Electrical bypass.

Since mentioning it in my last two posts, several people have reached out for more information about the electrical "upgrade" that I've been working on. Rather than send the fairly detailed answer to everyone individually, I am just going to write it up here. This post will contain nothing else, so anyone not interested in electrical details can just skip it; I will return to our regular travelogue in the next post.

As a background, Vector has an isolation transformer, added by her last owner whilst upgrading the shore power system to 50-amp, 240-volt from its original dual 30-amp, 120-volt inlets. Vestiges of the original dual-30 configuration persist in the layout of the main panel at the helm, which is now actually a subpanel.

New breaker "panel" for the secondary inlet. The J-box below it will get a non-metallic cover; the stainless one was all I had in my parts bin.

The isolation transformer, which turned out to have been incorrectly wired and thus not doing its job, is principally to protect the boat from galvanic corrosion when connected to shore power. The way that corrosion happens is that the boat's underwater metals, including the anodes for the cathodic protection system, the propeller and running gear, and the hull itself, once connected to the shore grounding system, can suddenly become the "protective anode" for anything on shore, such as dock pilings, and anything else connected to it, such as other boats with poor cathodic protection.

The isolation transformer solves this problem by severing the ground connection to the shore altogether. The boat serves as its own ground for the onboard electrical system, which is isolated from the shore ground, hence the name. The other way to solve this problem is to use a device called a galvanic isolator, which goes in between the boat's ground and the shore ground and prevents the flow of current unless the voltage difference between the two rises above about 1.4 volts. That does happen sometimes, in what is often referred to as a "hot marina" — the isolation transformer does not have this problem.

Gratuitous photo of the transfer switch that connects the main panel to the isolation transformer, which only works on 240vac input. I wrote this project up in a Facebook post and not here on the blog.

One consequence of an isolation transformer on a 50-amp boat is that not only does the ground not connect to the shore, but neither does the neutral. Only the two hot legs are connected; the isolation transformer secondary has a center tap and the boat's own neutral is generated there, where it is also bonded to the boat's ground. This makes for a very safe system, but it means that the boat's 120-volt (nominal) system is always exactly half of the input voltage.

This means that we must connect our 50-amp shore cable to a 240-volt (nominal) supply. Nothing will work at all if we tried to use one of those adapters that are especially common in the RV world, but can also be found in the boat world, which connects both hot legs together to a single 120-volt hot and passes the neutral through on a 30-amp plug. The isolation transformer sees that as zero volts. We do have an adapter that lets us use two 30-amp receptacles together, so long as they are on two different legs of power.

Another consequence of this arrangement is that when we find a marina that uses commercial three-phase power instead of split-phase, we have low voltage throughout the boat. While the hot-to-neutral voltage on such as system is 120 volts, and boats with conventional split-phase power input arrangements will see that on all their 120-volt circuits, the phase-to-phase voltage is just 208 volts, rather than the 240 of split-phase, and thus all our circuits now see just half that, or 104 volts. It takes a really, really long time to toast a bagel, and all our lights are dim.

A few years back, to cope with the inability to use any shore outlet with only 120 volts, I resurrected parts of the old dual 30-amp shore system. I reinstalled a 30-amp inlet on the aft deck, where there was a an abandoned 30-amp line down to the engine room, and I set this up as an alternate input directly to our inverter-charger, which is also a 30-amp device. To deal with the galvanic issue I used a galvanic isolator, two of which were also abandoned under the helm. And to make it seamless, I installed a 30-amp, 3PDT relay that automatically switches the inverter over to this alternate 30-amp input whenever it is live.

The enclosure for the 3PDT transfer relay. This has been in place for nearly a dozen years.

We've used this numerous times and it has mostly worked well when we've needed it. But the way the inverter is wired to not just one, but two hot legs on its normal feed (from the generator or the 50-amp shore cord) meant that the "max shore amps" setting on the inverter control could not account for the loads to back the charger off correctly. Let me take a moment to explain that. The explanation is lengthy, but it is fundamental to the changes I've just made.

The galvanic isolator is under this mass of wiring for the voltage converter. Two exposed screw terminals are on the left side.

Our Magnum MS4024 inverter-charger, while basically a single-phase, 30-amp device, nevertheless actually has a split-phase input and output, which means there are two hots in and two hots out. One of those feeds the charger and any loads on Hot 1, and the other, Hot 2, is just a pass-through, until the input power goes away and the inverter starts inverting. Then both Hot 1 and Hot 2 are connected to the single 30-amp inverter output.

This is the wiring diagram for the transfer relay. The light blue lines are the neutral, which must always be switched along with the hots.

We have both inputs connected, but only Hot 2 is connected to the output. When shore or generator power is available, we thus have a full 30 amps available to the charger on one leg, and a full 30 amps for loads on the output on the other leg. In order to make the inverter work on a single 30-amp leg of shore power, the relay that switches the input simply connects both Hot 1 and Hot 2 to the single available leg.

