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Live Ground Shocked 5 Year Old

Hedges said:
I disagree. You put 12.5A in L1 of an auto-transformer. That flows and supplies 12.5A to center tap N of transformer and load.
Magnetic coupling sucks another 12.5A in L2 of auto-transformer (note I said "in", not "out", at any given instant it is flowing in opposite direction not same direction as current in L1), and supplies a second 12.5A for 25A in N.

The key here is to think in terms of current which is not "in phase" as you like to say about split-phase systems, but rather current at 0 degrees phase or 180 degrees phase. Same direction or opposite direction.

There is never 25A flowing in L1 and out L2. It is one big inductor, can't get more than a fraction of an amp to flow at 60 Hz. The only way to have 25A flow in L1 is to have 25A flow in L2. They are flowing in opposite directions. When the meet in the middle, they both flow out N for 50A. And that is 2x the transformer's rating.

12.5A I say it is.

This is NOT an isolation transformer, where current flowing in secondary allows current to flow in L1, out L2.
Click to expand...

Well... The conclusions are right but I'd phrase it differently.

Confusion arises because the two lines are at opposite phase, so if power IN on L1 is current IN, power OUT on L2 is ALSO current IN. Total current on L1, L2, and N has to be zero, so the current contributions from load-balancing power being transferred between L1 and L2 are twice on N what they are on each of L1 and L2.

The other current contributions come from losses, reactive magnetization currents, less than perfect balance between the two halves of the autotransformer winding, and less that perfect "mutual inductance" coupling between the two halves. These currents only flow between L1 and L2, not through N. But these currents are a drop in the bucket compared to those from correcting the maximum rated load imbalance. So let's ignore them for now. Without them the size of the L1 and L2 currents would be precisely equal and half the size of the N current.

But an imbalance of, say, 3000 watts means Lx is being loaded with 3000 watts more than Ly. To correct it the transformer needs to take half that, 1500 watts, from Ly and give it to Lx, letting Lx provide the other 1500. So the size of the current in L1 and L2 corresponds to the 1500 / 120 = 12.5A, in N it's 25A.

Worst case imbalance could be generated by a single 120V load pulling >25A and thus, with the autotransformer, putting a 240V load of >12.5A (plus a tad for transformer losses, etc.) on the inverter. So a dual-gang 13A breaker on the feed from the combiner breaker box to the 120V split-phase breaker box should protect the autotransformer - at the cost of limiting the total load on the 120V box to about 3000 watts (equivalent of about a single 25A 120V circuit).

It would be nice to have a remote trip breaker driven by a current sensor around the autotransformer's N wire and set to trip at 25A there. That would let you run all the loads the inverters would power and still drop power to the 120 and 240-with-neutral loads if the imbalance became excessive. But in a few minutes searching I didn't find anything ready-to-go.

I note that the only place a greater-than-L1-or-L2 current appears in the autotransformer is from the center-tap tie between the two coils out through the N terminal. Everything else carries either L1's or L2's current. Since the two coils seem to be joined AT the N terminal, and N's current is essentially JUST proportional to the imbalance, which is rated, I don't see any need for special thinking about protecting N's current specifically, rather than just meeting the imbalance specification.

I also note that this is basically exactly a transformer. Like a motor winding, it has some thermal mass and takes a bit of time to warm up. So motor-rated breakers that allow a half to a few seconds of moderate overload should be fine.
 
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Hedges said:
Did you post this regarding breakers in a split-phase panel, possibly MWBC? Or regarding auto-transformer?
Click to expand...
Just split-phase in general. Because people think that it's two phases 180 degrees out of phase with each other.
Hedges said:
In an auto-transformer, only low (magnetization?) current flows from L1 to L2. Whatever inductance allows.
Click to expand...
Absolutely
Hedges said:
With a 120V load between N and one of the two Ln, current flows from L1 to N and into the load, and simultaneously flows from L2 to N (swimming upstream) and into the load. When polarity of AC voltage reverses, current flows from load and then from N to L1, and same/similar amount of current flows from load and then from N to L2.

Kirchhoff's current LAW holds true. Two coupled windings and the center tap N join at a node. Sum of currents into that node is always zero. If 25A flows out of N, then 12.5A flows into L1 and 12.5A flows into L2.

I've demonstrated this on the bench. Cmiller is going to replicate or refute my results. You previously refuted them, but I need to see your no-load current (center tap N disconnected from load) to see if you just had a very non-ideal transformer and too light a load.

I think we will all agree about Kirchhoff's current law. Exact values of L1, L2, N current will vary. The fundamental behavior of coupled windings is what I have been arguing about.
Click to expand...

