Outback Skybox wiring and load questions

[df0rster]

I hope I can form this question to make sense..

I am in the process of installing my critical load sub panel. I've found that I can pretty much move all my circuits over to the subpanel, except maybe the heat pump and electric water heater. I have a lot of circuits but most are very lightly loaded. I normally have about 10-15 amps or less going through all of them combined. at times it might reach 20 amps if a couple fridges and the toaster and microwave are all running together.

My question is, the way I see the wiring diagram for the Skybox, it only handles loads in the AC subpanel with solar and/or battery. The diagram doesn't show any loads in the AC distribution panel, it only shows loads in the AC subpanel so that makes me think it doesn't send power back towards the main panel side (i'm not going to use grid export). So even if i am barely using any power in my subpanel , my main panel with the heat pump and water heater won't be able to use PV power from the batteries or skybox? Or am I thinking about it wrong?

I guess my question should be, will the Skybox feed power back to loads in my AC distribution main panel on the grid side? I am not going to export power but would like to have the heat pump and other main panel loads use the solar power if there is enough.

[Mike90250]

You set up your critical loads panel, so that you do not overload the source (skybox, generator.......) not the loads that you like. A toaster and microwave on at the same time will consume more than 20A, unless they are very tiny RV models.

I would consider critical loads the overhead lighting circuit, water pump, Fridge/Freezer, sump pump, and communications systems - cable modem, laptop, cell phone charger, TV, ignition circuit for gas range. next would add heat pump or gas furnace to keep house at 50F to prevent pipes from freezing..

I don't know what the output limit and surge limit for the skybox is, you have to add up your loads and calculate it for yourself. The good news is if you do it wrong, you house is the only one that suffers.

[df0rster]

When I spoke to the engineer at the solar company, he told me the skybox could manage the loads using a mix of grid and solar or battery. Up to 5kw for this inverter. I just want to make sure to utilize all 5kw of pv that I am installing.

as far as the critical loads panel, I can modify the circuits however I need if the loads get too high. I’ve got it set right next to my main panel. my toaster and microwave are 900 and 1200 watts, so right at 18 amps together. And I’ve got wood and propane heat with those fans on the critical loads panel, and propane range and all fridge/freezers on the panel. But that’s really not my question.

im just not understanding the setup of the skybox yet. The manual does not seem very comprehensive. I don’t have the actual unit yet. Maybe there is a better install manual included.

my big loads like the heat pump for ac and electric water heater will stay in the main distribution panel and I don’t need to back those up. But would like to be able to use pv power to supplement them like the sales guy told me I could.

thanks!

[Ampster]

There are several webinars and other online setup guides available. There is no robust manual however. The user forum can be helpful.

As far as supplementing your other load panels is concerned the only way to do that would be to export power. Essentially the Skybox would see everything beyond its own Grid In connection as the grid. However, if there is production beyond that it will get exported to the grid. The Skybox is fully UL 1741SA and CA Rule 21 compliant so no worries there. You would need an interconnection agreement with your Power Company to do that.

There may be workarounds by using external CTs to limit export. I can limit the amount of export but have not used external CTs. That is a technical question that might be good for the user forum. The external CTs are expensive if you purchase the Outback ones but one user found generic ones that can be configured. On the other hand, if you buy them from your dealer with the condition that they provide support it may be worth the price. I do know that the Skybox does measure power internally for solar, grid and batteries so conceptually you should be able to do it. Since my Skybox is configured for export and I have an interconnection agreement with my Power Company I have not needed to experiment with external CTs.

[df0rster]

There are several webinars and other online setup guides available. There is no robust manual however. The user forum can be helpful.

Thanks for the info. I am definitely going to be looking up some webinars or other guides.

I am surprised that the Skybox can't do what I'm thinking, and was told it could do. They claim it was designed just for this kind of purpose, hybrid and grid interaction, etc. It doesn't seem that complicated to be able to back feed a load into the main panel. It would be doing that for net metering anyway. I would think it could determine the load and just not supply more than that. But I'm not an electrical engineer so I don't know what is involved there.

[Ampster]

Thanks for the info. I am definitely going to be looking up some webinars or other guides.

