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Converting residence to Off-grid solar power, how to handle 56kW inverter output

Converting residence to Off-grid solar power, how to handle 56kW inverter output


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Old 10-22-14, 03:26 PM
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Question Converting residence to Off-grid solar power, how to handle 56kW inverter output

Hello! Hopefully I don't sound too crazy here.

Long story short, I was able to acquire a large amount of solar panels (~30kW) along with seven relatively new stackable 8000W off-grid inverters, a ~3600Ah / 48V battery bank, and charge controllers to go with all of this.

In any case, I'm in the starting stages on planning the installation of all of this equipment in an effort to convert my entire home to off-grid solar power. (Please no attempts to persuade me to do grid-tie...)

The home currently has two 200A grid fed panels (320A meter split to the two). The largest load is an 80A/20kW EV charger.

Long story short, a slightly over-estimated load calculation puts me at around 225-250A service needed. (All lighting have been converted to LED, high efficiency AC/heat pumps installed, hot water possibly being converted to LP if needed).

I figure the 56kW/233A continuous available from the inverters would be sufficient. They are also about to do 10% more for 15 minutes, 20% more for 30 seconds, and 100% more (102kW!) for 0.1 seconds for surge power needs. If it seems necessary I can purchase and stack additional inverters, up to 10 of them, for 80kW total... seems like overkill, though, but not unreasonable if I were to add a second EV charger.

For those wondering, these are Outback Power Radian GS8048A inverters. (They are stackable and sync the output waveform.)

I would also like whatever setup I install to be expandable if I did decide to add additional inverters.

I'm pretty savvy, but I've not personally worked with anything much beyond 200A service.

Ideally, I would like to move the grid incoming service to its own new panel and leave the two existing 200A panels where they are, or close to it.

The inverters accept grid A/C input and can be utilized as DC chargers in the event that the battery pack is depleted, and pass through the grid AC power to the load side when this happens. Seems unlikely with ~170kWh of storage, but, who knows. Would like the option. So, I would have the new panel simply feeding the inverters' AC inputs and nothing else.

This part seems pretty straightforward. Have electrician move the existing panels, install a new panel, wire inverter AC inputs to the grid panel. Piece of cake.

My question comes up where I want to combine the output of the inverters to feed the two existing, now disconnected from the grid, 200A panels. Generally I would figure I would need a panel to combine the output of the inverters into a large output. (An example diagram from Outback power of this setup for four inverters is detailed here: http://www.outbackpower.com/download...line_Rev-4.pdf )

Is there a simple way to do this? Each inverter has it's own 50A AC OCPD at the inverter already. I'm just not sure the most sensible way to feed the two 200A panels from these outputs. I'm not against converting these panels, to say 150A panels, if needed either. I'm just looking for the best solution.

I have several friends who are connected with the contractors I would need to get all of this done officially, I just want to have a more detailed plan to present before I make the calls on those favors.

A rep at Outback suggested I combine the inverter outputs in a large 400A panel and output to the two 200A panels from there. I'm not sure if that makes sense, however, and I'm unsure how this works code wise or with physical space requirements. (Don't 400A panels require 2x10 walls?)

Could I combine the inverter outputs in a second meter housing similar to the existing grid-tied meter housing, then feed the two 200A panels from that? Having a meter on the inverter output would be a nice touch.

I'll note that everything for this part of the project will be installed indoors in a finished 2-hour rated room in the basement of the house, close to the existing service entrance. (~8 ft) The existing service entrance is at the basement level as the rear of the house is lower than the front with the rear wall of the basement being an outside wall.

Anyway, beyond this particular area I think I have the rest covered. I'm just not sure the best way to combine the inverter outputs in a way that allows a legal energizing of two 200A (or 150A if needed) panels.

2011 NEC applies in my area with no local or state changes that should matter here I believe.

Thanks in advance for your advice!

-k
 
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Old 10-22-14, 05:59 PM
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Welcome to the forums!

Your post is a bit over my head but I can tell you this: You are better off installing 200 amp panels than 150 amp. The panel rating is the max amps total the panel can output to the branch circuits. It is not what the panel is using. Larger is better.

That said, I would not suggest installing over a 200 amp panel. 400 amp panels will be panel boards, require bolt in breakers, and would be much more expensive then what you need. (Panel boards will fit in a 2x6 wall.) I would stick with multiple 200 amp panels.