This is my understanding.
240v Input current flows between L1 and L2.
120v Output current flows between either L1 or L2 and N.
I will describe one side (L1) and just say that the other is happening in reverse at the same instance.

On the first half of a cycle.
Current flows in L1 and out N.
Half of the load current is physically flowing through the L1 coil, and the other half is induced from the L2 coil.


On the second half of the cycle. (Reverse flow)
Current flows in N and out L1.
Half of the load current is physically flowing through the L1 coil, and the other half is induced from the L2 coil.

When the loads are balanced, an autotransformer is just idle.
No current flows in or out of L1, L2, or N.
Other than a small amount of saturation current.

We agree that the 120v loads current inside the autotransformer is divided in half on the 240v feeding the autotransformer.

Were we disagree on the feed breaker size, is on how the actual loads will be presented to the breaker.

I say that it depends on how the autotransformer is connected.
In line with the feeder, I say protect the feeder. (As it will be carrying the most current)
Or "T" tapped at the loads, I say protect the neutral. (As it will be carrying the most current)
You say protect at the neutrals rating in both cases.
 
ULR said:
Like a motor winding, it has some thermal mass and takes a bit of time to warm up. So motor-rated breakers that allow a half to a few seconds of moderate overload should be fine.
Click to expand...
This is why the Victron Autotransformer has a solenoid trip for its main breaker based on toroid temperature, not Neutral current.
Short excessive unbalances over 30A do not trip it. They claim 28A continuous depending on ambient temp.
 
timselectric said:
If using the pass through, it can be as much as 200a.
Click to expand...
Correct. The issue lies in a power outage scenario. I don't believe that it is best practice to have more passthrough loads than what an inverter can handle during a grid outage. You run the risk then, of having a complete power failure when grid is down, due to inverter overload. Kinda defeats the whole point of a "hybrid" inverter with batteries.

Just because it's possible doesn't mean its practical, advisable, or best practice.

We do, by the way, from time to time run 200A passthrough on the 15ks. In fact today we did a 15k "whole house backup" setup. We also "locked out" the aux heat coils (during grid outage) on the furnace to avoid overloading the 15k!
timselectric said:
It just depends on what you feed it from.
You could limit the input with a reduced feed.
And this could keep you from overloading the panel.
Click to expand...
You are right that it depends on what you feed it from. I would put money on it, that the average double or triple 15k install does NOT have more than a 200A electric service to start with. So then you may as well only run 100A through each inverter... and in the downstream panel you won't want more than 200A anyway, unless you are splitting to 2 panels.
timselectric said:
Every situation is different. And anything can be made to work.
I'm not saying that you can't make it work. I'm saying that I wouldn't recommend doing it that way. And passing inspection with it has been difficult for people.
Because there are no provisions in the NEC for it. So, most inspectors don't accept it. If you are not getting it inspected, then you can of course do whatever you want. It can be done safely.
Click to expand...
If I had an issue with an inspector, I would bring up the NEC articles on how to size breakers for PV inverters. (Rated output x 1.25 and round up to nearest breaker size.) Sorry I don't know where it's found offhand. Don't have my code book in front of me. That would make the relevant breaker size for a 15k an 80A. (62.5 x 1.25 (78A) == 80A breaker)

Shouldn't make a difference if you are feeding "forward" or "backward" for lack of better terminology.

I have run into where the engineers said we need to install (2) 15Ks because the total loads were ~22kW. Since we felt confident that the loads wouldn't exceed one 15Ks capabilities (they overrated load draw for a number of items, plus not everything is on at the same time, ever...) we got them to combine the 15K rating with the standby gen rating (14kW) to get the number we needed. (Gen assist via grid peak shave....) I should note, this is a complete off grid install.
 
Hmm.... I better move on for now, but I'm thinking now that @Hedges is right on that 12.5A input to the autotransformer....

Now this will bug me until I get to the test tomorrow! 😆 I'll report back with numbers, pictures..... and idk what else. But all the data that I can easily and quickly provide!
 
timselectric said:
We are in total agreement on this.
Click to expand...

Specfically:
- MUTUAL inductance between the two halves forces the current from transferring power between L1 and L2 to be equal and both the same direction (and in N to be twice that and in the other direction), while:
- TOTAL inductance forces magnetization current (the bulk of the current in L1 and out L2) to be small, while:
- The remainder of the current in L1 and out L2 represents losses (resistive, hysteresis, eddy current, coil imbalance), which are all small.
 
ricardocello said:
This is why the Victron Autotransformer has a solenoid trip for its main breaker based on toroid temperature, not Neutral current.
Short excessive unbalances over 30A do not trip it. They claim 28A continuous depending on ambient temp.
Click to expand...

