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Old 06-10-2016, 23:10   #61
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by DotDun View Post
This would be easily detectable. If the neutrals measure 120v, reverse them on one transformer. You should be able to combine the two and have 60A of power on the same line.
From what I have been told the issue is minor differences in manufacturing between two similar transformers results in significant unbalanced loads. The second transformer will often only be loaded to 60 percent or so.

For split phase power reversing the outputs would bring them into phase, but don't just hook up as could be a big pop.

For 208v circuits they will never be in phase as one is 120 degrees out from the other rather than 180.


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Old 07-10-2016, 07:34   #62
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by botanybay View Post
From what I have been told the issue is minor differences in manufacturing between two similar transformers results in significant unbalanced loads. The second transformer will often only be loaded to 60 percent or so.

For split phase power reversing the outputs would bring them into phase, but don't just hook up as could be a big pop.
For twin independent 30A shore cords I would NEVER suggest you try to parallel the transformer output to create a "60A" service. How would you know ahead of time whether the two 30A shore recepticles were on the same leg of the same phase, opposite legs of the same phase, or a completely different phase altogether? Sounds like a recipie for blown breakers and burned wiring.
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Old 07-10-2016, 09:49   #63
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Re: Question On Twin 30 Amp Shore Power Service

I learned quite a while ago that a weakness in my background for AC is in the business of phases, motors, and such for voltages other than 220 and 110.

I therefore am not an expert on any of this. That's my disclaimer. So I appreciate reading your various comments, but since there are some disagreements, I am left to decide if any of them pertain to me or not, and if they do, which are correct and which aren't. Another example of the usefulness of the KISS principle. The PO of my boat evidently did not subscribe to that principle so I am going through the boat and simplifying things as I go, where possible.

Currently I am leaving in the two Charles Instrument ISO transformers. Each is connected to separate 30A 110v inlets, each with separate double pole master breakers. And both are used to power completely different downstream panels where the hots are never combined. The neutrals are because both are bonded to ships ground which is something that seems to me to be the correct installation and the one that CI says should be done.

I guess I can see how the two inlets could be out of phase depending on how the dock is actually wired. My dock does not have 208v anywhere to my knowledge. I am not sure why it would and even if it did have for some use why they would wire the 30A or 50A circuits to that part of the system. There may be a reason which I am sure one of you will explain. Of course, other marinas and other countries may or may not do the same, even if not here.

My question if whether I would have a problem in any case when the two hots would never be combined under any scenario. I do have separate neutrals for each all the way to the panels and thus to the loads, but, as noted they are still both bonded to the same ground. I can see the value in having GFCI master breakers but I don't have them and may or may not ever put them in.

So, could two neutrals at different phases with separate hots which are both bonded to the same ground cause a problem if they never power the same circuits or share a panel?

That would be a good question for Charles as well and I may do that. Their customer support seems to be outstanding.
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Old 07-10-2016, 09:57   #64
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by DotDun View Post
This would be easily detectable. If the neutrals measure 120v, reverse them on one transformer. You should be able to combine the two and have 60A of power on the same line.
How would you do this in a practical manner?
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Old 07-10-2016, 10:37   #65
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by boatbod View Post
For twin independent 30A shore cords I would NEVER suggest you try to parallel the transformer output to create a "60A" service. How would you know ahead of time whether the two 30A shore recepticles were on the same leg of the same phase, opposite legs of the same phase, or a completely different phase altogether? Sounds like a recipie for blown breakers and burned wiring.
Exactly my point! While also noting to those that say why would it not work the reasons to never do this.

In violent agreement here😀

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Old 07-10-2016, 11:01   #66
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by exMaggieDrum View Post
I learned quite a while ago that a weakness in my background for AC is in the business of phases, motors, and such for voltages other than 220 and 110.