I am surprised that the Skybox can't do what I'm thinking, and was told it could do. They claim it was designed just for this kind of purpose, hybrid and grid interaction, etc. It doesn't seem that complicated to be able to back feed a load into the main panel. It would be doing that for net metering anyway. I would think it could determine the load and just not supply more than that. But I'm not an electrical engineer so I don't know what is involved there.

It can do what you were thinking and that is exactly what it was designed to do. It is not complicated to back feed a load to the main panel. The complication is what to do with the excess power that is not used by the main panel. Either you send it to the grid which is electrically the simplest because the grid presents a nice infinite load and the right impedance so it can take anything that your throw at it within reason.You just need an interconnection agreement like I have to do it legally. Or you program the Skybox to not send anything to the grid by configuring external CTs at your main panel so the Skybox does not export anything past those CTs. Hypothetically the CTs would have to be placed at the point that your main panel interfaces with the grid at the meter. That can be complicated and often not something that many people have had experience with doing. There are also policy reasons that that kind of advice is not encouraged on this forum. Perhaps there is someone else on this forum that has done exactly that and can advise you in more detail.

Your dealer did not explain those details and that you need to have an interconnection agreement for it to work easily. Often sales people and the marketing people at manufacturers leave out those kind of details. It is not unique to the Outback Skybox. That is just how it is in a highly technical industry like this. It is not a problem for anybody who is off grid. All you have to do to make it work is to turn off your main breaker and nothing will go back to the grid. I do understand your frustration and wish I could give you the exact process to accomplish what you set out to do. Again, I suggest you try the Outback user forum, I know there is at least one poster there who has has some luck using external CTs. It is probable that at some point someone from Outback support may post with the details of exactly where to place the CTs and how to set the Skybox so you can accomplish your goals.

[df0rster]

It can do what you were thinking and that is exactly what it was designed to do...

Your explanation makes sense. You're right, the sales engineer didn't really go into that kind of detail. I did explain to him that I didn't want to export power but wanted to be able to power all of my loads and he indicated that could be done.

The reason I dont' want to export is because the interconnection agreement is not very customer friendly for my COOP. They add another $30 monthly fee for net metering and require a $1m liability policy that will cost me about another $50/month. So it doesn't pay to sell back the power at this point.

When you say "CT" are you talking about the current sensors? https://www.solaris-shop.com/outback...t-transformer/

They aren't too expensive. I will try to get on an Outback user forum and get some answers as well. Are you referring to a forum on Outback power's website? I'll poke around until I find something.

Thanks!

[Ampster]

Yes, current transformers like that. They come in pairs, one for L1 and one for L2.

[Ampster]

In reviewing this thread i realize some assumptions may have been made about the capacity of the Skybox. Some are hard limits like the 5 kW of the inverter, the 125 Amp DC breaker and the 60 Amp AC breakers (14 kW). As mentioned in other threads it is common to have DC to AC ratios as high as 1.5 to 1, so you may not be limited to 5 kW of DC solar but the voltage limits should still be respected.


There is a good discussion about that here:
https://forum.outbackpower.com/viewt...47&t=15987
Also on that forum there is one user that is using CT clamps that cost less than $20 each. I believe he may be using them in a similar application to yours.

[df0rster]

In reviewing this thread i realize some assumptions may have been made about the capacity of the Skybox. Some are hard limits like the 5 kW of the inverter, the 125 Amp DC breaker and the 60 Amp AC breakers (14 kW). As mentioned in other threads it is common to have DC to AC ratios as high as 1.5 to 1, so you may not be limited to 5 kW of DC solar but the voltage limits should still be respected.


There is a good discussion about that here:
https://forum.outbackpower.com/viewt...47&t=15987
Also on that forum there is one user that is using CT clamps that cost less than $20 each. I believe he may be using them in a similar application to yours.

Awesome! Thanks! I'll read that for sure. Been doing quite a bit of reading to prepare for installation.

