A rewire begins with arithmetic, not with cable. Calculate the real electrical load — continuous, intermittent and peak — before selecting a single component. Systems specified the other way round end up with batteries that cannot support the equipment, charging that cannot replace what is used, and cable sized by guesswork.
Why older vessels accumulate electrical chaos
Few boats are rewired; most are added to. A pump here, an inverter there, a new chartplotter spliced into whatever circuit was nearest — and after fifteen years the vessel contains several generations of wiring, multiple standards, unlabelled runs and abandoned cable that may or may not be live. The result is a system nobody fully understands, which is a safety problem before it is an inconvenience.
A refit that opens the hull is the moment to reset this, because access to routes behind ceiling and under sole will not exist again for years.
Step one: the load calculation
| Category | What to establish | Why |
|---|---|---|
| Continuous loads | Refrigeration, instruments, lighting, monitoring | Sets the baseline daily consumption |
| Intermittent loads | Pumps, winches, watermaker, galley equipment | Determines peak demand and cable sizing |
| Duty cycle | Hours per day at anchor, underway, alongside | Converts loads into daily amp-hours |
| Autonomy target | Days without charging | Sizes the battery bank honestly |
| Charging sources | Engine, generator, solar, shore | Must replace consumption in available hours |
Routing in a wet, moving, wooden vessel
Cable routes should be high where possible, continuous, supported at regular intervals, and protected wherever they pass through structure. Chafe is the enemy: a cable resting against a frame in a hull that works will wear through its insulation eventually. Runs should be kept clear of bilge water paths and separated from plumbing, both to avoid wet cable and to prevent a plumbing failure becoming an electrical one.
Every connection is a future fault
Connections corrode, loosen and heat. Minimise them, make them properly, place them where they can be inspected, and keep them out of the bilge and out of unventilated voids.
Isolation and bonding
Two related disciplines, frequently confused. Isolation is about ensuring circuits — particularly shore power and any AC system — cannot introduce current into the vessel’s structure or the water around it. Bonding is about deliberately connecting underwater metals so they share a common potential and can be protected together by anodes.
Done well, the combination protects skin fittings, shafts and fastenings. Done casually, it can accelerate exactly the corrosion it was meant to prevent, and on a timber hull it can degrade the wood around fastenings as well. Where a vessel spends time on shore power in marinas, this deserves specialist attention rather than a general-purpose approach.
Switchgear, protection and labelling
- Protect every circuitAppropriately rated, at the source of supply, with the rating matched to the cable rather than to the appliance.
- Group logicallyPanels organised so a person under stress can find the right switch quickly.
- Label at both endsEvery cable, at the panel and at the device.
- Document as builtA schematic reflecting what exists, updated when anything changes.
- Provide isolation pointsSo an individual system can be worked on without shutting down the vessel.
Batteries and charging
Battery location matters structurally as well as electrically: the weight is significant, it must land on structure that can carry it, and the installation must restrain the bank against motion in a seaway. Ventilation requirements depend on the chemistry chosen. Charging must be capable of genuinely replacing daily consumption in the hours realistically available — a calculation that frequently reveals a proposed system to be under-specified before any cable is bought.
Removing the old system
Redundant cable comes out. It is tempting to leave it — removal takes time and disturbs nothing — but abandoned wiring is a lasting source of misdiagnosis, and it is not always as dead as assumed. Removal also gives the clearest possible picture of where water has been travelling inside the hull, which is useful evidence for the structural side of the refit described in tankage, plumbing and grey water.
Where it sits in the programme
Rough-in during the systems phase, with structure signed off and joinery not yet closed. Final termination, panel work and commissioning follow once the interior is substantially complete. The phase logic and reporting discipline are covered under yard project management, and the wider refit sequencing under refit and restoration.
Testing and handover
A completed rewire is proved rather than assumed. Insulation resistance is checked, polarity confirmed, protection devices verified against the circuits they defend, and every device operated from its own switch to confirm the labelling matches reality. Voltage drop at the far end of long runs is measured under load, because a circuit that works with everything off is not the same as a circuit that works when the vessel is busy.
Handover includes the as-built schematic, a circuit schedule listing every breaker with its rating and what it feeds, and photographs of cable routes taken before joinery closed. That last item is quietly valuable: the next person who needs to trace a cable behind a bulkhead will otherwise be cutting access panels to find out what a photograph could have told them in seconds.