Isolation Transformer

by Adrian on February 20, 2013

We’ve had a galvanitic isolator (protecting the boat’s hull against earth current leakage) installed since launch, but I’ve recently taken shorepower isolation and galvanitic protection to the next level.

Isolation TransformerI’ve had a custom isolation transformer made which like all things on the boat, performs multiple functions:

1) It physically isolates the boat’s 240V electrics from the shore power by the use of physically separate transformer coils which transfer electricity from one coil to another via electromagnetic induction.

2) The primary coil has switched multiple windings which allow either 110V or 240V input, and a constant 240V output.

This provides me with complete physical isolation from the shore power in marinas. If there are any wiring faults or current leakage they do not affect our boat or aluminium hull.

Works in the UK and the USIt also converts 110v to 240V, which allows us to plug in to either 240V mains or 110V mains when travelling around the world, and still run all our 240V on-board appliances.

IsolationBasics.jpegIsolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected.


The transformer is rated at 5kw, which means it’ll handle up to 20 Amps at 240v. This far exceeds our boat’s power requirements, but to be safe, I also upgraded all the main 240V supply wiring to 6mm cable, which will easily handle the load.

I have also bought a replacement 6mm shore power cable to handle the additional current draw when connected to 110v. To supply the theoretical maximum 20A at 240V, the transformer could potentially draw approximately 40A from the 110V supply, so it requires a very chunky cable.

The unit itself is quite bulky and heavy, so I bolted it securely to a wooden bulkhead in the transom area. This keeps the cable run from the shore power connection to a minimum, keeps the weight low, keeps it mounted safely and secure.

  • Inside, looking at coils and connectors

  • Ready to install

  • Connected up and ready to go

The transformer has two cable glands – one for the supply input from the shore power, and one output feeding to the supply crossover switch (for selecting either the shore power or generator supply), on to the breaker switches through the inverter and out to the 240V circuits.

Costs and Supply

There are plenty of production marine isolation transformers on the market, but because of my dual 110V/240V supply requirement, I found a UK company Airlink Transformers Ltd that could produce a custom unit at a reasonable price.

Michael Harris at Airlink Transformers Ltd discussed my requirements and designed and made the unit for me. They provided a very professional and helpful service. The transformer cost just under £500 including VAT. It is solidly built and is CE certified.

Isolation Transformer
The shorepower cable came from a company called, who have a great stock of quality cables at non-marine inflated prices. Their 25m off-cuts of 6mm Arctic Yellow flex are ideal for a shore cable, and instead of costing several hundred pounds, cost me just £65.

{ 6 comments… read them below or add one }

Mangesh Jadhav December 24, 2014 at 05:20

Dear Sir/Madam,
We required Isolation Transformer Pri. 0-230V ,Sec. 0-230V @ 3Amp
With Earting Terminal
Neutrail Earth Leakage 0V, Capacitor mounting should be proper assembled product send this data sheet per month required qty 15 to 25 nos.

With best regards,

Mr. Mangesh jadhav
Cell +91-9226261159

Utopia Optovision Pvt Ltd
P-4/1, Information Technology Park, OLD MIDC, Satara – 415 004 Maharashtra, INDIA
Telefax 0091-2162- 246821, 246829, 246921, 246828


tecmax January 28, 2014 at 08:00

The isolation transformers are known to consist of two coils that help in the transferring of electronic devices from one device to another. Contact the Isolation Transformer Manufacturers for further details.


Steve March 10, 2013 at 10:29

Hi Adrian, great post, however without getting too geeky myself, it might pay to mention the whole 50hz/60hz dilemma. I’ve going to do the same install on ours, but i’ve already spec’d my 240v electrical components internally at 60hz wherever possible, (A/C, watermaker, inverter etc, 240v 60hz kit dosnt mind running at 50hz in the UK but as you know 50hz kit will not like 110v @ 60hz.



Adrian March 11, 2013 at 23:48

A very good point. Both the Studer inverter/charger and my backup Phillipi charger run off 60 or 50hz. I’m not sure about the microwave, but everything else is pretty basic and shouldn’t have a problem. If necessary I can run the microwave from the inverter anyway. Our water maker will be a 12v Desalator model.


Francisco Moreno March 2, 2013 at 13:38

Hello, your transformer looks exactly like we would need.How much does it weigh?


Adrian March 11, 2013 at 23:50

the package slip says 32kg, but I’m not sure it was that heavy. Felt more like 20-25kg to me. Pretty heavy as it is a reasonably large metal box with a chunky electro magnet inside.


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