transfer/disconnect/cross connect switch

nichohel

Hello,

I have big thick 200A aluminum cables from my service disconnect to my house panel. I want to put the house on an Outback Skybox for normal operation (the house electrical demands are well below 5kW peak) but retain the option of directly reconnecting grid power to the house if, for example, in case of problem with Skybox or if I want to temporarily run some extremely power hungry things. Ideally there would be some sort of transfer switch that could intercept the existing big aluminum wire, and provide a place to connect thinner copper wires to and from the Skybox, and with a simple three position switch do one of the following:

1) completely disconnect incoming grid from everything, for maintenance
2) connect input grid directly to output house load as things are now, completely disconnecting the Skybox
3) route input grid to Skybox input (via smaller copper wires, which Skybox can accept), and route Skybox output
  on smaller copper wires to the existing larger aluminum wires for the house load

An issue is that the existing large aluminum wires that connect the service disconnect to the house panel are rated for 200A. Skybox can pass through at most 60A itself. (I am not clear if the current is actually going through the Skybox inverter when the Skybox is in pass through mode--does anyone know? I don't think it matters for my main question but I am curious). So it is a little odd to have the 200A aluminum wires connected to thinner, probably 115A-capable copper in the hypothetical transfer box, and then to the 60A-capable Skybox and then out more copper to the hypothetical transfer box and then on to the existing 200A aluminum wires to the house. But I am hoping that this is actually OK, because the Skybox would simply act as circuit breaker and shut down if house current draw exceeded 60A.

This seems like a desirable setup to me because the whole house could be powered without rewiring the house panel into a "critical circuit" subpanel, and yet I could retain the ability to easily use more than 60A in the future by temporarily bypassing the Skybox.

So, questions: Does my hypothetical "transfer box" exist? If so, who makes it? Is this a very unusual or undesirable way to approach things?

Thanks for any suggestions!

Nic

BB.

Welcome to the forum Nichohel,

I am not quite sure I understand your wiring plan... Normally, your utility would drop power to your meter input leads, go through your meter. Then to a main disconnect or main circuit breaker.

One option is a direct plugin for your meter socket (intended for legal genset installation):

http://www.generlink.com/generlink.html

Depending on how your home is wired--You may be able to put a transfer switch to replace a mains disconnect, or downstream of the main disconnect and put the transfer switch before the main panel.

https://www.solar-electric.com/midnite-solar-60-amp-240-volt-dual-ac-manual-transfer-switch.html

Another possibility is to purchase a external panel that wires up to your main panel and lets you (your electrician) basically wire from 6 or more branch circuit breakers (120 single pole or 240 double pole) and you "choose" which circuits you want to be on genset/AC inverter backup power.

https://www.homedepot.com/p/Reliance-Controls-50-Amp-10-Circuit-Manual-Transfer-Switch-A510C/206503336

And there is simply installing a second "protected" sub panel that would be connected to the loads you want to have backup power too.

And there are more complex transfer switches--From simple relay (genset or inverter AC power on, transfer relay connects to backup power source to sub panel):

https://www.solar-electric.com/pmts-50.html

And you can get automatic transfer switches (detect AC fails, start your genset, switch over, switch back when mains working again, cycle genset once a week, etc.).

And you can get AC inverter-chargers that have both AC mains and Genset AC inputs, and will automatically control the AC inverter/Genset as needed...

This is not necessarily a simple choose A and install. It really depends on what you want, your local building codes, and how sophisticated you want your system to be.

I believe you are in Nevada (USA)? Adding genset/backup AC inverter systems to your home requires complying with local building/electrical codes and probably even require a licensed electrician.

-Bill

nichohel

Thanks for your response. You offered lots of useful information but unfortunately I don't see the answer to my question in there. None of the devices you linked to seem to do exactly what I'm looking for. That may be my fault for not including a diagram, or being too verbose. I am including a diagram below.

Essentially what I would like is to have a swtich that could either route grid input through a flexible inverter device (Skybox) on the way to the house, OR completely bypass the inverter device and DIRECTLY connect the grid input to the house as it is now, with the SKybox completely removed from the system.

So in the first case, this would require breaking the connection from grid to house (Bypass in the digram), and making a connection from the grid to the input of the Skybox (SBIn), and making a connection from the output of the Skybox to power the house load (SBOut).

In the second case, SBIn and SBOut would be disconnected and Bypass would be connected so the house would be powered by Grid alone.

In my post I specifically mention a desire to avoid a critical circuit subpanel. I would like take over all the house loads in one step. This may raise issues, which I brought up in my first post, but it also may be something that is commonly done; I just don't know.

Yes of course electricians will be involved at appropriate times and codes complied with. But I have learned it is best to figure out what I want to do first, and that's my goal now.

Thanks,

Nic

mcgivor

An automatic transfer switch (ATS) would work, depending on what is prime power or backup either inverter or grid, is a matter of choice, some have a disable switch with manual operating features.

The capacity of the ATS would need to be compatible with the incoming 200A grid feed, it would need to be installed after the main disconnect with a  60A branch circuit feeding the inverter bypassing the ATS to allow pass through, the main disconnect would ensure complete grid isolation. When the Skybox is in control of the loads the limitations of the loads will be determined by the Inverters maximum capacity.