Everything works, but now the loads are connected directly to the input power, bypassing the inverter logic, and thus the inverter control has no way to see how much load there is and back off the charge rate to fit it all within a given number of amps. In order to do that, the output would have to be wired to the Hot 1 output. I had to stare at the schematics for power-input mode and inverting mode for a very long time to understand this, so don't feel bad if I've lost you along the way. You can see those diagrams for yourself here; the relevant diagrams are figures 3-1 and 3-2 on pages 42 and 43.

I want to take a moment here to say that I really, really like this dual-input single-output feature, so much so that when the inverter crapped out a year ago, I bought the exact same model to replace it, after first evaluating everything else on the market including Victron models. So changing this arrangement by, for example, switching to single-input single-output wiring was not a desired option.

Connection diagram for the new setup, with arrows to show flow. Innards of 3PDT box are detailed in a separate diagram above. The SPDT is actually inside the inverter enclosure and is straightforward.

With the output bypassing the inverter logic this way when single-leg shore power was connected, I could not set the "max shore amps" to 30 when on 30-amp power, or else we would trip the shore breaker as the charger came in on top of the loads. I had to take a guess at our total load, and set the max amps to the difference between that and 30, less some safety factor. Often I was setting input to as little as five amps, which is just enough to keep the batteries from depleting but not enough to charge them.

We actually have a second battery charger connected to the house batteries, a non-programmable charger good for about 30 amps DC and drawing maybe 8 amps at 120vac. It's there as a backup for the inverter/charger if needed, and normally it runs as supplemental to it when were are running the generator. To avoid the problems with tripping the shore breaker on 20- or 15-amp shore outlets, instead of using the arrangement I have already described to connect the inverter/charger directly to this shore power, we've taken to just running an extension cord out of the boat and plugging this auxiliary charger into it directly, and letting the inverter just run the 120vac loads from the batteries.

With all that as background, I am working on two separate projects to improve things. The first, now completed and pictured above, was to simply split that 30-amp shore inlet line, which previously went directly to the relay ahead of the inverter, into two circuits — one for the inverter relay as before, and a second for a power outlet to run the auxiliary charger. No fancy relay here; I will just move that charger's standard plug from one outlet to the other when needed.

This duplex receptacle has one outlet on the regular shore/gen system, and the other on the aft deck feed. There is no danger or real downside to "forgetting" to move the plug so no need for automation.

This lets us do what we've already been doing, but without having to prop the back door of the engine room open to run the extension cord out of the boat. There's no way to lock up the boat with that arrangement, which has given us pause to use that method when we have to leave the boat someplace. Also, it bypasses the galvanic isolator. Of course, you can't just parallel a 15-amp outlet to a 30-amp circuit, so using the single 30-amp inlet line for this meant adding a small electrical panel with separate breakers for the 30-amp circuit to the inverter and the 15-amp circuit to the charger outlet. They are not intended to ever be used simultaneously.

New MCBs in their enclosure. 16a at left goes to charger outlet, 32a at right goes to the inverter transfer relay.

I did not have the room for full-size NEMA electrical panels or even some of the Blue Sea stuff, spendy as it is. I opted to go with the European-style "MCB" items, which meant a 16a and a 32a because those are the closest available ratings. They make a miniature enclosure for a pair of these, really intended for a single RCD or two-pole breaker. Bridging the input side between the two breakers was a tight squeeze, and there was not enough room at the other end of the enclosure to make all the neutral and ground connections, so I had to add another J-box below it.

J-box for making the ground an neutral connections. Ground comes in from the isolator. You can see the two hots going up to the breaker enclosure. The Wago connectors accept up to 10 AWG.

You may recall I said I used a galvanic isolator on this bypass arrangement, and previously the ground wires to and from the isolator ran from the J-box where the bypass relay is located. With this new arrangement I had to move those wires over to the new breaker box.

This new arrangement facilitates using 15-amp circuits when that's all that is available, but it does not solve the 30-amp problem, and for that I have sourced a 30-amp DPDT relay that will be mounted inside the inverter enclosure in the wiring junction area. This relay will switch the output from Hot 2 to Hot 1 whenever the bypass relay that switches the inputs is active. The big 30-amp, 3PDT power relay that switches the inputs actually has a 12VDC coil, due simply to that configuration being the only one readily available when I built it. A small transformer supplies the 12v when the input is hot, and I've run that same 12v signal over to the new relay, which also has a 12vdc coil.

This is the relay that will move the loads from the bypass side to the controlled side of the inverter output. It will go inside the wiring box to the bottom right. 12vdc control wires are hanging loose. If you look carefully you can see there is nothing at all connected to Hot 2 Out right now.

This final piece of the project is waiting on, of all things, more AWG 12-10 quick-connect crimp terminals (the ones with the yellow barrels). I'm out of plain ones and the fully-insulated ones don't fit the wells on the relay. Previously I was delayed by getting all the way as far as making neutral and ground connections, only to find my giant supply of Wago lever-lock connectors only went as large as 12 AWG. It's always something.

Once this is complete, with charger management working correctly on a single 30-amp circuit, we will most likely opt for 30-amp power instead of 50-amp in marinas that use three-phase power when we do not need either the big air conditioners or the clothes dryer. And we have a marina stay coming up where 50-amp power is $50 per night, whereas 30-amp is just $25 per night, so we will take advantage of it then, too.