50 amp 250V isolation transformer options

[Mischief Managed]

Rather than hijack the other isolation transformer thread, I figured I'd start a new one.



My boat has two 120VAC 50 amp shore power inputs that are required if I use all three of my reverse cycle air conditioners and other loads like my water heater, at the same time. I do have a switch that will allow one of the shore power inlets to supply all the AC circuits if I wish, but I have to carefully manage loads when I use that option, especially since it's always meant a single 30 amp source, so far. At my home dock, I have the shore power inputs connected to a standard 50amp 240V shore power source (it's really 208V since it's actually two phases of a 208V three-phase but since all my loads are 120V, it works fine) on the pedestal. I use a Pro-Mariner 60 amp GI and that seems to keep corrosion at bay nicely.



My marina is way overdue for new electrical services and when they do eventually replace them I am concerned that I may "need" an isolation transformer. I think they are a good idea regardless, and I have plenty of room to install one or two, so I am exploring doing so. I think the only obvious choice for my boat is to go with two 120V 6kVA units so that I can always use a single 120V service without difficulty. 3.6 kVA ITs are available from many sources, but 6 kVA units seem much harder to source. Charles and Bridgeport Magnetics seem to be the only options. I am leaning toward Bridgeport due to cost and weight. Are there any other options out there?


If I do buy a couple of isolation transformers, can I safely mount them 30ish feet from the shore power connectors? My boat is already kind of stern heavy and I'd like to mount them forward of my engines to balance the boat out.

 

[O C Diver]

Why do you feel you may need an isolation transformer?

Wire distance should have no effect on the transformer. If it's more than 10 from the inlet, fuses or breakers will be required at the inlets. You should check wiring requirements for whether a disconnect is required between the inlets and the transformer.

Ted

 

[tiltrider1]

It sounds like your boat is all 120v and you need two legs of 50a to have adequate power. You did not mention a dryer. Often dryers are 220v and that makes a bit of difference on what I am going to suggest you look into.

Bridgeport magnetics makes a round isolation transformer. It comes in two sizes. 50a and 30a. Installing two of the 50a models and wiring it correctly you will end up with two 50a 125v services, even plugged into your 208v dock. Wiring is critical so thAt the neutral does not become overloaded. If you add a dryer in the future you will need to add a 125 to 250v transformer. However, you will still be isolated and you won’t suffer any 208v issues.

A this moment Torrid style isolation transformers are not ABYC approved, they are currently writing rules to make these round style isolation transformers ABYC approved. I have not talked to anybody who sees an issue with the Bridgeport Magnetic transformers.

 

[tiltrider1]

As long as you have a circuit breaker mounted close to your shore power cord socket, the 30’ run from there won’t matter.

 

[Mischief Managed]

Why do you feel you may need an isolation transformer?

Wire distance should have no effect on the transformer. If it's more than 10 from the inlet, fuses or breakers will be required at the inlets. You should check wiring requirements for whether a disconnect is required between the inlets and the transformer.

Ted

Thanks for the reply. My boat will trip a standard 15 amp 120V GFCI while the boat is on shore for the Winter so I figure it will likely trip the latest GFCI power pedestal stuff, that's even more sensitive, when it's in the water. I have never had an opportunity (that I know of) to connect to the newest shore power pedestals, so maybe it won't trip them, but an isolation transformer (or two) would make that all moot and future-proof my boat. I would hate to be caught urgently needing 6kVA isolation transformers and finding out there's a 3 month lead time because they are such low volume.



That said, I have never done any work to see why my boat trips the 120VAC 15 amp GFCI. It might just be the long extension cord that causes it, I've witnessed that at home at times. It might also be something I can fix.

 

[Comodave]

First I would troubleshoot the reason you trip the 15 amp GFI. Your boat is probably wired incorrectly. Most older boats are. You should be able to run selected things on the GFI without tripping unless you draw more than 15 amps. Our last boat would trip GFIs just by plugging it in. The builder put all the neutrals on one bus bar. I separated the neutrals and it would then work without tripping the GFI. I also replaced the main panel but not for that reason.

