SolarEdge Auto-Transformer W/ Sunny Island 6048

For SI to 120 VAC loads, the AT "does not do anything".

Where current (power) will flow is if you have, for example, your GT Inverter connected to L1/L2, and you pull power from L1/Neutral, to either backfeed the SI to recharge the battery bank, and/or to supply your 120 VAC loads on L1/N. And if you have 120 VAC loads on L2/N circuit too. As long as L1/N/L2 current flows are never more than 25 Amps each, you should be OK.

Note that only L1 and L2 need a double pole breaker on the Transformer leads (the type used on 120/240 VAC split phase North American branch circuits). You do not need a breaker on the Neutral Line, as N=L1-L2 currents.

The SE Inverter configurations show that they placed the double pole breakers on the L1/L2 output to the AC loads (not on the Auto Transformer directly.

Probably need to think about the A or B location of the 25 amp breakers... The 6048 SI Inverter is capable of something like 50 Amps max continuous output, and I would be suggesting something like 50a*1.25NEC-derate=~63amps (to avoid false trips if you really did have loads in the 40-50 amp range on the 120 VAC circuit. Following the Sunny Island Inverter manual/installation guides as always to ensure SI is "safe".

If you had a 25 amp single pole breaker from the 50 Amp SI Inverter output, that 25 amp branch circuit goes to the Auto Transformer, And L1+L2 auto transformer to loads/GT inverter 240 VAC output... That limits the SI Inverter to 25 Amps @ 120 VAC maximum on the AT branch circuit (combination of 3,000 Watt output "on battery" and a maximum of 3,000 Watts charging from the GT inverter(s) (25 amps @ 120 VAC).

You do have the possibility of SI Inverter 3,000 Watts + another 3,000 Watts (or more) from the GT inverters on the 240 VAC L1+L2 circuits (max of 25 amp difference between L1 and L2 for Auto Transformer limit)... But that is only when there is enough sun (mid-day), and the SI Inverter is not drawing 120 VAC power for charging.

Are you planning on having "oversized daytime only loads" (like water pumping with a 240 VAC pump)? The configuration between the different inverters and externally supplied power--Gets complicated. The Sunny Island is certainly very configurable... I don't know about the details or limitations.

-Bill

I want to be very clear... The SB Inverter is 120 VAC... And in the USA, we would say there is an L1 and a N (Neutral) output). Neutral is typically ground bonded (grounding rod, cold water pipe, etc.). L1 (and L2, etc.) are "Hot" with respect to ground/neutral...

So, from the SB Inverter, you would have L1 and Neutral, 50 Amps maximum or 6,000 Watts (P=V*I=120 Volts * 50 Amps = 3,000 Watts).

The 240 VAC GT inverters have an L1 and L2 output, 240 VAC between them. And usually no Neutral output (there may be a neutral connection for a 120 VAC to Neutral "sanity check", but there is not "split phase" transformer output to Neutral). So, if you had 6,000 Watts of solar, that is 6,000/240 VAC = 25 Amps in L1 and L2.

The Auto Transformer has three power connections. L1, L2, and Neutral. The SB Inverter connects to L1 and Neutral (at 120 VAC), and the GT inverter (high current connections) connect to L1 and L2 (at 240 VAC).

And the transformers are constant power devices... Power In = Power Out... And Power=Voltage*Current.

So, if you dump 6,000 Watts @ 240 VAC into the Auto Transformer, that is 25 amps on L1+L2. If you try to pull 6,000 Watts out, then that is 50 Amps on L1+N (6,000 Watts / 120 VAC = 50 Amps) going to the SB inverter/120 VAC "main panel". But the Auto Transformer is only rated for 25 amps per leg maximum.

I have to be a little careful here... Your GT inverters are rated at 6000 Watts, and the Auto Transformer is only rated at 5,000 Watts total/full load (or 5,000w/120=20.8 amps across L1+L2). Or 3,000 Watts (I think) across L1 and Neutral (120 VAC).

Note that L1 and L2 lines are just arbitrary designations... L1 is "no different" than L2 functionally... Just trying to keep using the same designations in the discussion to avoid even more confusing (like saying L1 and L2 on the SB Inverter AC output--I would call them L1 and Neutral @ 120 VAC... Where as L1 and L2 are 240 VAC across them... And L1 to Neutral, or L2 to Neutral is 120 VAC. And Neutral being the "center tap" of a transformer (as used in North America).

The only way you can get 5,000 Watts "through" the Auto Transformer (not 6,000 Watts), is to connect a 5,000 Watt source across L1 and L2 (240 VAC). And connect your 3,000 Watt (maximum) 120 VAC loads across L1 and Neutral for Branch Circuit 1, and a second Circuit 2 across L2 & Neutral--Also 3 kWatt maximum @ 120 VAC.

If you have a 240 VAC load and a GT Inverter @ 240 VAC source, then there is (near) zero current flowing through the auto transformer. L1&L2 current from GT Inverter to 240 AC load (such as your inverter).

