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Second post - Installing a subpanel and using battery backup for grid outage

gwcook

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Dec 6, 2020
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I am slowly putting this puzzle together, both mentally and literally. Reading owner manuals, internet articles/posts and watching videos. I've even called tech support for AIMS inverters.

Stuff: four 6 volt, 370ah batteries, a 3000 watt, 24 volt, single phase inverter and a 20 slot 125 amp subpanel - it's a Square D.

My plan:

1. Install the subpanel. Add a 125 amp breaker in the main panel. Run two hot leads using 3 gauge cable over to the subpanel main lugs. Run a ground from the main to the sub. Run a neutral from the main to the sub - these last two I am not 100% sure of as I watched a video but there were no batteries or inverters involved.

2. Move my critical circuits over to the subpanel

3. Install some kind of breaker in the main - maybe 50 amp, not sure here. Maybe I'll find that in the manual. This breaker would be used to feed power to the input side of the inverter. The manual shows two options: L,N,G or L,L,G.

4. Here's where I'm struggling: can you run output from the inverter to the subpanel while it has input power from the main panel? I mean, I would think it is designed for this but I can't find any information on this. Again, the manual has three options: L,N,G, L,L,N or L,N.

My though is that if main power is lost, the ATS would activate and power up the subpanel.

Thoughts?
 
You can have grid and main panel feed input of your inverter/charger.
Output of inverter/charger feeds sub panel.
You now have a UPS providing battery backup.

Inverter is single phase 120VAC. You can use half the sub panel. You can wire to both legs of the sub panel. You can install a 120/240V transformer.

You can install a manually interlocked backfeed breaker. Main panel feeds 125A to the 125A main breaker in sub panel. Next to that is a 30A breaker (fed by inverter), and sheet metal interlock prevents both from being on at the same time.

"can you run output from the inverter to the subpanel while it has input power from the main panel?"
Run to a breaker which is off (and interlocked). Not both wired (on) to the busbars of panel at same time. If grid goes down you backfeed the grid. If grid comes up it burns up your inverter.
 
"can you run output from the inverter to the subpanel while it has input power from the main panel?"
Run to a breaker which is off (and interlocked). Not both wired (on) to the busbars of panel at same time. If grid goes down you backfeed the grid. If grid comes up it burns up your inverter.

For a setup where someone wanted some circuits powered by grid first, battery second (and the switchover between grid/battery to happen automatically), would the sub-panel be installed with no direct connection to the main panel, instead only being powered only by the output of the automatic transfer switch?
 
For a setup where someone wanted some circuits powered by grid first, battery second (and the switchover between grid/battery to happen automatically), would the sub-panel be installed with no direct connection to the main panel, instead only being powered only by the output of the automatic transfer switch?
Yes, that's what I was thinking of. For an inverter/charger that works as a (not "on-line") UPS. Power goes through the inverter, grid feeds loads. If power fails, after 30 ms the inverter opens a relay to disconnect from grid and makes power from battery.

I suggest also having a direct connection to main panel through manual transfer switch. When electronics fail, just flip a switch and power is restored.

For other inverters or generators, you might use a separate automatic transfer switch.

I do a bit of both. House is on output of Sunny Island, automatic backup.
With manual transfer switch I can power garage as well. Because it has arc welder and shop tools, can bring total current higher, I don't have it wired in all the time.
 
I have a subpanel with a transfer breaker setup. 60amp main feed from the main panel and 60 feed from the inverter. Only one can be on at a time, as there is a mechanical interlock. I put a plug in where the inverter feed is so I can unplug the inverter and plug in a generator if needed.
 
I called AIMS support again and they said the inverter/charger/ATS just passes power through from the input to the output, so from the main to the sub panel in my case. Then when main input power fails, the batteries take over. That's basically what I was looking for. But I have no experience with any of this so I'm learning as I go. When Hedges said "UPS with battery backup," the lightbulb went on and that triggered the callI to tech support to verify.