I used the 15amp GFI for verifying that I had the problem fixed. I went through the boat one circuit at a time checking that each circuit would work without tripping. The hard part on that boat was that the white wires were not labeled and were individual wires so we had to ohm out each circuit to see which white wire went with each inlet.

I have fixed about 7 boats for this problem. Each had at least the neutrals on one bus bar. But one had the volt meters for each inlet with the neutrals daisy chained so we had to separate them. It also had one outlet where somebody switched the neutral and ground wires. It also had a short inside the water heater between neutral and ground.

If you fix the base problem then you don’t really need the isolation transformers.

 

[O C Diver]

That said, I have never done any work to see why my boat trips the 120VAC 15 amp GFCI. It might just be the long extension cord that causes it, I've witnessed that at home at times. It might also be something I can fix.

Try turning ALL the breakers including the main shore power breaker off. Then connect the shore power cord and turning on the main breaker. Then, one by one switch on and off individual other breakers. You may have luck finding the offending circuit.

Also worth looking to see if the bonding system is connected to the 120 VAC ground (it shouldn't be). Other areas include the inverter and the generator. It's fine to have a generator engine tied to boat's bonding, but not the 120 VAC power ground. If you have the manual for the inverter, check their instructions for tying to the bonding system.

Ted

 

[Mischief Managed]

First I would troubleshoot the reason you trip the 15 amp GFI. Your boat is probably wired incorrectly. Most older boats are. You should be able to run selected things on the GFI without tripping unless you draw more than 15 amps. Our last boat would trip GFIs just by plugging it in. The builder put all the neutrals on one bus bar. I separated the neutrals and it would then work without tripping the GFI. I also replaced the main panel but not for that reason.

I used the 15amp GFI for verifying that I had the problem fixed. I went through the boat one circuit at a time checking that each circuit would work without tripping. The hard part on that boat was that the white wires were not labeled and were individual wires so we had to ohm out each circuit to see which white wire went with each inlet.

I have fixed about 7 boats for this problem. Each had at least the neutrals on one bus bar. But one had the volt meters for each inlet with the neutrals daisy chained so we had to separate them. It also had one outlet where somebody switched the neutral and ground wires. It also had a short inside the water heater between neutral and ground.

If you fix the base problem then you don’t really need the isolation transformers.

Thanks for the detailed explanation. Lets assume all my neutrals are on one bus bar, what is the actual fix for this? is it just splitting the L1 and L2 neutrals? If so, won't they just be connected back at the pedestal?

 

[Mischief Managed]

It sounds like your boat is all 120v and you need two legs of 50a to have adequate power. You did not mention a dryer. Often dryers are 220v and that makes a bit of difference on what I am going to suggest you look into.

Bridgeport magnetics makes a round isolation transformer. It comes in two sizes. 50a and 30a. Installing two of the 50a models and wiring it correctly you will end up with two 50a 125v services, even plugged into your 208v dock. Wiring is critical so thAt the neutral does not become overloaded. If you add a dryer in the future you will need to add a 125 to 250v transformer. However, you will still be isolated and you won’t suffer any 208v issues.

A this moment Torrid style isolation transformers are not ABYC approved, they are currently writing rules to make these round style isolation transformers ABYC approved. I have not talked to anybody who sees an issue with the Bridgeport Magnetic transformers.

I was under the impression that it's impossible to overload the neutral on a two phase connection because the most current the neutral can ever carry is the difference between L1 hot current and L2 hot current. Thus, no more than 50 amps.

 

[Comodave]

Thanks for the detailed explanation. Lets assume all my neutrals are on one bus bar, what is the actual fix for this? is it just splitting the L1 and L2 neutrals? If so, won't they just be connected back at the pedestal?