To connect the inverters to the panels, I would think you could either split the load between the two, or combine them all in one (AC Out Bus in the diagram) and tap a larger feeder to each 200 amp panel.
 
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Old 10-22-14, 06:44 PM
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To connect the inverters to the panels, I would think you could either split the load between the two, or combine them all in one (AC Out Bus in the diagram) and tap a larger feeder to each 200 amp panel.
As an option you might be able to install double barrel lugs on the inverter and wire the two 200 amp panels in parallel.
 
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Old 10-22-14, 07:07 PM
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400 amp panels will be panel boards, require bolt in breakers, and would be much more expensive then what you need.
Good call. I'll stick with the two 200A panels on the home loads for sure, assuming it's doable. I was thinking I may need the 400A for the AC Out Bus, though. See below.

To connect the inverters to the panels, I would think you could either split the load between the two, or combine them all in one (AC Out Bus in the diagram) and tap a larger feeder to each 200 amp panel.
Splitting the load between the inverters is not ideal since this would limit the power saving capabilities of using them with a combined output (most inverters can "sleep" under low load, for example), plus would complicate that setup requiring additional control hardware.

The AC Out Bus in the diagram I linked is basically a 200A load center, according to the rep I spoke with at Outback Power. To do the same with seven inverters I believe I would need the 400A panel, since the total output would exceed the bus bar of every 200A panel I've found.

As an option you might be able to install double barrel lugs on the inverter and wire the two 200 amp panels in parallel.
Well, there are seven outputs. I need to combine them and split off to the two 200A panels, legally... that's the dilemma.

If there is a legal way to combine the outputs of the inverters using some kind of AC Bus, that would be ideal. Since they have their own local 50A OCPD on each inverter that would be possible from a technical standpoint, just not sure about code wise.
 
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Old 10-22-14, 07:36 PM
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How does each inverter mach each others sine wave output?

Splitting the load between the inverters is not ideal since this would limit the power saving capabilities of using them with a combined output (most inverters can "sleep" under low load, for example), plus would complicate that setup requiring additional control hardware.
As electricians, we split up loads between panels all the time, and by splitting up the loads, your power saving would be better rather then powering a single 400 amp feeder. Since the inverters would be in parallel wouldn't they all try to power the feeder when there is a load?

If there is a legal way to combine the outputs of the inverters using some kind of AC Bus, that would be ideal. Since they have their own local 50A OCPD on each inverter that would be possible from a technical standpoint, just not sure about code wise.
I am only going by your diagram. It shows the output of each inverter to a 50 amp breaker in a 200 amp panel. Your are basically back feeding the 200 amp panel through the bus bars to a 200 amp feeder, then to a 200 amp panel main breaker, then to your branch circuits.

You could do the same thing but this is where the 400 amp panel would come in. It would go:
Inverter(s) -> 50 amp two pole breaker(s) -> 400 amp panel board with main breaker -> 400 amp feeder in a gutter -> 200 amp panel(s) taped off of 400 amp feeder.

To answer your code question, look at Art 690 of the NEC. The only codes I can think of right now is the requirements for disconnects and overcurrent protection devices, which I think you have covered.
 
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Old 10-22-14, 07:38 PM
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You have about 1 day of battery backup? I hope you live in San Diego...
 
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Old 10-22-14, 08:28 PM
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1 day? Definitely not using 170kWh per day... probably closer to 80kWh per day at the top of the mark, which gives me a minimum of two days. In practice probably more like 3-5 days.

Keep in mind that these inverters can also be used for recharging the battery pack from the grid in the event that is needed. Essentially my own net metering, without ever feeding power to the utility.

How does each inverter mach each others sine wave output?
I suppose a little background on the inverters is in order.

The inverters are all connected to a hub and a central controller. All inverters match the sine wave of the others while powered up.

To save power and increase efficiency, the inverters stay powered down (sleeping) until the load reaches certain thresholds, generally in increments of 50% of the rated output of the inverter. Each inverter has two 4kW modules.

So, for example, with a minimal load, say 1kW, only one 4kW module on the first inverter would be active, the other in that inverter and all of the rest of the inverters would be sleeping.

As the load passes 2 to 3 kW, the second module would power on in the first inverter and share the load between those two 4kW modules.

As it approaches 4-5kW, the first module in the second inverter would power on, etc etc and so on and so forth until all modules are powered on in all inverters.

The modules can power on nearly instantly to meet demand. They will all be powered on at 50% load and remain powered until the load decreases. Obviously they can handle 100% load, but at less efficiency overall.