I therefore am not an expert on any of this. That's my disclaimer. So I appreciate reading your various comments, but since there are some disagreements, I am left to decide if any of them pertain to me or not, and if they do, which are correct and which aren't. Another example of the usefulness of the KISS principle. The PO of my boat evidently did not subscribe to that principle so I am going through the boat and simplifying things as I go, where possible.

Currently I am leaving in the two Charles Instrument ISO transformers. Each is connected to separate 30A 110v inlets, each with separate double pole master breakers. And both are used to power completely different downstream panels where the hots are never combined. The neutrals are because both are bonded to ships ground which is something that seems to me to be the correct installation and the one that CI says should be done.

I guess I can see how the two inlets could be out of phase depending on how the dock is actually wired. My dock does not have 208v anywhere to my knowledge. I am not sure why it would and even if it did have for some use why they would wire the 30A or 50A circuits to that part of the system. There may be a reason which I am sure one of you will explain. Of course, other marinas and other countries may or may not do the same, even if not here.

My question if whether I would have a problem in any case when the two hots would never be combined under any scenario. I do have separate neutrals for each all the way to the panels and thus to the loads, but, as noted they are still both bonded to the same ground. I can see the value in having GFCI master breakers but I don't have them and may or may not ever put them in.

So, could two neutrals at different phases with separate hots which are both bonded to the same ground cause a problem if they never power the same circuits or share a panel?

That would be a good question for Charles as well and I may do that. Their customer support seems to be outstanding.
It turns out that about half of the USA marinas are 208 and the other half 220

The reason is that it is about 300 dollars per slip cheaper to use two phases of a three phase transformer to provide two 110v legs which are 120 degrees out of phase.

The alternative is to use a transformer at the head of the dock (or somewhere else) to go from three phase to "split phase" which is what you have in your house. It is two 110v legs which are 180 degrees out of phase.

When keeping the two 110v panels separate there is little difference (there is a residual neutral current even if the loads are the same on each leg for the 208v case)

Now if you have a high voltage appliance (i.e. 208v or 220v) the equipment will see a voltage difference.

As for ELCI circuits (big gfi which allows a bit more differential on a primary feed) these are great.

For your system treat each transformer+panel as an independent system. Don't use the two hot wires to drive any 220v equipment.

Each system gets its own ELCI which monitors the current difference between that systems neutral and hot.

The neutral output of each isolation transformer is connected to ground right at the transformer. So yes, both neutrals are at the same potential and are tied to ground.

Beyond that point (prior to the independent ELCI units), each panel's neutral needs to be separated to ensure that the hot and neutral currents are equal except when the ELCI trips.

If you try to pull power from both hots to get high voltage in this case the ELCI will trip (a good thing).

Because you don't know (unless you test it) the relative phase of the two 30amp inlets the voltage between the two hots might be 0v, a few volts, 208v, or 220v.

ELCI are your friend and I highly recommend them😀

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Old 07-10-2016, 13:54   #67
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by botanybay View Post
It turns out that about half of the USA marinas are 208 and the other half 220

The reason is that it is about 300 dollars per slip cheaper to use two phases of a three phase transformer to provide two 110v legs which are 120 degrees out of phase.

The alternative is to use a transformer at the head of the dock (or somewhere else) to go from three phase to "split phase" which is what you have in your house. It is two 110v legs which are 180 degrees out of phase.

When keeping the two 110v panels separate there is little difference (there is a residual neutral current even if the loads are the same on each leg for the 208v case)

Now if you have a high voltage appliance (i.e. 208v or 220v) the equipment will see a voltage difference.

As for ELCI circuits (big gfi which allows a bit more differential on a primary feed) these are great.

For your system treat each transformer+panel as an independent system. Don't use the two hot wires to drive any 220v equipment.

Each system gets its own ELCI which monitors the current difference between that systems neutral and hot.

The neutral output of each isolation transformer is connected to ground right at the transformer. So yes, both neutrals are at the same potential and are tied to ground.

