I see so much confusion about this… and get questions on the issue as well so I decided to write this post.
Attached is the diagram that I have posted often. It shows the shore power
inlet, isolation transformer and an inverter/charger that has an internal transfer switch. It also shows the distribution side with the breaker panel. Here you must determine if you have single
pole or double pole breakers. This matters later on.
There are guidelines that you need to adhere to. First the location of shore power
inlet, breaker box and isolation transformer. The wiring
between the inlet and the breaker box must be as short as possible and not longer than 3 meter / 10 feet.
The second photo
shows the breaker box that I recommend. These are very affordable, as are the breakers. Install both double pole breakers that connect to the transformer in it, as well as some DIN rail terminal blocks, then wire the inlet to the input of the first breaker, which must be 30-32A. The ground conductor is wired to a terminal block.
Wire the transformer input to the load side of the input breaker and the ground conductor to the same terminal block that the inlet ground was connected to.
Wire the transformer output to the input side of the output breaker , which is 30A for 120V boats and 16A for 230V boats and the ground conductor to a second terminal block, that is not connected to the first.
Last but not least is the cable towards the inverter/charger or your system, whatever you have aboard. Connect it to the load side of the output breaker and the ground conductor to the same terminal block that the output from the transformer connects to.
In case you have single
pole breakers on your AC panel: connect a jumper between the transformer output Neutral and the transformer output Ground. Sometimes the transformer has a jumper for this, do it in the breaker box if there is no room at the transformer.
In case you have double pole breakers on your AC panel: you can choose to ground the neutral like for single pole breakers, which is recommended by the ABYC in the US. I prefer not to but only leave it out when you understand this floating neutral concept
. It’ll probably get discussed in this thread again.
Above I wrote that the input breaker must be 30-32A, even for 230V boats. This is because the transformer can take 110-120V shore power
and double that for use aboard, which is very convenient. As the voltage double, the current
is back to the regular 16A for 230V boats so that is why the output breaker is 16A in that case.
Now comes the testing. There must be no connection between the input ground and output ground. None at all. Same for the other conductors, they should all be completely isolated between input and output.
Feel free to ask about any additional testing etc. in this thread