Live Ground Shocked 5 Year Old

Eliminate the worry of lopsided split phase loads from an auto-transformer . . . Here you go, E-bay, 25KVA, tiny little thing $562.50, $2000 shipping (just kidding). Totally isolates one side from the other, just don't pull over 25KVA all together and you are good to go!

You wire the 240 to both of the feed windings in parallel take your output in series, and grab the CT for Neutral! I helped someone wire up a smaller one for a machine shop with a finicky CNC moons ago. If you have room it's as robust as you are going to get. Find someone tearing down a building you might find something nice-priced. They make "smaller" ones 15KVA, and down. Some of the guys here might have some reason to shoot it down, dunno, fixed the CNC issue back in the day.

ksmithaz1 said:

Eliminate the worry of lopsided split phase loads from an auto-transformer . . . Here you go, E-bay, 25KVA, tiny little thing $562.50, $2000 shipping (just kidding). Totally isolates one side from the other, just don't pull over 25KVA all together and you are good to go!

You wire the 240 to both of the feed windings in parallel take your output in series, and grab the CT for Neutral! I helped someone wire up a smaller one for a machine shop with a finicky CNC moons ago. If you have room it's as robust as you are going to get. Find someone tearing down a building you might find something nice-priced. They make "smaller" ones 15KVA, and down. Some of the guys here might have some reason to shoot it down, dunno, fixed the CNC issue back in the day.

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Price is good.
Other than size and weight, it's perfectly fine.

timselectric said:

Price is good.
Other than size and weight, it's perfectly fine.

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Some units are outdoor rated. When under consideration, I was going to pour a small pad and put it outside. If your stuff is "in the barn" in a rural setting, and you are committed to a 220v system and the space is not an issue, I'd poke around where people are remodeling/demoing commercial buildings. The one I found a while back, was literally sitting in a pile to go to the scrap yard. I didn't have any way to lift it on my truck, or it would probably be sitting in my back yard rusting

ksmithaz1 said:

Some units are outdoor rated. When under consideration, I was going to pour a small pad and put it outside. If your stuff is "in the barn" in a rural setting, and you are committed to a 220v system and the space is not an issue, I'd poke around where people are remodeling/demoing commercial buildings. The one I found a while back, was literally sitting in a pile to go to the scrap yard. I didn't have any way to lift it on my truck, or it would probably be sitting in my back yard rusting

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Yeah, I used to come across them all of the time.
When remodeling a space. The existing equipment is rarely reused. Unless it can all remain in its original location.
Now, everything I deal with is 3-phase. (Large commercial and industrial)
But, I could have a buddy grab me one, if I wanted it.

ksmithaz1 said:

I had a quote for about $1500 shipped for an actual dry transformer,brand new, 200A, 480/240 with CT output ... that gets you 240 with a center tap, split phase out.

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Any typical transformer you buy is likely to be a nasty load on your inverter, excessive no-load current:


Only a few designed for the purpose draw low current. I could use two 240/480V to 120/240V to perform the isolation function more efficiently, but operating them at 1/2 of rated voltage.

ksmithaz1 said:

I never did get a definitive answer on whether it would help with the HVAC surge,

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A transformer won't help with surge current, except if you're trying to start a 120V load from two 120V inverters stacked for 120/240V, because then the transformer takes power from both and delivers to the 120V load.

ksmithaz1 said:

Eliminate the worry of lopsided split phase loads from an auto-transformer . . . Here you go, E-bay, 25KVA, tiny little thing $562.50, $2000 shipping (just kidding). Totally isolates one side from the other, just don't pull over 25KVA all together and you are good to go!

You wire the 240 to both of the feed windings in parallel take your output in series, and grab the CT for Neutral! I helped someone wire up a smaller one for a machine shop with a finicky CNC moons ago.

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Works fine fed by the grid.

ksmithaz1 said:

If you have room it's as robust as you are going to get. .. Some of the guys here might have some reason to shoot it down

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Me! Me!

Hedges said:

I agree measurements are most important, "ground truth".
Theory can be entirely wrong.

I just did measurements to back my theory:

Auto-transformer L1/N/L2 current

Transformers aren't well understood by most people who use them. It is well known that centertap "Neutral" connection carries the difference between L1 and L2, but that is not at all true for autotransformers. It is only true for the case of an isolation transformer where all power is delivered...

diysolarforum.com

Please link your measurements there, I can't remember where they are.

Once we work out how these animals actually behave and how to protect them, we can give OP our suggestions.

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In this load test, did you try putting TWO loads, on opposing hots then measuring the load on N?

Supervstech said:

In this load test, did you try putting TWO loads, on opposing hits then measuring the load on N?

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Doing that is kinda pointless. That just routes current directly into loads and bypasses the transformer.

AntronX said:

Doing that is kinda pointless. That just routes current directly into loads and bypasses the transformer.