Also, this particular inverter I'm looking at has an auto start for a generator in the event I ever decide to get one.


Thanks again for the help in understanding this stuff. I'm sure this won't be the last question I have.
 
I called AIMS support again and they said the inverter/charger/ATS just passes power through from the input to the output, so from the main to the sub panel in my case. Then when main input power fails, the batteries take over. That's basically what I was looking for. But I have no experience with any of this so I'm learning as I go. When Hedges said "UPS with battery backup," the lightbulb went on and that triggered the callI to tech support to verify.

Also, this particular inverter I'm looking at has an auto start for a generator in the event I ever decide to get one.


Thanks again for the help in understanding this stuff. I'm sure this won't be the last question I have.
If we lose power it is normally a winter ice storm. You may or may not see the sun the next day or two. With heavy overcast, you might as well not have solar panels. The generator is a good "plan B".
 
I have two 6V, 370ah batteries. I have returned my 12/120V inverter in order to purchase the 24/120V inverter and therefore need two more of these pricey batteries. The 370ah batteries are hard to come by. I can drive 4 hours to Huntsville AL. or I can buy four available 230ah batteries - this is what I am doing. But I still want to be able to utilize my 370ah batteries. Is there such a thing as a 12-to-24 volt transformer that would make it possible to use those two 370ah batteries?

I have been told and read that it is not a good idea to use different capacity batteries in the same bank. So I guess I can't connect them all together in a 24 volt configuration since they have the different C20 ratings.

Also, do I need a breaker from main to inverter or inverter to sub panel? If so, how do I calculate size? Would it be a 30 amp breaker in the main since I am using a 300 amp fuse between inverter and batteries on the positive connection.

Thanks
 
Probably should use the two 370 Ah batteries for a separate 12V system, unless you get two more just like them (not advisable if these are older, degraded from new.)

No great way to have them feed your 24V system. If this was in a vehicle, maybe you would want 12V for some DC loads and 24V for the inverter. You can get DC-DC chargers, such as to charge a 24V house bank from 12V alternator.

Maybe get a 12V battery charger/maintainer that runs off inverter AC, if 12V is good for something. For a house, I think it would be good for internet, cell phone charging, alarm, etc. and would last for days even if inverter had depleted the 24V bank. I plan to do that with a spare 12V 100 Ah AGM battery I've got.
 
Also, do I need a breaker from main to inverter or inverter to sub panel? If so, how do I calculate size? Would it be a 30 amp breaker in the main since I am using a 300 amp fuse between inverter and batteries on the positive connection.

This sounds like a grid-backup system (which is what the thread title says)
Looks like this is the 3000W inverter with 120 VAC in, 120 VAC out.
Should be a 40A breaker in main panel feeding input to inverter (unless otherwise specified in manual) because 3000W/120V = 25A, add 25% for 32A, round up to 40A.
Same 40A would be good in sub panel where fed by inverter. This is a backfed breaker, so ideally held in place by a clamp of some sort so it can't pop out and have live exposed contacts.

No particular relationship to 300A fuse at 12V DC, except 30A 120V, 300A 12V.
Perhaps you should have a bit higher than 300A fuse, because 3000W/12V/0.85efficiency = 294A, add 25% to avoid nuisance trips = 375A
Unless you never put more than 2500W continuous on the inverter.

As I said earlier, I suggest setting up the sub panel with interlocked main breaker (direct from main panel) and backfed breaker (my 40A suggestion, fed from inverter). The interlock kit includes a sheetmetal bracket to hold down the backfed breaker.
 
Probably should use the two 370 Ah batteries for a separate 12V system, unless you get two more just like them (not advisable if these are older, degraded from new.)
These are actually new batteries as I am just getting started on this project. But see the wisdom in using them for a separate 12V system.
 