If you have no GT inverter power (dark, etc.), then the SB inverter would supply 1,200 Watts to L1&N, (1200w/120v=10amps) and the Transformer would supply 1,200 Watts to the AC load through L1&L2 connections (1200w/240v=5amps through L1&L2).

And since the Auto Transformer is limited to a maximum of 25 Amps per leg, the input power the transformer can manage is 25a*120v=3,00w from the SB Inverter.

https://circuitglobe.com/what-is-an-auto-transformer.html

This drawing shows a "stepdown" confuguration, but Transformers are bi-directional, so it can also be a step up too. Your GT Inverter is on the left side (120/240 VAC L1/N/L2) and the right side is just 120 VAC to your SB Inverter (L1/N).



-Bill

Just for information... Here is a nice Outback Application guide for their auto transformer:

http://www.outbackpower.com/downloads/documents/integration_products/autotransformer/manual.pdf

Note that the windings used for the "Center Tap", are both brought out separately, and tied together at the bus bar. So, in this case, there are only 25 Amps on each lead entering/leaving the auto transformer. And not violating the 25 Amp per leg rule... (I think that is correct, I have not worked very much with Auto Transformers, so I cannot claim that this is an accurate statement or not).

Note that the "50A=I2" current path in the "Center Tap" connection (Neutral, typically "ground bonded neutral") in our discussion.

Note too that there are no breakers in the Center Tap/Neutral leads... In general, you do not want to circuit breaker "neutral" leads, If the neutral connection is opened, your 120 VAC appliances (if any) on the L2 and L1 to Neutral circuits can see anything between zero volts and 240 VAC. This is true of 120/240 VAC split phase power... A bad neutral connection in the main panel from the utilty drop to the main neutral bus bar will cause the same problem (0-240 VAC across any 120 VAC branch circuit).

The reality is that you will probably never use 100% of the 3/6/whatever kWatt ratings for your inverter/hardware (other than starting a well pump, etc. for short periods of time). I like to be conservative, so suggest that wiring/breakers/etc. be designed as if you are using 100% of rated power--Just to be safe. HOWEVER, when working with solar power (like your GT inverters, Battery Charging, etc.), you can expect many hours of running at rated output (i.e., 6 kWatt from a 6 kWatt GT inverter with >6 kWatt Pmp rated array, charging a battery bank from 50% state of charge, etc.)--In those cases, then it is very easy for an off grid battery based power system to run at rated load for an extended period of time (and why I highly suggest 80% or 1.25x NEC deratings. NEC is pretty conservative, but not for 100% of rated power for hours on end). For example, the typical NEC type breaker will usually never trip at 80% of rated current, but is supposed to trip at 100%+ of rated current (may take many minutes to many hours to trip). Don't want "false trips" to happen to you or your system:

• 15 Amp Breaker * 0.80 NEC derating = 12 Amps max continuous circuit loading
• 15 Amp Desired continuous current * 1/0.80 = 18.75 Amp minimum rated branch circuit/wiring/breaker (round up to 20 amps)
• 1/0.80 = 1.25 derating factors

In general for engineering, things that are less than 2x different are "almost the same". Things that are 10x different, that is a "night vs day" difference (and usually can ignore the 1/10th portion for calculations).

Also, be aware that Power equations for Voltage and Current are "squared" (P=V^2/R=I^2*R)... So that if you "double the current", you get 4x the power (or self heating effects in wiring/transformers due to resistance). So, in those cases square root of 2 (=1.414) can be used as the ceiling estimate of "don't care"...

In the case of a 30 lb vs 28 lb transformers--That is pretty much no difference. Of course, the design matters too (type amount of steel, amount of copper, cooling air flow, optional fan, etc.).

It would appear that your usage of 25 Amps max on the L1/L2, and 50 Amps on the Neutral/Center Tap can be correct... As long as the Auto Transformer center tap wiring comes out on "Two Wire" to a bus bar that is rated to be connected to a 50 Amp Center Tape Load. And, of course, the 240 to 120 VAC to Auto Transformer connection is rated for 50 Amps (50 amps from L1 120 VAC side, 25 Amps to Auto Transformer Wiring, and 25 amps to 240 L1 wiring).

And it probably does not matter in a larger installation like yours... But you do have transformer losses... If you assume 20 Watts typical no-load power consumption for the Auto Transformer, and run it 24 hours per day:

• 20 Watts * 24 hours = 480 Watt*Hours per day

That is about 1/2 the amount energy a refrigerator uses, or a good size laptop could consume if running most of the day.

With all of the above caveats, it does look like you could do what you needed.

Of course, this is your system and your home. If anything we talked about here does not look right, ask more questions, do further research. I (we) want you to be safe, and not having the experience/being there, operating at the "edge", etc., I cannot be sure of your installation/assumptions made/etc.--You are the person that needs to be 100% sure of what you do is correct and safe for you and your family.

-Bill