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No, not pointless, it would show how much is on the neutral alone, with balanced loads of 120V

He is suggesting using 12.5 to 15A breaker to feed the autotransformer, and i am asking for his data to show if loads are balanced is there any load on the neutral.

Supervstech said:

and i am asking for his data to show if loads are balanced is there any load on the neutral.

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There would not be load on neutral if loads are balanced. But @Hedges point is making transformer protected against worst case unbalance. Without clever use or triple pole 25A L1,L2,N breakers or some electronic trip solution the only simple electro-mechanical way to ensure autotransfomer is not overloaded is 12.5A L1,L2 breaker. Another solution is to parallel 2 identical autotransformers.

Why is this so difficult, double pole breaker immediately after the inverter before the transformer then everything is protected. Why are y'all making this so complicated.

Quattrohead said:

Why is this so difficult, double pole breaker immediately after the inverter before the transformer then everything is protected. Why are y'all making this so complicated.

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I think that everyone agrees with that.
The difference of opinion is regarding that breakers size.

No opinion required, the size of the wire determines that, as we all should know.
I just do not know why we are still talking about transformers, all this is bloody dangerous in the hands of the general public.

Quattrohead said:

No opinion required, the size of the wire determines that, as we all should know.

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The equipment connected to the wire must also be protected.

Supervstech said:

No, not pointless, it would show how much is on the neutral alone, with balanced loads of 120V

He is suggesting using 12.5 to 15A breaker to feed the autotransformer, and i am asking for his data to show if loads are balanced is there any load on the neutral.

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Of course that would be no current on neutral. I didn't test it, would have seen some current due to slight differences in load resistance or winding turns count.

The goal is to protect transformer while allowing user a wide range of 120V loads. They could put 4x 1500W space heaters on L1 and 4x 1500W on L2, zero current on N from auto-transformer. But as thermostats cycle, imbalance appears and transformer is overloaded.

Quattrohead said:

Why is this so difficult, double pole breaker immediately after the inverter before the transformer then everything is protected. Why are y'all making this so complicated.

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12kW of inverters, 3kW transformer.
If you put 12.5A 2-pole breaker after inverter, user can have up to 3000W load total on any combination of 120V L1/N, 120V L2/N, 240V L1/L2. But they can't use more than 3000W, and they bought 12kW of inverters.

If you put larger than 12.5A 2-pole breaker in L1/L2 then they can have larger loads. But imbalance greater than 3000W causes transformer overheating and failure.

timselectric said:

I think that everyone agrees with that.
The difference of opinion is regarding that breakers size.

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Although some people thing 25A 2-pole breaker would protect transformer.

AntronX said:

Without clever use or triple pole 25A L1,L2,N breakers or some electronic trip solution the only simple electro-mechanical way to ensure autotransfomer is not overloaded is 12.5A L1,L2 breaker.

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Clever is what we're all about.

AntronX said:

Another solution is to parallel 2 identical autotransformers.

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Or one big honkin' autotransformer.

My 25kVA 240/480V to 120/240V transformer can carry 50A in each of two 240V primary windings.
So I connect them in series (480V configuration), apply 240V up to 50A (12.5kW), and draw up to 100A at 120V between center-tap N and either L1 or L2.

That solution is 100% rated for his 2x 6kW inverters.
Cost me somewhere between $300 and $600 used, plus a day's drive each way to San Diego and back.

Quattrohead said:

No opinion required, the size of the wire determines that, as we all should know.

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Although ampacity of enamel wire wrapped in a tight coil isn't immediately obvious to me.

And remember each coil has a wire pigtail on each end. Do you pull center tap "N" out as those two wires, or join together and bring out one (undersized) wire.

Quattrohead said:

I just do not know why we are still talking about transformers, all this is bloody dangerous in the hands of the general public.

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Because Victron did it right, then Solar Edge, Growatt, and others said, "Me too!"

timselectric said:

The equipment connected to the wire must also be protected.

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The equipment should have specifications for ocp min/max...

Supervstech said:

The equipment should have specifications for ocp min/max...

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Do we see that for the Growatt autotransformer, or any others that don't have protection built in?

If whoever we are talking about has the wrong size transformers for the inverters then I don't know why we're having this discussion.
European/rest of world growatt inverters and transformers have no place in the split phase world, it's inherently too dangerous to make work properly.

Hedges said:

Although some people thing 25A 2-pole breaker would protect transformer.

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Absolutely
If the transformer is wired in series with the load. Like in this thread.
Maybe not if "T" tapped at the loads panel.

Hedges said:

Do we see that for the Growatt autotransformer, or any others that don't have protection built in?

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The Growatt doesn't have an input rating, on the unit. (Maybe listed online somewhere)
Solar Edge does.

Someone correct me if im wrong as ive no experience with growatt inverters or interconnecting them but from my knowledge this is how it should be wired in its current config id refer to @timselectric for proper bonding to earth. fuse as needed with your 2 growwatts feeding 1 2 pole breaker, then a single pole breaker feeding the auto transformer, autotransformer feeding the main lug of the right hand panel.