You would need to add a second neutral bus bar. I have worked on multiple boats from the 80s and 90s and this was the major problem with each. Trojan, Tiara, Carver and some far east built boats all had all the neutrals from both inlets on one bus bar. It would work when the boats were built but not with the GFI on the dock. So first thing is to look and see if you have 1 or 2 neutral bus bars. If you have only 1 then you can’t proceed with any other troubleshooting until you fix that.

The hard part is figuring out which white wires go with inlet 1 and which go with inlet 2. If the wiring is done with individual wires it is much more difficult than if they are a triplex wire in a sheath, white, black and green. If they are in a sheath then you know which white goes with which black. But if they are individual wires you have to dig out the ohm meter and ring them out. I use an extension cord that will reach to all the outlets on the boat and back to the panel. Then cut off the female end of the extension cord. Have power disconnected obviously. Plug the extension cord into an outlet and have all the neutrals disconnected at the bus bar. Run the ohm meter from the white wire in the extension cord and down each neutral in the panel. You will find out which neutral wire is associated with that outlet. Mark it as either inlet 1 or 2 depending on which it gets power from. Do all the outlets first since they are easy to do with the extension cord. Then you have to get creative as to how to ring out the remainder of the neutrals. I still use the extension cord since it will reach all over the boat and just clip onto the neutral tip and then proceed as the outlets. Eventually you should find all the neutrals and figure out where they should go, either on bus 1 or bus 2.

Then use your 15 amp GFI to test the boat and see if you have other problems. First turn everything off and plug the boat in and see if it trips, if not turn a circuit on and draw power on it and see if it trips. Work through all the circuits and if you don’t have any trips then take the boat to a dock with a GFI and test it again but with everything turned on.

Good luck!

 

[tiltrider1]

I was under the impression that it's impossible to overload the neutral on a two phase connection because the most current the neutral can ever carry is the difference between L1 hot current and L2 hot current. Thus, no more than 50 amps.

What I meant to say was, be careful how you wire it up so that you get two 125v lines and not two 104v lines. I got distracted while typing and when I came back my brain went to overloaded neutrals.

 

[twistedtree]

Here's another approach that you might consider..


1) Install an Acme/Hubbell 120/240V isolation transformer.


2) Change your power inlet to a 120/240V 50A inlet.


3) Add a second inlet that is 120V, 30A.


4) Bring both inlets to a three position selector switch. The selector positions will be:


4a) "240V Normal" this will wire the 120/240 inlet to the 240V input of the isolation transformer


4b) "240V Boost" this will wire the 120/240V inlet to the 208V input of the isolation transformer. This will boost 208V shore power and give you 120/240V onboard.


4c) "120V 30A" this will wire the 120V30A inlet to the 120V input of the isolation transformer and will give you 120/240V onboard, though limited to 3.6kw


5) On the secondary side of the isolation transformer, wire each leg to each of your boat's existing 120V loads. It's fine to retain any common neutrals, and in fact you need to join them if they are currently separate, but a single bridge wire is fine.


With this setup you will always have power on both panels/legs onboard, though the need for power management will vary. You only need a single isolation transformer rather than two, You can still run off either 120/240V or 120V shore power. For 120V shore power you will be set up for the much more common 30A service, but if you really wanted to accept 120V 50A, you could always add another inlet and position on the selector switch to accept 120V 50A.


When adding a 120/240V isolation transformer, I think it's essential to include a way to boost 208V. Otherwise you end up with 120V onboard service that is half of the incoming voltage, and typically ends up around 100V. But the boosting doesn't have to be automatic. The Acme/Hubbell (and other) transformers have input taps for 208V and it's pretty simple and very cost effective to wire a manual selector switch.

 

[luna]

This thread has been idle for a few days so I thoughI would chime in with an answer and some thoughts.