Good news/bad news.

First the good news: I've found a Square-D QO series shunt trip breaker!

QO2301021 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in, AC shunt | Schneider Electric USA

Schneider Electric USA. QO2301021 - Mini circuit breaker, QO, 30A, 2 pole, 120/240VAC, 10kA, plug in, AC shunt.
www.se.com www.se.com

Bad news: at $256 msrp It costs about as much as the Growatt autotransformer.
Like everything Schneider, street price usually less. $120 at the slowest shipping supplier I have ever encountered:



Victron auto-transformer with 100A pass-through is a good way to go at $509

www.currentconnected.com

Victron Autotransformer - 120/240V - 32A or 100A | Current Connected

Enhance safety and efficiency with the Victron Autotransformer. Step up, step down, and balance your split phase output effortlessly.
www.currentconnected.com www.currentconnected.com

Or for similar money, pick up a used 25kVA transformer and use its 240/480V primary windings as auto-transformer. Then you can have 100% of inverter wattage feed 120V load. Just make sure breaker panel and wires can handle the current (2x the inverter current.)
 
If you want to protect at the neutral output current. A 3-phase motor starter could be cheaper. Using the center pole for the neutral.
 
timselectric said:
We agree that the 120v loads current inside the autotransformer is divided in half on the 240v feeding the autotransformer.

Were we disagree on the feed breaker size, is on how the actual loads will be presented to the breaker.

I say that it depends on how the autotransformer is connected.
In line with the feeder, I say protect the feeder. (As it will be carrying the most current)
Or "T" tapped at the loads, I say protect the neutral. (As it will be carrying the most current)
You say protect at the neutrals rating in both cases.
Click to expand...

Maybe the difference is circuit topology.

I've been saying 12.5A 2-pole breaker feeding autotransformer + loads was necessary to protect it. (e.g. only the 2-pole breaker on left of this drawing.) that's upstream breaker.

What I just realized is, if we feed the autotransformer only with 240V from source, then daisy chain off its L1/L2 to a sub-panel with backfed 25A transformer breaker, and also connect centertap of transformer to neutral bus of that panel, then autotransformer is protected. Current drawn from that panel will always have |N| = |L1 - L2|, Neither L1 nor L2 can exceed 25A so N can't exceed 25A. That allows loads to be 25A 120V on L1, or 25A 120V on L2, or 25A 240V on L1 & L2. that's downstream breaker.

While it doesn't let you run loads willy-nilly like the Victron transformer with 100A pass-through breaker, it is probably good enough for most DIY work.


1712705056701.png
 
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ricardocello said:
Never mind! $120 is actually not awful at Gordon.
Click to expand...

No, but (sometimes) 6 month delivery time is.

Their policy is to ship next day for about $250 from any given supplier, after several weeks for smaller orders.
But if one little item is out of stock, the entire order is held back without any communications.
 
Joshua787 said:
Exactly the problem I face here in Puerto Rico. Ive called multiple electricians and as soon as i say offgrid or solar theyre turning their backs on me. This Forum has been the BEST source to fix my problem. This is my very first time posting or reading from a Forum and its been an amazing experience.
Click to expand...
When I first got started in 2019-20 buying stuff and watching Wills early videos , I didn’t have a clue what I needed to buy… . so I bought all Victron .BB and Blue Seas stuff as he was into that stuff at that point..

After I got it, 100% of the electrical companies I called to hook it up Said NO… or never called me back.. didn’t even want to talk about it…and there was no solar companies at all other than in Charlotte … 2+ hours away.

the one veteran electrician ( nice guy sorta) who said he would come out , wanted to see it “after I got it hooked up and finished” as he was unaware how to even mess with solar ..and was curious …but there would be a 85 dollar trip fee for coming out…😵‍💫.

Holy Moly ..

I decided to just dive in , learn and do it my self…
With a lot of help from many here ,it has worked out fine…

J.
 
Hedges said:
Maybe the difference is circuit topology.

I've been saying 12.5A 2-pole breaker feeding autotransformer + loads was necessary to protect it. (e.g. only the 2-pole breaker on left of this drawing.) that's upstream breaker.

