Rewiring a boat is necessary for any yacht older than 20 years that still has its original wiring, especially if you’re keen to avoid endless problems and constant troubleshooting and temporary repairs.

A few decades ago, boat owners typically had minimal electrical requirements, allowing boatbuilders to get away with providing the most rudimentary installations.

Today, however, boat owners appear to want the same level of gadgetry on board that they enjoy at home, which usually requires rethinking the boat’s entire electrical system from batteries to devices, together with a serious upgrade to both cabling and circuit protection.

The following is a guide to the main considerations when overhauling your boat’s wiring system.

Rewiring a boat: Cable Type

Choosing the correct cables for the job is the single most important factor when rewiring your boat as undersized conductors can overheat under load, creating a dangerous fire risk.

Firstly, all marine cable should be multi-stranded, with tinned copper conductors.

The flexibility of the strands compensates for any movement or vibration typical of a vessel at sea, and the tinning protects the copper wire from oxidisation, which often results in increased resistance and faulty connections.

Ambient heat will also increase the resistance of a cable, so those running through engine compartments will have a reduced current-carrying capacity.

For this reason, they must be of a larger capacity and covered with fuel-resistant, fire-retardant insulation.

Rewiring a boat: Cable Size

Electrical cables are specified by their cross-sectional area (CSA), not their thickness or diameter (although the two are interrelated).

Multi-stranded conductors increase the available CSA for a specific cable diameter.

The CSA dictates the amount of current that can safely flow through the conductor.

The smaller the CSA, the greater its resistance and the larger the voltage drop over its length.

Rewiring a boat: Voltage drop

All cables have a resistance that increases or decreases according to cable length and CSA.

The result is a drop in voltage over the length of the cable.

A 10% voltage drop is considered acceptable in most non-critical applications, but 3% is preferable for essential equipment such as radios and navigation equipment.

It is also important when choosing cable for high-power items such as winches and windlasses.

The temptation is often to use smaller, cheaper cable to feed along the length of the boat to, say, a bow thruster or anchor windlass.

However, if the CSA is too small for the length required, the voltage can drop noticeably at the device.

This not only slows the device up, but also increases the current being drawn through the cable due to Ohm’s Law.

If this current exceeds the rated cable spec then there’s a good chance it could melt down and start a fire.

Rewiring a boat: Planning

For cables powering a number of different devices you need to calculate the maximum current that could be flowing with everything fully on, then add a good 30% safety/expansion margin.

In order to calculate the total current loading for each cable in amps (A), divide the power of a device in watts (W) by the circuit voltage (V). You also need to estimate as accurately as possible the total circuit length, which will be the sum of the distance from the source of the power to the device and back.

For the mathematically challenged, there are numerous websites and apps that provide easy calculators for wire sizes, otherwise refer to our Wire Size Calculations box (below).

Rewiring a boat: Good Connections

The most common cause of electrical failure on boats is poor connections.

In such a salty environment it is essential to ensure all the terminations are clean, firmly attached and the adjacent cable properly secured.

The best way to terminate multiple cables is to use a good quality bus bar (Blue Seas or similar) and crimped cable terminals.

Before you start wiring you will need to invest in good quality wire cutters, strippers and a crimper.

Decent cutters will make an even, square cut, allowing the wire to feed all the way into a crimp terminal.

Buy a wire stripper that has marked dies for each cable size to ensure you get a cleanly stripped cable without losing any of the fine wire strands.

Finally, a ratchet-type, double-action, parallel jaw crimper that has a double die (one side for strain relief on the cable outer, the other for the bare wire being crimped), which ensures correct and even pressure is applied to the terminal and crimps the cable firmly into the joint while ensuring the all-important insulation remains totally intact.

Note, though, that there are two different ‘double-jaw’ types – one for heat sealed crimps and another for straightforward, strain relief-style insulated crimp terminals.