For MM, as you run 2 independent power sources, in order to pass the GFCI test you need to ensure that whatever circuits are powered by source No. 1, have all of the neutrals from those circuits connected only to the neutral of source No. 1. Same thing for source No. 2
This will typically take 2 neutral buses.
Even after this is done you can still trip the GFCI at the pedestal as it is common to find neutral to ground connections inside the cabinet of residential appliances. My washer and microwave had such connections, as might your Air Cond. system. You will need to rewire these to their respective conductors. Finding them is likely best done by plugging in with everything off and then turning on individual breakers until the GFCI opens. Then the dismantling to find the culprit(s) begins.
Additionally, your inverter should have a neutral to ground connection. Just turning the inverter itself off prior to plugging into the grid usually works or turn the inverter's breaker off, plug in and then breaker on.
To answer your question, yes the neutrals are likely connected inside the pedestal but that is on the other side of the GFCI so it does not matter. You will need your neutrals separated.

To TT. There is no way that you can wire 2 of the 3 phases (which is what the Marinas around here typically provide) to produce 104V AC. Each phase to ground has 120V AC on it.
Any 2 phases being 120º apart, results in 208V AC. 104V, just like overloaded neutrals is nowhere to be found.

To Twisted. I agree with almost all of what you write, except the part about the boost transformer. Here is why. Typically, you as you state you are going to run this boost when you come across a 208 V feed, and yours, like mine, likely raises the voltage across the 2 ungrounded conductors to about 230V, fine. It does so by raising the voltage on the 2, 120V ungrounded conductors to about 130 V AC, as that is all it can (and what it is designed to) do.
This 130V is not good for any motor designed to run on 120V, think Refers, fans, Air Cond, watermakers or anything that runs for a long time, as the additional 10V is simply turned into heat, which is not good for longevity of the appliance.

The boost should be reserved for those times when you experience voltage loss due to line losses as you are at the end of the dock and all of your dock mates get power first.

Granted, I'm in the PNW, have no need for Air Cond. and can live with the oven or clothes dryer taking a little longer to heat up, all the while saving the fridges.
I would not run an inductive load designed for 240V on 208V. If you need to run the Air Cond. it's by genset unless you have a solid 240V supply from the grid.


As with boats, electrical systems are a compromise, unless you want to spend big dollars.

 

[twistedtree]

This thread has been idle for a few days so I thoughI would chime in with an answer and some thoughts.

For MM, as you run 2 independent power sources, in order to pass the GFCI test you need to ensure that whatever circuits are powered by source No. 1, have all of the neutrals from those circuits connected only to the neutral of source No. 1. Same thing for source No. 2
This will typically take 2 neutral buses.
Even after this is done you can still trip the GFCI at the pedestal as it is common to find neutral to ground connections inside the cabinet of residential appliances. My washer and microwave had such connections, as might your Air Cond. system. You will need to rewire these to their respective conductors. Finding them is likely best done by plugging in with everything off and then turning on individual breakers until the GFCI opens. Then the dismantling to find the culprit(s) begins.
Additionally, your inverter should have a neutral to ground connection. Just turning the inverter itself off prior to plugging into the grid usually works or turn the inverter's breaker off, plug in and then breaker on.
To answer your question, yes the neutrals are likely connected inside the pedestal but that is on the other side of the GFCI so it does not matter. You will need your neutrals separated.

To TT. There is no way that you can wire 2 of the 3 phases (which is what the Marinas around here typically provide) to produce 104V AC. Each phase to ground has 120V AC on it.
Any 2 phases being 120º apart, results in 208V AC. 104V, just like overloaded neutrals is nowhere to be found.

To Twisted. I agree with almost all of what you write, except the part about the boost transformer. Here is why. Typically, you as you state you are going to run this boost when you come across a 208 V feed, and yours, like mine, likely raises the voltage across the 2 ungrounded conductors to about 230V, fine. It does so by raising the voltage on the 2, 120V ungrounded conductors to about 130 V AC, as that is all it can (and what it is designed to) do.
This 130V is not good for any motor designed to run on 120V, think Refers, fans, Air Cond, watermakers or anything that runs for a long time, as the additional 10V is simply turned into heat, which is not good for longevity of the appliance.

