panel which contains a 200A breaker and the neutral-ground bond for the house. The xfer switch operates on only L1 and L2, neutral passes through and the bare ground bonds to the switch case before going back to the main panel.
FWIW the generator is just a 5500W portable - suitable for a day if the power is out but not much more than that.

As backup power through a manual switch, I had not intended to connect the AC mains to the inverter at all. Operating off of DC inputs only, the inverter will bond N-G and as the transfer switch does not disconnect the neutral, I've got a loop. I had also not intended to have a separate critical loads panel. With the generator, I just kill the breakers in the sub panel for the loads I'm not powering with the generator prior to connecting with the transfer switch.

Option 1 is to get a new transfer switch which switches neutral as well as the lines. This seems to be the correct solution but likely the most expensive.

Option 2 is to disable the N-G bond in the inverter. Easier and cheaper, this seems like it would work for a back up power scenario but I'm not sure if this causes a real safety issue as:
1, The neutral would still be connected to the utility even with the lines disconnected. A danger external to the house???
2, I'm uncertain whether or not there's an issue with a breaker being able to clear a ground fault that occurred in the inverter. With no AC input to the inverter (and the internal N-G bond disabled) I'd have the N and Grnd from the inverter both connected to the point where the N-G bond in the main panel occurs. Wouldn't that provide a sufficient pathway back to the source?

Option 3, abandon the back up power only idea, run AC to the inverter and add a separate sub-panel that the inverter is always powering some of the loads in the house. This seems like the most common way of using these things but in going with this option, It dawns on me that if the utility power goes out, I'm kind of right back to square one.

I'm going to get a price on changing out my transfer switch. Thoughts on options 2 and 3?
Could you add a “global” neutral lift switch?
 

BrianH

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Joined
Mar 28, 2022
Messages
8
Yes, that's an option but then whomever is operating the unit needs to know to do one more thing...
 
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