Another thing I found on their setup sheet, they say the AC wire sizes are #8 to #3. Normally I would think a 5kw 240v circuit would be sized at 25 amps, #10 wire. and without having any surge capacity I wonder why they call out #8 as the minimum wire size. Its not a big deal, I'm only going 10', but I don't want to use #10 wire if there is a reason to use #8 min. I'll probably just stick with their minimum recommendation. But I've been noticing some odd things like this in their setup guides..

Still haven't gotten over to the outback forums, but I will soon.

Thanks!

[Ampster]

...........

Another thing I found on their setup sheet, they say the AC wire sizes are #8 to #3. Normally I would think a 5kw 240v circuit would be sized at 25 amps, #10 wire. and without having any surge capacity I wonder why they call out #8 as the minimum wire size. Its not a big deal, I'm only going 10', but I don't want to use #10 wire if there is a reason to use #8 min. I'll probably just stick with their minimum recommendation. But I've been noticing some odd things like this in their setup guides..

......

The Grid In, AC out and Generator breakers are all 60 Amps so I sized my wire for that. That is 14 kW which would allow you to feed your subpanel with 5kW from the Skybox inverter plus another 8 or 9 kW of energy from AC coupled GT inverters. That way you could feed any excess to your main panel to cover some of those loads. I described the breaker as Grid In but it and the AC out to the critical loads sub panel are both Bidirectional, My AC coupled GT inverters come through the critical loads subpanel and they can produce as much as 5 kW on their own plus the 2 kW I have connected directly to my Skybox.
I have asked the question on that other forum to confirm the process about backfeeding your main panel and using external CTs to have zero export. If I get a chance I am going to get some of those inexpensive CTs and try it myself because i want to be sure if that is a viable option in case the economics of my next installation warrant that alternative. To be clear, I am backfeeding my main panel now because I see the power going to the grid. The issue I am not certain about is whether the external CTs can be used to limit that backfeed to create zero export.

[df0rster]

The Grid In, AC out and Generator breakers are all 60 Amps so I sized my wire for that. That is 14 kW which would allow you to feed your subpanel with 5kW from the Skybox inverter plus another 8 or 9 kW of energy from AC coupled GT inverters. That way you could feed any excess to your main panel to cover some of those loads..

so the skybox will “pass through” the extra ac coupled power if you have another inverter? That’s handy.

so I’m installing my main panel (grid) and subpanel wires now. I did go ahead and size them for all 60 amps (4awg) Did you breaker yours in the sub panel? Or just to the main lugs since it is breakered in the skybox? I’m guessing it doesn’t make much difference but just curious if there is a particular code regarding this. In the main panel I’ll just connect it to a 60 amp breaker since I don’t think you’re supposed to double up wires on the main lugs.

[Ampster]

I followed my county electrical code which required a breaker from the main panel to the Skybox. The AC out from the Skybox went to the busbars of the subpanel. Later I wired a Midnite Solar interlock because I had to completely take the Skybox offline for a few weeks and had no other way to power the subpanel. Next time I am installing a subpanel with a breaker to feed the busbars on the subpanel. and then will use an interlock to have that manual capability if I ever need to take the Skybox off line again. It is also convenient and may be required in some jurisdictions to have a breaker on the input of the subpanel for safety reasons when you want to work on it if you are not with some distance of the disconnect at the main panel.

[df0rster]

I followed my county electrical code which required a breaker from the main panel to the Skybox. The AC out from the Skybox went to the busbars of the subpanel. Later I wired a Midnite Solar interlock because I had to completely take the Skybox offline for a few weeks and had no other way to power the subpanel. Next time I am installing a subpanel with a breaker to feed the busbars on the subpanel. and then will use an interlock to have that manual capability if I ever need to take the Skybox off line again. It is also convenient and may be required in some jurisdictions to have a breaker on the input of the subpanel for safety reasons when you want to work on it if you are not with some distance of the disconnect at the main panel.

thanks. That is exactly how mine is setup now with the breaker in the main and on the subpanel it’s going to the main lugs. I may put a main breaker in the sub though so I can install the interlock. I currently have my subpanel powered with a 60 amp breaker from the main panel. I’ll have to interlock that as a backup like you mention. I don’t want any possibility of powering the subpanel both ways!