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Edit--this appears to be a duplicate of this question, which remains unanswered.

Summary: I'm trying to find a way to feed a 50A load panel from two different sources, each with an existing neutral/ground bond. I'm looking for a cheap, safe, code-compliant way to switch the hot legs and neutral on the feeder to my load panels (un-bonding the generator is undesirable). The only off-the-shelf solution I can find is prohibitively expensive, so I'm wondering if my alternative ideas are feasible.

Background: I have a hybrid solar inverter that I intend to use as the main power source for the home, fed by my main service panel. Complicating matters, I have a neutral-bonded diesel generator (Separately Derived System) I'd like to use as a backup power source to the input of the inverter (per drawing), but without switching the neutral at my feeder to the load, it would add a second neutral bond, which is not permitted by code.

Although there's no permitting process to follow in my location, I wish to abide by the NEC.

I've read:

OK to start a house circuit wire with a plug, to plug into circuit panel outlet or generator breakout box?

To use any generator with the house inlet, may one leave the generator ground wire unconnected in the inlet?

Can a circuit connecting to an outlet be branched as a subpanel?

https://forums.mikeholt.com/threads/nec-400-8.9203/

To use any generator with the house inlet, may one leave the generator ground wire unconnected in the inlet?

Generator - bonded neutral connecting to house

a terrible diagram (apologies for the quality of the sketch)

Since my main service panel has a bonded neutral, it seems the only "right" way to connect the generator would be through a transfer switch that also switches neutral: a 3-pole double throw safety switch seems ideal--aside from the $639 price tag! (I had also encountered and considered a Reliance X Series transfer switch kit, but stock levels are an issue, it's more complicated than what I need, and the price tag is still high, compared to my proposed solution)

I'm looking for another safe, code-compliant (and hopefully, affordable) way to switch neutral on the feeder to my load panels. I'd rather not un-bond the mobile generator if it can be avoided, in order to retain its standalone capabilities.

I considered using a pair of 60A 3-pole safety switches (@$60 each), but would lack a way to mechanically interlock them, so that one is always in the OFF position.

Then I had an idea: the power input to the inverter is switched by unplugging the grid source, and plugging in the generator. Maybe I could do something similar with the feeder of the load panel: wire a 50A receptacle from the inverter output, and a grid-sourced 50A receptacle next to it in case I need to bypass the inverters (again, per drawing). Then, a matching plug would feed the load panel. *This would be a cheap, safe, and durable solution, but I'd like to know if it could be implemented in a code-compliant way.

If that's not possible, I've considered using a 3P contactor ATS in a DIN rail box, which would still be <$100, but I think I'd prefer the simplicity, durability, and UL listing of a plug and pair of 50A receptacles. An RV transfer switch appears to be a similar alternative solution, and has ETL certification. But can it be used in residential house wiring within the constraints of NEC?

edit/update -- Although I no longer intend to try to switch neutral for a Separately Derived System generator, I'd still really like to know if the 50A plug/socket or RV transfer switch would be disallowed by NEC.

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  • I'm confused about a couple of things: 1 - Do you actually have utility power or not? Diagram shows "Grid 200A"but text talks about main power from hybrid solar inverter. 2 - Why can't you remove the bond in the generator? Oct 30, 2022 at 22:42
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    @manassehkatz-Moving2Codidact Thanks for asking for clarification. 1-Yes, I have grid power, and the ground bond with neutral is in the meter. 2-the generator is a mobile unit, and I’d like to maintain its standalone capability without having to reverse the unbonding every time it’s used in that capacity. Oct 31, 2022 at 1:12
  • What make and model is your inverter? Oct 31, 2022 at 1:41
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    It's a Sunny Island 6048-US-10. Two of them, actually, in split phase configuration. It only has one AC input that is presently connected to the genset, but I'd like to connect grid power, and fall back on the genset during outages. Oct 31, 2022 at 1:42
  • No matter what the power source (grid, generator, solar), you're only supplying a grand total of 50A to your house? That seems awfully small for any sort of a modern house, unless you have gas heat, hot water and cooking, with no air conditioning.
    – FreeMan
    Oct 31, 2022 at 12:08

3 Answers 3

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Anytime you focus on doing things in a complex or unusual way, you're going to pay for it. Martin of Wintergatan had this problem with his Marble Machine X, and talked about it here. Listen to what Elon Musk says around 1:30.

