Most boats run two electrical systems that share the same battery bank but behave very differently: a low-voltage DC system that runs almost everything on board, and an AC system that only exists when shore power, a generator or an inverter is supplying it. Understanding which system a fault sits in is the first step in fixing it, and the first thing a competent marine electrician establishes on a call-out.
The two systems are not separate in practice. An inverter turns DC into AC, a battery charger turns AC back into DC, and both draw from or feed the same bank. A fault in one often shows up as a symptom in the other, which is why boat wiring gets diagnosed as one system rather than two.
This guide sets out how the DC and AC sides are built, how they are protected, and the standards that govern how they should be installed on a UK leisure boat.
Almost every boat runs a 12V or 24V DC system from a battery bank, feeding lights, pumps, instruments, navigation electronics, fridges and often an autopilot. The bank is charged from one or more of three sources: the engine alternator, shore power through a battery charger, and solar or wind where fitted.
Distribution runs from the bank through a main isolator and main fuse to a DC distribution panel, then out to individual circuits, each with its own fuse or breaker sized to the cable and the load. Cable sizing on DC circuits matters more than owners often expect, because low voltage means higher current for the same power, and voltage drop over a long cable run to a bow thruster or windlass can be significant even when the cable looks generously sized.
ABYC and RCD/ISO 13297 guidance both point to the same practical rule: every conductor is protected close to its power source, sized for the actual run length as well as the load, and the negative return is kept as a proper conductor rather than relying on hull or engine block bonding.
The AC side only carries mains-equivalent voltage, 230V in the UK, when something is actively supplying it. On a marina berth that is shore power, brought aboard through a shoreline, inlet and a galvanic isolator or isolation transformer, then into an AC consumer unit with its own RCD protection and individually fused circuits.
An inverter lets the boat run AC appliances from the battery bank when there is no shore power, and an inverter/charger combines that with a mains battery charger in one unit, switching automatically between the two. Wiring an AC system on a boat is not the same job as domestic house wiring; the marine environment, the mix of ELCI/RCD protection, bonding and the consequences of a wiring fault in a boat sitting in water mean this work should be BMEA or equivalent qualified.
The battery bank is the meeting point. A charger takes AC in and puts DC out; an inverter does the reverse. Anything wrong with the DC bank, low voltage, a loose terminal, a failing cell, shows up as poor AC performance too, an inverter that trips under load or a charger that never reaches float.
Bonding and earthing is the other place the systems interact. AC earth, DC negative and the underwater metal bonding system need to be correctly related to each other, not simply tied together, or you introduce corrosion risk or a shock hazard rather than removing one.
ISO 13297 for AC systems and ISO 10133 for DC systems on small craft set out expectations for cable sizing, overcurrent protection, switching and labelling that go beyond what many boats were built to, particularly older or DIY-modified ones. RCD compliance for boats sold in the UK and EU references these standards.
Warm cables or connectors, a persistent smell of hot plastic, breakers that trip intermittently, corrosion at crimped joints, or additions made with automotive-rated cable and connectors are all signs the system has drifted from a safe, documented state. Boats accumulate wiring in layers over decades of ownership, and it is common to find circuits with no clear origin and no fuse at all.
A full survey traces every circuit back to its source, checks fusing against cable size, and produces a wiring diagram, something surprisingly few boats actually have, which then makes every future job faster and safer.
A basic understanding helps you describe symptoms accurately and do simple checks safely, but any AC work and anything beyond swapping a fuse on the DC side is best left to a qualified marine electrician given the consequences of a fault on a boat in water.
Usually low battery voltage under load rather than the inverter itself, often caused by undersized cable between the bank and inverter, a poor connection, or a bank that is not holding charge.
No. Marine cable is tinned to resist corrosion, connections are typically crimped and heat-shrunk rather than twisted, and the standards governing protection, bonding and earthing are specific to a vessel afloat.
Corroded or loose connections, particularly at battery terminals, isolators and earth points, causing voltage drop that mimics a dozen other faults.
An annual visual check plus a full survey every few years, or whenever you buy a boat, catches most developing problems before they become failures.
We survey and document the whole electrical system, DC and AC, so you know exactly what is on board and why. Get in touch for a free assessment.