This is not something I would recommend an unqualified person to undertake, as there are code rules and regulations to follow in most circumstances, the potential to make fundemental mistakes is very high.

 Transfer switchs are made by many manufacturers such as Schneider, Siemens, ABB, Cutler Hammer, Telemecinique, Eaton etcetera.

nichohel

[Reposting. My post seems to have disappeared when I was making an edit for clarification]

BB: Thanks for your response. You offered lots of useful information but unfortunately I don't see the answer to my question in there. That may be my fault for not including a diagram, or being too verbose.

Essentially what I would like is have a swtich that could either route grid input through a flexible inverter device (Skybox) on the way to the house, OR completely bypass the inverter device and DIRECTLY connect the grid input to the house as it is now, as if nothing had changed, with the SKybox completely isolated.

So in the first case, this would require breaking the connection from grid to house (Bypass in the diagram), making a connection from the grid to the input of the Skybox (SBIn), and making a connection (just one) from the output of the Skybox to power the house load (SBout).

In my post I specifically mention a desire to avoid a critical circuit subpanel. I would like to do this for the whole house in one step. This may raise issues, which I brought up in my first post, but it also may be something that is commonly done; I just don't know.

Yes of course electricians will be involved at appropriate times and codes complied with. But I have learned it is best to figure out what I want to do first, and that's my goal now.

nichohel

mcgivor: Thanks for your response. I think I understand from your post that a better way of doing what I want is to simply keep the Skybox powered from Grid at all times and not try to switch that off when in Bypass--just move the loads from Skybox to Grid. This makes a lot of sense. If maintenance needs to be done on the Skybox, everything could be depowered at the existing upstream service disconnect. So another set of smaller gauge copper (compatible with Skybox) could go from the service disconnect to the Skybox input, and then a simpler switch could just move the loads (only) from Grid to Skybox as desired.

mike95490

Regardless if you use 200A wire, connecting 200A service panel to a 60A source is asking for trouble, at some point, the 60A source will get overloaded, and often the internal transfer relay gets fried.

mcgivor

Adding a generator will make the exercise more complex, a second ATS would be needed to provide isolation between grid and generator. Depending on the generator capacity this could be done either at the main, with a large generator, or the input to the inverter, with a  small generator, the latter being the cheaper easier option..

BB.

You can use two sets of manual transfer switches... The first:

From Meter/main disconnect to A: bypass, and B: Sky Box

A second manual transfer switch: A; Bypass, and B: Sky Box; output to main panel.

I am not an electrician, but the above would need an upstream double pole breaker from meter -> transfer switches -> main panel

As far as I can guess, you cannot connect the transfer switch directly to the the grid/meter--If there are no breakers/fuses in the transfer switch. 

However you can get Circuit breaker panels that have interlocks and will take standard breakers, like this one:

https://www.homedepot.com/p/Generac-200-Amp-7-500-Watt-Non-Fuse-Outdoor-Manual-Transfer-Switch-6382/203835694

And you can get Generator Lockout kits (breaker interlocks) for various brands of panels:

https://www.homedepot.com/b/Electrical-Power-Distribution-Circuit-Breakers-Lockout-Kits/N-5yc1vZblzg
https://www.geninterlock.com/faq/
https://www.geninterlock.com/product-category/generator-interlock-kits/

Looks like EBay has a lot of these Generator Interlock kits too:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=Generator+interlock+kit&_sacat=0

I have also seen a large industrial inverter/genset backup system where (more or less) they used standard breaker panels but had keyed locks that only allowed the site electrician unlock the breaker for inverter/genset backup or bypass (when servicing the backup power system). I think they were similar to these:

https://www.ebmag.com/products/kirk-key-interlock-safeguards-small-circuit-breakers-17346

Since your Sky Box is only rated at 60 Amps, the circuits too/from the Sky Box will probably require 60 amp breakers too.

-Bill

nichohel

mcgivor: I think that adding a generator will not be complex and can be done as I show in my diagram. The Skybox itself can take a generator input and I believe can mix the power from grid, PV, generator, and battery, in pretty much any way you want or need. The Skybox is a very smart and flexible device! If I am wrong about this, someone please correct me!

nichohel

mike95490: Yes, I am concerned about feeding the house load (a 200A panel that typically only draws a very small fraction of that) on copper rated for much less (probably 115A) and through the Skybox rated for 60A max. But my assumption is that the Skybox would simply go into load protect shutdown of some sort in the unlikely event that the house actually drew more than 60A. If necessary another 50A or 60A breaker could be put on the line from Skybox to load to make sure the load did not exceed the Skybox capacity but if the Skybox must be designed to shut itself down if overloaded ... right?

nichohel

BB: thanks for the additional information. I am wary of any solution that involves multiple switches that must be moved independently to accomplish a cutover and could allow for undesirable connections to be made if switches weren't thrown in the right order. It seems like the right solution is to keep the Skybox powered at all times and just switch the load from Grid to Skybox with a single switch. For maintenance, everything could be shut off with the (existing) upstream service disconnect breaker.

mcgivor

nichohel said:

mcgivor: I think that adding a generator will not be complex and can be done as I show in my diagram. The Skybox itself can take a generator input and I believe can mix the power from grid, PV, generator, and battery, in pretty much any way you want or need. The Skybox is a very smart and flexible device! If I am wrong about this, someone please correct me!

