Home Crafts & Hobbies How to install shore-power on a boat

How to install shore-power on a boat


Marina AC is a safe and convenient option, but you must know how to properly install and maintain it aboard your boat. Duncan Kent gives expert advice

A few years ago, it was uncommon to find any AC power on a small-to-medium sized recreational boat. Now, shore power on a yacht has become a common practice.

The 240V power supply is an essential part of any marina. This allows you to use heaters as well as kettles, water heating systems, microwaves, and much more. In fact, it’s becoming increasingly rare to find any boat relying on its own power resources whilst marina-based, even when they’re paying for it on a meter.

This new abundance of AC power also comes with a problem: how to safely manage and handle it on a yacht. It’s important to remember that AC can be a killer, particularly when you’re floating on a highly efficient conductor such as salt water. It’s vital, therefore, that great care should be taken over an onboard AC installation, which, for many, will mean calling in professional assistance.

Cruisers’ essential kit has become shore power cables

Installing AC begins with the shore-power cable, plugs, and onboard sockets.

In UK marinas and boatyards, the connection points are now pretty much standardised so it’s possible to buy a kit with a pre-terminated cable and a suitable socket for the boat. The boat socket is waterproof and has a sprung gasket. However, it should be mounted in an area which is protected.

The cable that runs between the shore power outlet and the consumer unit must have a current carrying capacity at least equal to or greater than the maximum current supply. It should not exceed 3m in length. Although most UK pontoon outlets are limited to 16A, it’s advisable to install a 30A, 3-core cable to the CU.

Most marine electrical suppliers offer cable that matches these ratings. However, it must be flexible, multi-stranded, and preferably tinned, not the solid conductor copper intended for domestic wiring. It is important that the conductor does not crack when it is exposed to vibrations or flexing.

Install a galvanic isolator

Installing Isolators

You should install a galvanic isolater (GI), which is a device that allows high voltage AC currents (up to 1.4V), to be carried to the ground, while stopping any DC leakage.

Alternatives to a GI include an isolation transformer. These are the best option to avoid stray voltages and other problems as they leave the boat completely isolated from shore while maintaining 230V AC. But, because they are heavy, expensive and bulky, isolation transformers tend to be fitted only on larger boats.

If the cable needs to pass through a cockpit lock or other vulnerable areas, it should be protected by trunking. Mini trunking made of plastic and epoxied on the hull is very useful. It allows you to easily add, inspect, or remove cables later.

It’s also important to be able to check and correct the polarity of the supply. It’s easy to find polarity detectors; some AC panels or CUs even have them built in. However, to change the polarity to what it should actually be, you need a switch or cable that reverses the polarity.

If the correct CU has been installed, it includes protection against polarity reversement. It will include double-pole circuit breakers that ensure that whatever conductor is active is always disconnected.

Overheated cable showing signs of damage

Safety and Circuit Protection

Installing a consumer unit (CU), which is suitable for the installation, will protect against electric shock hazards. It must meet the latest regulations for mobile installations, which means you can’t just buy a domestic unit or one designed for sheds and garages.

As a first line of defense, every AC installation must include a Residual Current Device. This is also called a Ground Fault Circuit Interrupter. It is important to note that the RCD used for mobile installations should be an A-type and not the domestic AC type. Also, it must contain double-pole Mini Circuit Breakers, rather than the common single-pole breakers.

A RCD is a circuit breaker with two poles that protects circuits from leakage of current to earthed sources. In normal operation, current should flow into the appliance through the live conductor and return via the neutral cable.

If it isn’t, then some of the current must be leaking out from the appliance via the earth wire, or through some other conductor (such as a human being), in which case the RCD will trip.

Install a MCB rated appropriately for AC socket outlets. Separate MCBs should be installed for devices that draw a heavy load, like an electric cooker.

When planning the system, calculate the total AC current you’re ever likely to draw simultaneously (bearing in mind most shore power supplies are 16A), and the maximum load on each branch off the CU, in order to select the correctly rated circuit breakers. An MCB protects the cable and not the appliance. The appliance should have a built-in fuse.

The RCD will trip when an appliance produces a natural Earth Leakage

Nuisance tripping

RCDs can trip without apparent cause. It can be caused either by a system with two neutral-to earth (MEN) connections at different potentials, (for example centre tapped inverters), or an appliance producing a small amount natural earth leakage.