The boost should be reserved for those times when you experience voltage loss due to line losses as you are at the end of the dock and all of your dock mates get power first.

Granted, I'm in the PNW, have no need for Air Cond. and can live with the oven or clothes dryer taking a little longer to heat up, all the while saving the fridges.
I would not run an inductive load designed for 240V on 208V. If you need to run the Air Cond. it's by genset unless you have a solid 240V supply from the grid.


As with boats, electrical systems are a compromise, unless you want to spend big dollars.

For a 120/240V split phase boat, the input to the transformer is nominally 240V (L1 & L2), with no neutral connection to the transformer. The output creates the split phase neutral, so at the output you have L1, L2, and N. N is midway between L & L2, so N-L1 equals N-L2, and if half of L1-L2.


When you plug that into a 208 dock, the secondary will be 208V between L1&L2, and 104V between L1-N and L2-N. In contrast, if you measure between L1-N on the shore pedestal you will get 120V as a consequence of the 3-phase Y. But the neutral isn't carried to the boat, only L1 & L2 at 208V. The transformer splits that in half when it creates split phase, so on board you get 104/208V.


It's black magic, but true.

 

[luna]

That's not black magic, what it is, is not something that those of us that don't have an isolation transformer would experience, although I should have thought about that before typing. I simply didn't think through what happens if you have an iso trans.

Your 104V lives and your boost works if you have an iso trans. I don't.

 

[sunchaser]

When plugged into Canadian docks rated at 208 each of our legs measures 110. We don't have an IT. A few years ago a BC Hydro guy told me that US code for GFIC is unnecessary and an unwarranted expense for 208 hookups.

Divers in our BC marina claim to have never experienced a "shocking" issue. So is the Canadian 208 different phase setup better or safer?

 

[tiltrider1]

Divers in our BC marina claim to have never experienced a "shocking" issue. So is the Canadian 208 different phase setup better or safer?

One needs to understand the difference between fresh water and salt water. A lot of people will argue that GFIC is not needed in salt water. No one will argue that GFIC is not needed in fresh water.

One of the interesting features of the Bridgeport Magnetics Isolation Transformer is being able to wire it so that you get two 110v legs even when hooked up to a 208v socket. This assumes you are either a 110v boat or you have a transformer down the line that turns 110v into 220v.

 

[twistedtree]

That's not black magic, what it is, is not something that those of us that don't have an isolation transformer would experience, although I should have thought about that before typing. I simply didn't think through what happens if you have an iso trans.

Your 104V lives and your boost works if you have an iso trans. I don't.

That's it. The comment was all in the context of adding a 120/240V isolation transformer, and the importance of a boost function if you do add one. I hear complaints all the time about low 120V service on boats with isolation transformers plugged into a 208V dock. I don't think these should ever be installed without a boost function, assuming you will use the 120V service. It doesn't need to be an expensive auto boost function - a simple switch works just fine. The irony is it all is the dock's 120V service will be 120V even when the high voltage is 208V. And most higher voltage appliances will work fine on 208V. But 104V for a 120V appliance, not so much, especially when the actual voltage droops to 200/100V. Adding an isolation transformer solves a bunch of issues, but creates this new one that really needs to be dealt with.

 

[twistedtree]

One needs to understand the difference between fresh water and salt water. A lot of people will argue that GFIC is not needed in salt water. No one will argue that GFIC is not needed in fresh water.

One of the interesting features of the Bridgeport Magnetics Isolation Transformer is being able to wire it so that you get two 110v legs even when hooked up to a 208v socket. This assumes you are either a 110v boat or you have a transformer down the line that turns 110v into 220v.

So you would do this using two of their transformers, one on each 120V leg of the shore power? One the secondary (boat) side you would have two separate 120V services. To then get 240V you would use a step-up transformer on one of the 120V services? I think between the two 120V services, you would still have 208V, right?