So. Apply the lessons of that video here. Here's what I see.

Number 1. Get rid of the bypass.

The line along the bottom of your drawing, the direct bypass from the main panel to the sub. Doesn't need to be there. It creates a redundant neutral routing (meaning you would need neutral isolation both places) and one combination would result in complete failure of neutral-ground bonding.

"What if I feel a need to bypass the Sunny Island inverters?" Then plan for that contingency in your cable routing and make sure to have 3 Polaris connectors sitting exactly where they need to be waiting. It will take you 5 minutes.

You say "the family needs to be able to do it" but first, is this likely to arise while you're away, and second are they likely to be able to work with the dual transfer switch system you propose? Or even the socket method? It does not seem realistic - it's already pretty complex for a non-you or me.

Also, you have a lot of neutral paralleling going on there - once through the inverters and again through the bypass. That switch on the right would need to be 3-pole and here we are again. I suppose you could do it by giving the inverter a plug and socket, with cords arranged so the inverter could be bypassed. That might fly under NEC 400.8 "frequent removal" and - well, I've seen that done in a video game so it might be accessible.

Again if you have very high demands you have to expect very high complexity/price. For a product to be affordable, it has to be in the mass market, and that means adapted by masses. Your desired setup is too bespoke for that.

Number 2. Generator isolation:

First, do like Elon Musk says and delete the requirement. You are perfectly capable of applying and removing the neutral-ground bond at the generator everytime you swap applications.

But even so, think about it. What kind of generators would ever need neutral-ground bonds? Portable generators. What do we know about portable generators? They're small.

So that means a bog-standard 240/480V -> 120/240V isolating transformer won't be terribly costly. Start trolling Craigslist and other marketplaces looking for a used one, they occasionally pop up in the $100-200 range. The generator side is 240V, and the inverter side is 120/240V. Straightforward stuff. Since the generator neutral connects to nothing, it does not matter.

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  • Thanks for your reply and knowledge, both here and on other posts I've encountered! I'm not dead-set on anything. I'm just weighing my options, and trying to come up with a balance of user-friendly, simple, safe, durable, and affordable. I considered #1, as it would simplify things immensely, but I really need something my wife/kids can handle if the inverters fail, battery dies, etc. #2--a transformer never even crossed my mind! I'll look into it. PS: really appreciate the video and your ideas, but not so much the condescending tone. Did I say/do something to deserve it? Oct 31, 2022 at 15:05
  • @ConsumerBot734277 Sorry, wasn't aiming to be consdescending, just trying to echo off what the video said. I could see how that could be taken; cleaned up. Oct 31, 2022 at 22:31
  • @ConsumerBot734277 -- do you plan to leave the generator plugged into its inlet on the house and ready to fire up, or are you keeping it in a portable config and dragging it out manually when the power goes out? Nov 1, 2022 at 2:10
  • @ThreePhaseEel, the genset is presently plugged into the inverter, so it can charge batteries, and would pass power through. Would want it to fall back on during a grid outage coinciding with dark/cloudy weather. Definitely need to keep it portable! Nov 1, 2022 at 16:32
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    Way to bury the lede there @ConsumerBot ... that changes everything. Cord and plug attachment in that application is necessary and permitted (but bonding must still be correct). Since it's unfair to answerers to change the question so much, you should ask a new question and specify what exactly is on the trailer and what is on the house. yeah, a couple years ago there was a glut on the market of those solar trailers... most people strip the valuable equipment and mount it in their home, and either sell the trailer or just use it as a solar panel rack. Nov 6, 2022 at 20:51
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Switching neutral isn't that expensive

The good news in your case is that switching neutral transfer isn't as expensive as you're making it out to be. You see, 3-pole double throw safety switches like the one you linked are really meant for three-phase applications, where all three poles are switching hot wires. However, since switching neutral transfer is a reasonably common requirement, there are a couple folks who have designed transfer switches specifically for the job.