This sounds like a grid-backup system (which is what the thread title says)
Looks like this is the 3000W inverter with 120 VAC in, 120 VAC out.
Should be a 40A breaker in main panel feeding input to inverter (unless otherwise specified in manual) because 3000W/120V = 25A, add 25% for 32A, round up to 40A.
I know I've seen that math somewhere - just hard to remember everything I've read and watched. So 40A it is.
 
Same 40A would be good in sub panel where fed by inverter. This is a backfed breaker, so ideally held in place by a clamp of some sort so it can't pop out and have live exposed contacts.
Not sure I understand this part. This is probably an ignoramus question, but is it any different than a breaker in the main panel?

As I said earlier, I suggest setting up the sub panel with interlocked main breaker (direct from main panel) and backfed breaker (my 40A suggestion, fed from inverter). The interlock kit includes a sheetmetal bracket to hold down the backfed breaker.
Not sure I understand this part either. So two power sources then? Direct from the main but in a shut off state, as well as from the inverter but active? Why? Sorry to be asking so many questions but I'm struggling to understand some of this.
 
Use my example. I put in a 60amp sub panel. I pulled 4 critical house circuits out of my main panel and moved them to the sub panel. The sub panel came with two 60 "main breakers", but only one can be on at a time. Actually they are not "main breakers" but the input feed. Under normal conditions the 60amp that is connected to the main panel is where the sub panel gets its power. If the power goes out, I switch that 60amp off and switch the second 60 amp on. That second 60amp is feed from a inlet plug (power goes in, not comes out) on the wall. I can plug my inverter in there or I can plug my generator in there. But again, both 60amps can not be on at the same time. There is a mechanical device always blocking one of them.
 
Not sure I understand this part. This is probably an ignoramus question, but is it any different than a breaker in the main panel?


Not sure I understand this part either. So two power sources then? Direct from the main but in a shut off state, as well as from the inverter but active? Why? Sorry to be asking so many questions but I'm struggling to understand some of this.
Two power sources, either the inverter or the grid.
You have your "protected loads" on this panel, which you want to be able to use even if the grid fails.
But inverters fail too. Rather than having to rewire or move appliances back to outlets fed by the main panel, using a "generator" or similar switchbox lets you just flip a breaker to bypass the inverter and power your protected loads from the main panel.
You can buy a box with two interlocked breakers built in. You can buy an interlock and add it to an existing breaker panel.

The extra 40A breaker not really any different from the one in the main panels would be one of those two interlocked breakers.
(unless the panels are different brand or different series and don't accept the same breaker.)

Although you may decide to use a single pole breaker in the main panel to feed inverter, you might want to use a double-pole in the sub panel, both legs wired to the single hot output of your 120V inverter. There are some issues with this (depending on how the circuits on protected load panel are wired, I wouldn't want two circuits, black and red wire each on their own breaker sharing a single white neutral. But 3000W inverter limits how much current. I suggest ONLY connected protected circuits which each have their own white neutral.)

Here's one, which I'm using for my house, to bypass inverter if it fails.
It came with 100A and 30A breaker installed. I replaced those with 70A and 70A

Here's an interlock for the OUTDOOR 100 to 125A load panel. Different part for the INDOOR panel

Here's one for the OUTDOOR 150 to 200A load panel. I use it to manually connect inverter to main panel and power my garage if the grid fails. Different one for INDOOR panel.
 
I can plug my inverter in there or I can plug my generator in there. But again, both 60amps can not be on at the same time. There is a mechanical device always blocking one of them.

Did you have to build a power cord for the inverter terminal block to connect the inverter to the wall outlet or did you use a "suicide cord" with two male plugs on each end to connect from an inverter outlet to the wall outlet? I saw a video on this:
 
@DThames, I'm guessing your plug is wired somewhat like below (where the single hot from the inverter is jumpered over to second hot in the plug) so when it gets plugged into the outlet, there's 120v going to both sides of the panel from a single cable?

1607893622335.jpeg
 
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