Beyond that point (prior to the independent ELCI units), each panel's neutral needs to be separated to ensure that the hot and neutral currents are equal except when the ELCI trips.

If you try to pull power from both hots to get high voltage in this case the ELCI will trip (a good thing).

Because you don't know (unless you test it) the relative phase of the two 30amp inlets the voltage between the two hots might be 0v, a few volts, 208v, or 220v.

ELCI are your friend and I highly recommend them😀

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Thanks for your comments botanybay. I understand the benefits of an ELCI. I don't have one. My question was whether there would be a problem if I had two different phase 110v inputs to two different transformers where the neutrals were bonded to a common ground, and the downstream circuits were independent - separate hots, separate loads, separate neutrals. I don't have, nor will I have, any 220v loads.

Thanks for the comments on marina power. I was not aware of that. My dock recently blew its big transformer at the head of our dock and we were out of power for a couple of weeks while they sourced and then installed a new one. I may ask the manager why kind of power it provided from it.
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Old 07-10-2016, 19:20   #68
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by exMaggieDrum View Post
Thanks for your comments botanybay. I understand the benefits of an ELCI. I don't have one. My question was whether there would be a problem if I had two different phase 110v inputs to two different transformers where the neutrals were bonded to a common ground, and the downstream circuits were independent - separate hots, separate loads, separate neutrals. I don't have, nor will I have, any 220v loads.

Thanks for the comments on marina power. I was not aware of that. My dock recently blew its big transformer at the head of our dock and we were out of power for a couple of weeks while they sourced and then installed a new one. I may ask the manager why kind of power it provided from it.
Sounds like a plan. Happy to talk more if you have further questions

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Old 07-10-2016, 22:57   #69
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by DotDun View Post
But the N(s) would be connected together at the ground bus bar.
they defiantly shouldn't be. with one shore plug or 2. this isn't house wiring.
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Old 07-10-2016, 23:09   #70
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by boatbod View Post
I wasn't suggesting you could parallel the output (hot and neutral) of two or more transformers, but you CAN combine the neutrals. In fact, assuming you have correctly polarized the transformer outputs (tied neutral to ground on each unit) then you have joined the neutrals together anyway.
it's true would have have continuity between the 2 neutrals because of the common ground and both being tied to ground. but you still only want the n-g bond at the transformer output. the source of power. because after the output you should have a double pole breaker. if you rejoined the 2 different N's after that point. you would have
A, ground loops and current flowing on the green wire, and B. nuisance tripping on the double poles as one N line may take more load then the other, and may exceed 30a on one N even though both hots are under 30a.

if you have GFI's or ELCI's after the transformer and before the 2nd bond, you'd have constant tripping.
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Old 07-10-2016, 23:40   #71
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by exMaggieDrum View Post
Thanks for your comments botanybay. I understand the benefits of an ELCI. I don't have one. My question was whether there would be a problem if I had two different phase 110v inputs to two different transformers where the neutrals were bonded to a common ground, and the downstream circuits were independent - separate hots, separate loads, separate neutrals. I don't have, nor will I have, any 220v loads.
you should end up with something like this
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Old 08-10-2016, 05:23   #72
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by smac999 View Post
on an iso transformer you do connect the N to G on board.
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Originally Posted by smac999 View Post
yes I have put 2 iso's on a boat before. but you certainlly wouldn't connect the 2 N's together as you posted earlier. they are treated totally separate. with separate double pole breakers and separate N buses.
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But the N(s) would be connected together at the ground bus bar.
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Originally Posted by smac999 View Post
they defiantly shouldn't be. with one shore plug or 2. this isn't house wiring.
Please explain!

You first agree that N and G are connected together on-board when using a iso transformer. Then you claim that when using (2) iso's, the (2) N's would not be connected together. There is only (1) G on a boat.
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Old 08-10-2016, 07:03   #73
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by smac999 View Post
they defiantly shouldn't be. with one shore plug or 2. this isn't house wiring.
In the example here with an isolation transformer the neutral side of the isolation transformer OUTPUT is connected to ships AC ground. Without this connection gfci or ELCI will not work.