In particular, for your bypass switch downstream of the inverter, instead of some cord-and-plug rig or homebrewed contraption, I'd recommend a Reliance Controls XRK0605D with a Siemens ECLK2125 subfeed lug kit installed to feed power out to a downstream main lug subpanel that serves as your critical loads panel. While you could use the XRK0605D directly, that limits you to 10 spaces, which is rather tight for your application, and the larger (26 space) XRR series panels aren't readily available at the moment.

Why not an ATS?

While 50A RV ATSes provide a UL-listed solution to switching neutral in a light duty (single-phase) ATS, there's a problem with that. Part of the reason for having a bypass setup is so that you can service various system components safely, and using an ATS for the bypass function defeats that provision since the ATS will be acting of its own accord to switch power between the battery inverter output and the bypass path.

That said, a RV ATS may be a good solution for the situation upstream of the inverter, where you're trying to transfer the inverter's grid-side input between grid and generator power. However, even if you do go with manual transfer between grid and generator, using another XRK0605D, you'll need a relay (a RIBH1C will do the job) to tell your battery inverters if the grid is present, as per the Sunny Island's installation documents. (This is because neither RV ATSes nor the Reliance Panel/Link X-series manual transfer panels are available with auxiliary contacts to signal what position the transfer switch is in to an external system, like your inverter.)

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  • thanks for the answer. I'd actually encountered the Reliance X Series whilst hunting for transfer switches that would work on Separately Derived Systems, but I had seen a note in red on the product page saying they've been discontinued! I appreciate the note about the ATS for the inverter's gen/grid input. Ideally, I'd wire it as you described, but I just learned the utility wants me to jump through the same hoops as Net Metering if I attach a grid-feed capable inverter to their grid at all, even if it's only to be used to charge batteries. Frustrating, but understandable. Nov 6, 2022 at 17:41
  • @ConsumerBot734277 -- the older XRC series parts are discontinued -- the XRK/XRH/XRR series are the replacements. Who's your electric utility btw? Most utilities don't require that many hoops as long as you're using UL 1741 listed (compliant) inverters Nov 6, 2022 at 17:48
  • See here. Not available, and still more money than I'd prefer to spend, just to switch a neutral... I still feel like the RV ATS might still work for me. I'd be feeding it with CBs from both sources, so I could pretty easily "control" it manually, by switching off the feed from one (or both) sources. Nov 6, 2022 at 20:04
  • Our utility is AVECC. There are very few people doing solar here. I think they might be overly cautious due to lack of experience. Net metering requires a contract, drawing, and state inspection, since we are outside city limits. Nov 6, 2022 at 20:09
  • @ConsumerBot734277 -- Reliance's website isn't all the way up to date -- the catalog link on their homepage is your best bet for current info Nov 6, 2022 at 22:19
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I think I've found a nearly perfect solution--a 50 amp RV transfer switch will isolate the neutrals, and is far more affordable than the double throw safety switch. I'm not 100% certain it would be NEC compliant, but it is UL listed, from a major US-based manufacturer with a good reputation, uses German contactors internally, and will be hard-wired.

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  • You probably don't want to use an ATS for the downstream-of-the-inverter (bypass) function though Nov 2, 2022 at 2:16
  • @ThreePhaseEel - I don't really understand why this isn't the obvious choice, assuming it's within NEC. I retain (most) control over the switch, since the breakers that would feed it from each source are right there. It would just prioritize one over the other, and be on a timer. Nov 6, 2022 at 20:19

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