The problem is not the voltage between the grid and generator inputs but rather frequency synchronization, it would be almost impossible to have the two synchronous, the two need to be isolated from one another. One waveform over another would create havoc, the Skybox no matter how inelegant it is, has no control of the grid, nor the generator, be very careful with assumptions. This is positive feedback, no disrespect intended.

Estragon

nichohel said:

mike95490: Yes, I am concerned about feeding the house load (a 200A panel that typically only draws a very small fraction of that) on copper rated for much less (probably 115A) and through the Skybox rated for 60A max. But my assumption is that the Skybox would simply go into load protect shutdown of some sort in the unlikely event that the house actually drew more than 60A. If necessary another 50A or 60A breaker could be put on the line from Skybox to load to make sure the load did not exceed the Skybox capacity but if the Skybox must be designed to shut itself down if overloaded ... right?

Inverters typically do shut themselves down in an overload condition, but that's not all the breakers protect against.  For example, the inverter could short internally, potentially causing a fire in affected circuit wiring.  The breakers mainly protect wiring, not so much the device itself.

nichohel

mcgivor: Thanks for your cautionary note. The Outback Skybox is designed to have both grid and generator input. The device *certainly* is at least smart enough not to try to combine out of phase sources. It may simply internally disconnect one or the other inputs as needed. But it also seems possible to me that it is smart enough to just take power from the generator and phase shift to match the grid with smart inverter electronics. I'm not sure about that, but it certainly won't cause mayhem by trying to do something dumb!

mike95490

My XW inverter sync's itself to the generator or the grid.   It cannot take generator power and sync that to grid

it is smart enough to just take power from the generator and phase shift to match the grid with smart inverter electronics

 That sort of shift isn't possible at consumer price levels.

BB.

As always, it is a bit more complicated with this modern AC inverter-charger systems...

For example, there is the generator support mode. This is where you can (for example) have an 1,800 Watt genset (~15 amps @ 115 VAC max) connected to an inverter-charger with generator support. You program the Inverter-charger for 13 amps max on AC. The inverter will take 0-13 amps to charge the battery bank, and if there is an AC load, will pass through the genset 115 VAC to the AC inverter-charger output. If the Load+Charger power exceeds 13 amps, the inverter-charger cuts back on charging current. And if the AC output exceeds 13 amps, the inverter-charger will actually "help" (or support) the generator and add enough current/power to keep the load voltage stable at ~115 volts, up to the AC rating of the inverter (it.e., you can have a 6 kW inverter-charger + 1,500 Watt AC genste powering a 7,500 Watt AC load (while placing the AC charging of the battery bank "on hold"). So, in this type of operation, the Hybrid inverter-charger does sync with an AC source (including less than accurate frequency control on the genset).

This is pretty neat because it allows you to use a smaller more fuel efficient between 50% to 80% of rated output (more fuel efficient range) for battery charging and during poor solar days... And the inverter-charger with generator support will temporarily supply additional power for starting a well pump or even running a stove/microwave.

Much better than running a 10 kWatt genset at 20% load for much of the time...

-Bill

nichohel

mike95490 said:

it is smart enough to just take power from the generator and phase shift to match the grid with smart inverter electronics

 That sort of shift isn't possible at consumer price levels.

It seems perfectly possible to me that the Skybox could use the generator to charge batteries while the grid is active, and then mix power from the inverter (fed by the batteries) in phase with the grid to the loads. But it doesn't really matter to my question. If the Skybox can't mix the two live power sources with some advanced phase shift mojo, or by charging the batteries which feed the inverter, the Skybox *certainly* is smart enough to not destructively and naively mix phases. I'm sure it just internally disconnects one of the input sources if it can't mix them.

The Skybox seems to be quite advanced.

Nic

mcgivor

nichohel said:

mike95490 said:

it is smart enough to just take power from the generator and phase shift to match the grid with smart inverter electronics

 That sort of shift isn't possible at consumer price levels.

It seems perfectly possible to me that the Skybox could use the generator to charge batteries while the grid is active, and then mix power from the inverter (fed by the batteries) in phase with the grid to the loads. But it doesn't really matter to my question. If the Skybox can't mix the two live power sources with some advanced phase shift mojo, or by charging the batteries which feed the inverter, the Skybox *certainly* is smart enough to not destructively and naively mix phases. I'm sure it just internally disconnects one of the input sources if it can't mix them.

The Skybox seems to be quite advanced.

Nic

The Skybox may well have the inputs for both grid and generator but are isolated internally with a transfer switch allowing only one to operate at any given time. The generator is usually a backup to the grid and or battery/solar, if the grid is present then it wii not connect to the generator. See page 14 in the Symptoms and Remedies, will not connect box.