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If you have solar, you might get a manual transfer switch between utility and solar. Both of those would be hard wired.

So let's say you had solar and utility power AND a generator ... I see there are three-way transfer switches - again, all three choices would be hard wired.

But what if you had solar, utility, generator AND you also had various other power inputs in the future that you could not predict ahead of time. Maybe someone shows up with a solar array on a truck or someone drives up towing a bigger generator ... or maybe you disconnect your electric car batteries from the car itself and cable it up to the house ...

The point is, you would need a transfer panel with various, diverse inputs (perhaps standard NEMA plugs ? Open bus bars ? Lugs to clamp onto ?) and it would need to be able to switch between, say, five or six different inputs. Basically, a swiss army knife manual transfer switch.

Do these exist ? Is it possible to wire a device (switch) into my mains power such that I could rent a big huge Ingersoll Rand Diesel generator and drive it up to my house and connect it with cables without rewiring my house ?

How can I create a collection of diverse power inputs for my house that would allow me to opportunistically use whatever power input happened to be available ?

Thank you.

  • Note: multimode solar just about always uses a single-throw automatic transfer means (either a Backup Gateway type device, or integrated into the battery inverter), as manual transfer would prevent the system from operating grid-tied when the grid is up, something you probably don't want to give up. Can you provide more information on your actual situation regarding power sources? Also, when you say "generator", are you talking about a permanent standby generator? – ThreePhaseEel Jul 2 at 3:06
  • When I speak of a generator, I am speaking of ANY of the following: perm-installed propane, like Generac ... temporarily connected gasoline generator ... or maybe somebody trucks in a big Kohler Power or Ingersol Rand Diesel generator like you see at a small hospital. The point is, I don't know what we will want, or what will be available - I just want a swiss army knife that allows me to take advantage of whatever power input happens to be available ... connecting up the Chevy Bolt battery pack is kind of a joke, but not really - why not have the ability to do that ? Lots of power there. – user227963 Jul 4 at 17:04
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The issue, overall, is transfer architecture

The idea of a "universal transfer switch" per se doesn't exist, at least not in the turn-key form you're after. Why? The issue is that multimode (grid-tied/off-grid switchable) solar systems use a different transfer architecture from standby generators. In particular, if you have a standby generator (save for some exotic systems), you will never be running it in parallel with the utility's generation, so the transfer switches used with standby generators, whether it's a 6kW light-duty portable genset or a 600kW emergency generator at a hospital, are double throw switches that provide total isolation between the utility mains and the generator connection.

However, solar power systems in this day and age are expected to be able to deliver power to the grid when a grid connection is available. In order to be able to do this when the grid is around, while still providing safety from backfeeding when utility power goes away, multimode solar systems use a single throw transfer means. This can either be in the form of a dedicated box, such as the Backup Gateway used with the Tesla Powerwall system, or integral with some other part of the system, typically the inverter-charger used to manage the batteries in "open architecture" multimode systems using 24 or 48VDC battery banks. While many of these inverter-chargers also support selecting a second AC input instead of the first, it does not change the overall thrust of the setup; namely, that when the transfer means is closed, the solar system is permitted to generate power in synchrony with whatever source is feeding the "mains" side of the inverter.

Trouble comes where neutral meets ground

A further issue comes from the fact that most portable generators are required to meet the OSHA specifications for construction use, which require neutral and ground to be bonded at the generator so that everything works as expected when you're using the generator as a sole source of power, say on a jobsite. However, your house also supplies a neutral-ground bond at its main breaker panel (service disconnect enclosure), so in order to avoid trouble with parallel bonding means when you add the generator to the picture, you either need to

  1. Remove the neutral-ground bond from the genset (something that's not always possible/provided for, and even when it is, creates a bit of a trap for someone who wishes to then use that generator for portable service), or
  2. Use a transfer switch that switches neutral, thus isolating the generator's neutral/ground bond from the bond in your main panel and eliminating the problem.