A MEN connection is required for any AC power source. This includes grids, generators, and inverters. Grids are usually connected at the substation. However, they can also be connected at distribution points such as the meter board.

In many cases, the link comes pre-wired in the generator. With inverter/charger combis, it’s more complex as the link is needed when inverting but not when charging, as that is provided by the grid or generator. Combi inverters usually come with a switch that allows them to toggle between the two.

An example of the correct wiring on a UK mains electrical plug

Distribution and wiring

The mains 230V AC system must have its own switch panel, separate from the DC wiring. This is to prevent any confusion or leakage to the DC systems. Some marine specific AC distribution panels have circuit protection built in, however, it’s important to ensure any circuit breakers supplied are double-pole and the trip-free type that cannot be held in the ‘on’ position in the event of it tripping out.

Regulations also state that the panel must be clearly marked with a ‘240V AC – risk of death’ warning sign, stating that the shore power cable should be disconnected before accessing the wiring. The panel should also require a special tool to reach the wiring in the back. All AC terminations must be covered by a screwed down cover with a warning label.

It is best to keep AC cables separate from DC wiring, and not bundle them together in a conduit. Although the regulations say that this is acceptable as long the cables are divided.

Separating them is important for safety, but it’s also necessary because the DC cable can induce an extra current in the AC cable which could interfere with sensitive electronic equipment.

The AC conductors should be clearly identified at regular intervals and supported by cleats.

Normal domestic AC sockets may be used around the boat (provided they have an earth terminal), although it’s preferable to use the type with a sprung protective cover to avoid any splashes getting into the sockets.

Take care when connecting mains electricity in a marina – damp fingers can kill

Connections and terminations

The AC wiring is no different from DC wiring. The quality of the terminations will dictate how safe and efficient the system will be.

It is best to use only connectors with flanged spades or ring terminals that are sized correctly for the studs. They will be physically resistant to being pulled off. Heat-seal connectors with adhesive integrated into the stem are ideal. Heat shrink should also be used around each joint if you use standard crimp-terminals.

This seals the joint against moist air and provides additional strain relief for the wire.

It’s a good idea to use write-on identification heat shrink above the connector to indicate which wire does what. It is debatable whether it’s better to actually state what the wire does, or where it leads to, rather than using a numbering system whose index is highly likely to get lost at some point.

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Nylon cable ties, on the other hand, are cheaper and better at keeping cables together. Insulating tape can become gooey over time and peel off. Self-adhesive mounts for cable ties are available to keep your wiring neat and organised.

Rubberised grommets can be used to protect insulation when cable is fed through panel and bulkhead holes. Flexible grommets are also available to trim around large or non-round openings.

Finally, make sure to leave enough cable behind the distribution panel to allow it to be hinged down or placed on a flat work surface.

Also, drip loops need to be integrated into any looms before they reach the panel, consumer units or any other electrical appliances.

It can’t be emphasised enough that AC (even the output of a small inverter) can kill someone who sticks damp fingers in the wrong place, so a clear distinction between AC and DC outlets must be made and the appropriate label applied.

The connections inside a 10kVA 230V switch

Multiple AC sources

It is important to never allow more than one AC source (grid or generator, inverter) to be used on the same circuit at once. It is important to feed different supplies directly into an automatic or manual multi-way transfer switches that can isolate the sources completely. It is possible to start a fire by connecting two or more AC supplies with different phases.

There are automatic relay-switched boxes that can detect another supply (perhaps the grid) and disconnect it. If you have any questions about how to connect more than one source, we strongly recommend consulting a professional.

Installing shore power on your yacht

You can do this by clicking on the link below.

  • Choose the right cable capacity and type for every job
  • Keep the cable as short as you can (max.3m) from inlet to RCD
  • Install separate switch panels and segregate all AC wiring.
  • Mark all AC cables and devices with warning labels
  • Protect and support cables when in exposed areas or engine compartments
  • Install a circuit breaker to protect each device that uses high current.
  • Install a switch and indicator for reversing polarity


  • Connect any device (other than an GI) to the incoming supply before the RCD
  • Connectors made from cheap terminal blocks
  • Install AC cables into the bilges
  • Use solder to connect any cable terminals

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