In any case, splitting the inverters into groups would require first getting a second hub and main controller. Second, it would decrease the efficiency of the system, requiring more inverter modules to be powered during average use than needed. It would also reduce redundancy, since if one inverter fails in a smaller group the % of max load decreases substantially more than if one inverter failed in the larger setup.

Also, in a split setup, the sine wave can not be sync'd between the two setups. I don't think this is an actual issue, but, just a note.

Finally, and probably most importantly, the reverse operation of the inverter as a charger for the battery pack from grid power becomes more complicated in a split setup since the two setups can't synchronize their outputs to the battery pack.

So, one single output split to the two panels would be ideal.
 
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Old 10-23-14, 08:21 AM
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If there is a legal way to combine the outputs of the inverters using some kind of AC Bus, that would be ideal. Since they have their own local 50A OCPD on each inverter that would be possible from a technical standpoint, just not sure about code wise.
You might consider a power distribution block for your 7 - 50 amp line connections where you can also get 2 - 3/0 connections for the load side to feed 2 - 200 amp panels. Here is a block you might consider. With this 1-pole block the Line side would become your Load and will accomodate 2 - 3/0 conductors. The Load side of the block will become your Line and will accomodate up to 12 - #6 conductors. That would give you room to add up to 5 more inverters for future expansion. I would use 3 of these 1-pole blocks for your 2 hots and neutral connections between the inverters and 2 - 200 amp panels. Take a look, what do you think?

569051 - ERICO - Power Distribution Blocks | Galco Industrial Electronics
 
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Old 10-23-14, 01:09 PM
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Wow, that is fantastic. Would be perfect for my project, assuming using three of those (one per phase + neutral) in a junction box would be permissible under code.

Inverters (x7)->50A OCPD (x7)->distribution blocks (L1,L2,N)->two 200A load centers. If this is legal it solves my dilemma, thanks!
 
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Old 10-23-14, 01:43 PM
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Might be overkill, but, this one piece product is stocked at a local supply house and may work also.

MPDB69113 - Mersen, formerly Ferraz Shawmut - Power Distribution Blocks | Galco Industrial Electronics

It's 3-pole, 760A, 12 outputs (14 to 4 AWG) 2 inputs (4 AWG to 500 KCM).

I'm still curious as to if this violates any code... been parsing through relevant articles in the NEC trying to figure it out but haven't come to a conclusion yet.
 
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Old 10-23-14, 07:02 PM
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I believe the 760A 3 pole distribution block would work just fine, but it is somewhat more expensive and larger then using 3 - 400A 1 pole blocks. Yes, the blocks would have to be mounted in a large junction box. A 36" X 24" box with hinged cover would probably work. I'd look at CT cabinets with a plywood backboard to make mounting the blocks easy. I dont know of any codes they would violate, this is what they are used for.
 
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Old 10-23-14, 07:40 PM
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I think I may try to find an enclosure that I could mount the block in along with two 200A OCPDs on the output side. Not sure if that would be required... but it'd make me feel better about the setup.

Any suggestions on such an enclosure?

Thanks again... do you accept tips?
 
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Old 10-24-14, 06:40 AM
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Any suggestions on such an enclosure?
Sure.

A 36" X 24" box with hinged cover would probably work. I'd look at CT cabinets with a plywood backboard to make mounting the blocks easy.
A 36" X 36" CT Cabinet might be easier to find at a supply house. I do recommend the factory installed paywood backboard for easy mounting if you can find one.

Hoffman: Product Catalog : Contractor : Current Transformer, Hinged Cover, Type 3R

Hoffman: Product Catalog : Contractor : Hinged Double-Door with Three-Point Latch and Wood Panel

I think I may try to find an enclosure that I could mount the block in along with two 200A OCPDs on the output side.
If the 200 amp loadcenters have main breakers, I'd forgo the 200A OCPDs in the junction box and mount the loadcenters adjacent to the junction box.
 
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Old 10-24-14, 08:18 AM
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Oh dang, thanks. I completely missed your earlier suggestion, sorry!

As for mounting it near the two 200A panels I'm not 100% sure that is possible without moving the panels much further than planned. The room where the inverters and everything is getting setup is not quite close enough to the original panels I'd think.

If the code is anything like how service entrance OCPDs/disconnects need to be, I'll probably need a 200A OCPD for each panel on each side to be safe since it will be more than 5ft from the panels. About 15 ft of cable probably.
 
 

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