You must form a neutral coming out of an isolation transformer by connecting transformer output neutral to ships AC ground just as one would with a generator aboard.

The input to the transformer is dock hot and neutral. Shore ground is NOT connected to ships ground in this case.

Wiring boat AC with and without a transformer are very different.

Without an isolation transformer you are correct.


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Old 08-10-2016, 08:43   #74
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Re: Question On Twin 30 Amp Shore Power Service

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you should end up with something like this
Smac, your diagram is exactly how my AC system is installed. I haven't got to the pretty drawings of it yet but have a sketch that is similar. The switches and circuit breakers I have break both the hots and the neutrals.

A complication I have is that the inverter/charger AC out can also power the heater, but it can only do so if the second shorepower/transformer hot/neutral is broken, just like when using a generator and breaking the hot and neutral for shorepower.

Thanks.
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Old 08-10-2016, 09:13   #75
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Re: Question On Twin 30 Amp Shore Power Service

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Originally Posted by DotDun View Post
Please explain!

You first agree that N and G are connected together on-board when using a iso transformer. Then you claim that when using (2) iso's, the (2) N's would not be connected together. There is only (1) G on a boat.
I just looked at my old (2009-2010) ABYC standards book, from when I was ABYC certified. I can't afford a new one now but I don't hire out either. I am not aware that the AC basics have changed since then.

With shorepower coming in, the hot and neutral have a master breaker. The ground goes only to the boat AC grounding bus and will never be connected directly to the shorepower neutral. The grounding bus is required connect to the boat main DC negative bus, with one and only one conductor. The ground/DC negative conductor is to be as large as the largest AC conductor from AC inlets/generators/loads.

For multiple shorepower inlets you would have the same wiring for each, except that you would have a separate neutral bus for each. These buses are not connected to the AC ground bus.

With a generator, the generator neutral is bonded to the AC ground. The gen ground goes to the main AC ground bus only. The neutral goes to the neutral bus that is served by the generator, which may be shared with all loads or separate. The grounds are always shared. If the generator powers loads that can also be powered by shorepower then one or the other is disconnected via a hot/neutral transfer switch. Both the hot and the neutral would be disconnected from one or the other so that they would never share neutrals even though their AC grounds would be connected at the main AC ground bus. ABYC 2009-2010 diagram 2 and diagram 4 per E-11.17.1 and E-11.17.2.

With an isolation transformer (as wired on my boat), the incoming shorepower ground goes to the iso transformer shield and does not go to the AC ground bus. The output of the transformer has the neutral and case ground bonded together and the ground is taken to the main AC ground bus. Diagram 6, E-11.17.4. And Diagram 5, E-17.3 for the case of a generator with an iso transformer. Both the generator and the iso transformer neutrals are bonded to the same AC grounding bus. And so are the neutrals. And, as before, there is a double pole breaker for both outputs and a transfer switch to prevent both powering the hots and neutral buses for loads. In this case it is not envisioned that the generator and iso transformer would be powering the same neutral bus at the same time. But grounds are still shared.

With two iso transformers, each would be wired the same, so that there will be a ground going to the main AC ground bus from each, so that each transformer neutral is in effect tied together. The shorepower grounds are not tied to the AC main ground bus. A transfer switch would be required to prevent each from powering the same load circuits. BUT the neutrals would still be connected via the ground bus in all cases. I think this is the issue at hand.

The ABYC standard from that year is silent on the specific situation of two iso transformers and two shorepower inlets. I am not sure if the current standard addresses that or not.

So even though I have separate neutral buses they are both electrically connected via the AC ground bus. To my understanding you always have just one AC ground bus and one and only one connection from it to the main DC negative bus, to prevent current loops from/to the DC system.

See smacc's diagram as to how mine is wired.
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