What I just realized is, if we feed the autotransformer only with 240V from source, then daisy chain off its L1/L2 to a sub-panel with backfed 25A transformer, and also connect centertap of transformer neutral bus of that panel, then autotransformer is protected. Current drawn from that panel will always have |N| = |L1 - L2|, Neither L1 nor L2 can exceed 25A so N can't exceed 25A. That allows loads to be 25A 120V on L1, or 25A 120V on L2, or 25A 240V on L1 & L2. that's downstream breaker.

While it doesn't let you run loads willy-nilly like the Victron transformer with 100A pass-through breaker, it is probably good enough for most DIY work.


View attachment 208294
Click to expand...
Yep. Lol. This is the direction I was starting to think. 👍
 
Hedges said:
Maybe the difference is circuit topology.

I've been saying 12.5A 2-pole breaker feeding autotransformer + loads was necessary to protect it. (e.g. only the 2-pole breaker on left of this drawing.) that's upstream breaker.

What I just realized is, if we feed the autotransformer only with 240V from source, then daisy chain off its L1/L2 to a sub-panel with backfed 25A transformer, and also connect centertap of transformer to neutral bus of that panel, then autotransformer is protected. Current drawn from that panel will always have |N| = |L1 - L2|, Neither L1 nor L2 can exceed 25A so N can't exceed 25A. That allows loads to be 25A 120V on L1, or 25A 120V on L2, or 25A 240V on L1 & L2. that's downstream breaker.

While it doesn't let you run loads willy-nilly like the Victron transformer with 100A pass-through breaker, it is probably good enough for most DIY work.


View attachment 208294
Click to expand...
This is were I was. Because of how the Growatt autotransformer is wired. Input and output, instead of "T" tapped at the loads panel.
 
timselectric said:
That's fine,
Except that the grey wire in the drawing, between the inverters and transformer, should be black. So that it's not confused with a neutral.
Click to expand...
That’s fine? I’m confused, in his drawing he has L2 going into neutral busbar. In my set up I have L2 going into a 30a breaker which feeds phase 2 on my left panel.

Did I do something wrong?
 
Hedges said:
I think the comments laid it out pretty clearly. But commenters aren't authoritative, just as we forum members aren't.

"I've heard them called "slash rated". The lower rating is phase to ground voltage, the higher is phase to phase. As Bird dog said, the issue comes up in wanting to use the high leg on a 3P4W delta for a single phase 208v load; the breaker has to have a phase to ground rating of at least 208v, which a slash rated breaker does not."

"120/240V Slash rated breakers are suitable for 1Φ or 3Φ loadcenters, where the highest voltage to ground is 120V and the max voltage phase to phase is 240V. Also used in 120/208V 3Φ 4W Wye systems.



240V Straight rated breakers are suitable for 1Φ or 3Φ loadcenters, where the highest voltage to ground is 240V and the max voltage phase to phase is 240V. Such as in a Center tapped delta system, where the wild leg is 208V to ground.


Most 3P breakers come standard with a straight 240V rating.
2P units can come either way, normally slash rated, but straight rating is required if your connecting to a 208V wild leg.


BR240H is an example of a 2P unit with a straight rating, for use on a center tapped delta system."
Click to expand...
Moot point, since slash breakers wont fit in a high leg panel...
 
What is a "high leg panel"?
How is it different from any QO 3-phase panel?
 
Joshua787 said:
That’s fine? I’m confused, in his drawing he has L2 going into neutral busbar. In my set up I have L2 going into a 30a breaker which feeds phase 2 on my left panel.

Did I do something wrong?
Click to expand...
Good catch.
You're absolutely right.
I missed that, thanks.
That drawing is all kinds of wrong.
 
Come to think of it. I have never specified slashed or non slashed breakers, when ordering.
I just order the brand, number of poles, and voltage level. (250v or 600v)
Maybe one or the other breakers are a special order thing?
 
 
Hedges said:
What is a "high leg panel"?
How is it different from any QO 3-phase panel?
Click to expand...

It's an ordinary 3-phase panel fed from a 240V "high-leg delta" utility feed.

The neutral is fed, not from the midpoint of the phase triangle, but from the
center-tap of one (the "lighting") pair of phase legs. So while those two
are at 120V from neutral, the third ("high") leg is at 208V.

By convention the high leg is wired to the middle (B of A-B-C)
feed of the panel, coming out on the second, fifth, seventh, etc.
row of breaker positions. You can feed 120 V circuits from A or C,
240V circuits from any pair (but usually A and C), and three-phase
from all three as usual.

You can usually spot a high-leg panel because it has 120V
circuits but the B leg positions do not have them, but only a
3-gang breaker or blank.

(High-leg color code is supposed to be orange.)
 
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