Boating Transformers FAQ

Question 1: Is only 40ohms resistance between the transformer case and hull normal?

While checking our isolation transformer, I was surprised to see that there was only about 40ohms resistance between the transformer case (earthed to shore) and the hull of the barge. Suspecting a fault, I did some test and found that the resistance to the ground pin of the shore power cable stays at about 40-50ohms relative to the hull regardless of where we tried, so nothing to do with the transformer. We also tested another barge which is moored about 100m from ours and it showed about 800ohms between hull and the ground pin of the shore power cable.  Is this to be expected? I was thinking we'd see either very high values or even infinite resistance.

Answer:   Yes it is correct that there is a measurable resistance between two electrodes either in water or soil, this is to be expected & explains why the use of isolation transformers are widely used. There will be a measurable resistance between the hulls of two nearby boats and if they are both connected to the same shoreline supply then the following circuit will result:

This is a typical earth loop, which means that all sorts of currents & signals will be circulating.

Boating isolated power supply

However if boat B has an isolated power supply, which will have its own functional earth not directly connected to the shoreline earth, the following will apply:


isolated power supply



Question 2: What size transformer should I order?

Answer: The size of the transformer is determined by the total power of electrical appliances which you may need to run at the same time. (This is not necessarily the same as the total of all your appliances.)

For example you may expect to operate the following

Fridge - 300watts

Lighting - 200watts

Heater - 2000watts

Total - 2500watt

In this case we would suggest the 3600va transformer (order a transformer a little higher than the expected load)

PLEASE NOTE: However you may also be limited by your shore power. If your shore power is 16Amps then the maximum size would be 3600va since a 5000va transformer would require a 32Amp supply (you could draw up to 7200va). And to benefit fully from the 10kva you would need a 50amp supply.


Question 3: I wish to install a galvanic isolation transformer at my mooring. Are there any special considerations regarding the earthing arrangements?

Answer: Our steel cased isolation transformers are normally supplied with the case earth wired to the output of the unit to coincide with the vessel earth. Should you wish to mount the unit on shore, the case earth would need to be wired to the input ground. This may be specified with your order and supplied as such from our factory. 

If in doubt you should consult with a local electrician.