 

[twistedtree]

When plugged into Canadian docks rated at 208 each of our legs measures 110. We don't have an IT. A few years ago a BC Hydro guy told me that US code for GFIC is unnecessary and an unwarranted expense for 208 hookups.

Divers in our BC marina claim to have never experienced a "shocking" issue. So is the Canadian 208 different phase setup better or safer?

I'm at a loss trying to understand how what he said could be true, or what difference there might be between US and Canadian 208V service.

 

[sunchaser]

I'm at a loss trying to understand how what he said could be true, or what difference there might be between US and Canadian 208V service.

As best I understand a 110/120 reading from 208 has to do with how 3 phase (unisolated) power works with one side reading 120 and the other 88.

 

[tiltrider1]

So you would do this using two of their transformers, one on each 120V leg of the shore power? One the secondary (boat) side you would have two separate 120V services. To then get 240V you would use a step-up transformer on one of the 120V services? I think between the two 120V services, you would still have 208V, right?

Yes.

 

[Mischief Managed]

Wow, Lot's of awesome information since I was last here.
Thanks for all the replies.


FWIW, my boat has two independent 120VAC inlets and all my AC appliances run on 120VAC. I don't have any 240VAC devices on my boat and I have no plans to add them. Even my 8k genset is wired for a single 120VAC output.



When connected to shore power, I typically use a splitter on a single 50 amp 240VAC (typically 208VAC due to the phase angle) outlet that provides two 120VAC legs. The neutrals all tie together in the splitter (and may also be on the same buss in the electrical panel). I attach the splitter to my boat, not the pedestal, specifically so I can use a single 50AMP/240VAC shore power cable instead of two 50AMP/120VAC cables.



If I decide to install isolation transformers, there will be two of them with no boost capability solely because I don't have any 240VAC loads to power and 208VAC is a non-issue for me.

 

[EngNate]

There is no difference between US and Canada electric services or dock wiring.

Tripping GFI is most often caused by a ground to neutral connection on the boat that shouldn't be there, or wrong ground connections at the transformer if equipped.

Keep in mind that a transformer will reduce your available amps, and increase your power consumption, by about 12%.

 

[BLUE STEEL]

I have the shore power inputs connected to a standard 50amp 240V shore power source (it's really 208V since it's actually two phases of a 208V three-phase but since all my loads are 120V, it works fine) on the pedestal. I use a Pro-Mariner 60 amp GI and that seems to keep corrosion at bay nicely.

My marina is way overdue for new electrical services and when they do eventually replace them I am concerned that I may "need" an isolation transformer.

First post on TF!


Echoing Twistedtree's recommendation as I just completed the 'boost' function wiring on my boat by reverse-feeding the existing Charles Iso-transformer. This was required because of the transformer splitting 208v shore power into 2x 104v circuits. Not good for my 120v loads, plus 208v is not ideal for 240v loads.


Your boat has no 240v loads, so no need for a split-phase system and no need to boost the shore power. You are running 2x 120v circuits coming from the shore power pedestal. You will always get ~120v phase to neutral on 3-phase power.

I'm curious about the wiring of your GI, since you have 2x inputs on the boat. Do the ground wires from both inputs go into the single GI?

As Comodave said, you might have a wiring issue if there's tripping. You 'could' benefit from this product to help indicate where the fault may lie.

IF you wanted to 'unload' or balance the incoming neutral, you'd want to add an autotransformer between shore power and the load (i.e. boat), not an isotransformer - unless your boat is steel at which point you probably want both.

 

[Mischief Managed]

First post on TF!




I'm curious about the wiring of your GI, since you have 2x inputs on the boat. Do the ground wires from both inputs go into the single GI?

Welcome!


My GI is indeed wired to both grounds from the two inputs. They were connected to each other on the boat prior to the GI installation though, and my single shore power connection obviously connected them together outside the boat, so I don't think anything really changed other than 1.4V of DC is now blocked on the ground connection.