However, while switched neutral manual transfer is quite feasible to achieve, switched neutral automatic transfer is quite hard. In particular, while 3-pole double throw enclosed (UL98) switches are available at a reasonable price and a few switched neutral interlocked-breaker manual transfer panels (Eaton CHGEN...RSN, Reliance Controls PanelLink X, and some setups using Square D QO panels and parts) also exist, single phase automatic transfer switches that are not made for RV use universally have solid neutrals, and so do most multimode inverters. In fact, there's only one UL 1741 compliant multimode inverter available on the market at the time of this writing, and that's the 120V version of the Victron MultiPlus/MultiGrid; however, that inverter has several downsides that make it difficult to use in North American applications, especially those involving multiple power sources:

  1. It's 120V-only, requiring two inverters to be stacked for split phase service
  2. It lacks a secondary AC input, unlike it's bigger brother (the Victron Quattro, which hasn't yet been UL 1741 tested/listed), or the competing solid-neutral UL1741 inverters for that matter
  3. It only supports 24V DC, again unlike the Quattro, as well as the competing inverters from Outback and Schneider, which support 48V DC
  4. Its listing itself is...questionable when it comes to US usage, as the test lab that listed it isn't OSHA-approved as a Nationally Recognized Testing Laboratory (they're CSA-accredited, but that may not be good enough for your Authority Having Jurisdiction).

Permanent standby generators, of course, can be wired either with a bonded or a floating neutral without these issues, and are shipped in a floating-neutral configuration to allow use with stock two-pole residential transfer switches.

There is a glimmer of hope here though

The good news, though, is there is something that comes close to a "universal inlet" for most generators upwards of 8-9kW, whether they be light-duty portables or heavy-duty mobile generators. In particular, while NEMA never wrote a standard for twist-lock receptacles and plugs over 30A, the State of California did, and it has become a de facto standard throughout the electrical industry for this application. The resulting inlet for 120/240V split-phase applications is known as a CS6375, and is available either wrapped neatly in an inlet box from multiple manufacturers (Reliance Controls PB50, Eaton EGSPIB50, Midwest U050N, GE/ABB T050N), or as a kit that fits a 2.5" KO (Reliance Controls PI50).

While this limits you to pulling 50A at 240VAC from your generator, no matter what size it is, it's an adequate solution for most residential standby power needs. (Larger commercial applications use single-pole cam-lock type connectors, but these require significantly more attention in handling than residential power inlets and outlets do, and thus are generally only used in environments where appropriate personnel training can be guaranteed.)

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  • This is very helpful and informative - thank you. I think I would be happy with a totally manual system - I don't care about feeding back solar power to the grid because by the time we go fully solar, we will disconnect from the grid anyway. I also don't care about a totally manual transfer - I don't mind if the entire house loses power for a minute or ten while we throw one or two levers ... so if that is the case, I wont be limited by the 50A @ 240VAC solution you describe - I could have a power inlet that would let me manually switch and manually switch over the neutral+ground, yes ? – user227963 Jul 5 at 5:06
  • @user227963 -- the issue is that they don't make a bigger inlet without going to camloks or pin-and-sleeve industrial connectors -- you can get bigger switches, sure, but it's much harder to use them for a portable connection. Note that even if you're fully offgrid, you'll still want to have automatic solar-generator transfer as good multimode inverters can manage using the generator to bolster solar/battery power automatically if the batteries run low. – ThreePhaseEel Jul 5 at 5:50
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I have looked into it quite deeply. There are some products out there, but I don't want to do a lot of manual operation of the system. It would be awesome if a company could create a device or compatible collection of devices that could integrate solar, battery, generator and mains. The long pole in the tent always seems to be capacity. While the POCO can reliably supply 200 amps at 240v to power your loads, batteries, solar and generator are hard to integrate and properly control to supply a varying load (think electric range, electric WH, electric dryer, etc.), it's not always practical to try and switch from one to the other if your electrical demands are high. If your electrical demands were consistently low, natural gas powered toaster, hair dryer, waffle iron, space heating, cooking, clothes dryer, water heater, it might be practical for a manufacturer to create an integrated controller, battery charge manager, generator manager, solar panel manager and asso inverters as well as determining when to connect to mains power. I'd love to see a device or collection of devices that could accomplish that.

The closest I could come to what you are asking about is the Conext XW Series: Off grid and Grid Interactive by Schneider Electric. If you don't mind manually controlling a lot of this stuff, you could get pretty close to what you want, just don't expect a turn key solution.

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You need a power inlet: if you want a Portable device like a truck mounted or trailer type to provide power. the inlet has To be protected by a transfer switch and the male prongs are deenergized when you connect the new source with a female cord end that mates the power can be provided now from the cord. This is how portable generators are connected to homes.

If you want fancy or multiple methods to power the same requirements exists but safety circuits are included to where the connection to the female cord cannot be connected until the cord is in place. I have only done one of these and the inspectors 3 of them went with my explanation because it was beyond them they had never seen a system like that it was my first but how I finally got it to pass ( the inlet can not be powered until the cord is physically connected pulling in a contactor connecting the inlet with the 2nd alternate power source once they were in phase. I should mention if the alternate power source failed or got out of phase the contactor on the transfer switch would open.

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