To ensure a watertight joint the use of adhesive-lined heat shrinkcrimps is highly recommended.

These are impregnated with an adhesive that solidifies when heated after the crimp joint is made. sealing the joint

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Alternatively, you can coat the whole joint in silicone grease before applying a heat shrink cover that generously overlaps the joint (at least 25mm each side if joining two cables with
a butt connector for example).

Personally, I prefer to use the former as they form a strong joint that is impervious to moisture.

When sealing, use a heat gun on its lowest setting as heating it too quickly will cause the adhesive to bubble up and create air cavities in the joint.

Never use a naked flame as this can contaminate the joint and damage the insulation.

I am frequently asked if you should solder an electrical joint to make it stronger.

Well, though it might sound like a reasonable idea, the answer is no.

You should never solder crimps or terminals on a boat as it solidifies the wire strands, making the joint less flexible and thereby more susceptible to shearing off with regular movement or vibration.

More importantly, in an overload situation the cable can heat up sufficiently for the solder to melt and the wire to simply fall out of the joint, with the possibility of it then shorting against another terminal or metal casing.

For a resistance-free crimp joint the terminal must be correctly sized for both the cable and stud, and preferably galvanically compatible with the wire cores – i.e. tin-plated copper terminals (not aluminium) onto tinned copper wire.

Always put a ring terminal directly onto the stud and never onto a washer, which can allow moisture and contamination into the joint, causing it to overheat due to the increased resistance.

If you really can’t crimp a joint for some reason then use a quality, clamp-style terminal block (Wago for instance), placed inside a sealed plastic box.

If you absolutely must use plastic, so-called ‘chocolate block’ style terminal strips, then at least make sure the bars and screws are brass or stainless steel and slather the block with silicone grease or they’ll corrode.

Finally, ensure all cables are firmly anchored down close to the terminal and that you insert a drip loop in every cable between the anchor point and the termination block or device to keep water out of the joint.

For panel wiring, remember to leave enough spare cable on the loom to allow the panel to be removed and worked on easily – you won’t regret it!

Keep wires out of the bilges wherever possible. If unavoidable, either use heat-sealed crimps or seal any joints or terminal strips in a watertight box.

Rewiring a boat: Circuit Protection

Once you have designed your wiring layout and selected your cable sizes, the next step is to work out how best to protect your wiring from shorts and overloads, and to determine how you will switch the circuits on and off.

One of the most useful improvements that can be carried out to a yacht’s electrical system is to upgrade the switch panel, particularly if many more electrical items have been added over the years.

Whilst simple toggle switches and cartridge fuses may indeed function to a point, they often introduce problems themselves as their terminals corrode and loosen over the years.

Boat owners are increasingly installing ever more power-hungry devices, including fridges, windlasses, thrusters, inverters, immersion heaters, water makers and even air conditioning, so it is essential to ensure the cabling to these high-power units is totally safe.

The essential point to remember when installing a Circuit Protection Device (CPD) in a cable is that its purpose is to prevent loading the cable beyond its maximum recommended current limit.

Drawing too much current through cable will cause it to overheat and the insulation to melt, possibly even causing a fire.

Protection for the equipment itself will either be integral to the device or installed separately.

A CPD can be in the form of a fuse or circuit breaker (CB), with many choosing the latter for convenience and disconnection accuracy.

High-load fuses, such as ANL (35-750A), T-class (1-800A) and MRBF (30-300A) types, are ideal for heavy current draws and battery protection, whereas fast action, low current fuses are more suitable for protecting delicate electronics as CBs are not available under 5A.

ANL fuses are good for short-circuit protection, but can take a long time to blow in the event of a sustained over-current draw, whereas a T-class fuse will blow almost instantaneously.

Switchable CBs are ideal for distribution consoles as they double up as switches, simplifying the wiring and reducing the overall panel size.

A CB must be sized to match the smallest conductor in the circuit.

So, if a 30A cable leads from the main CB, say, to a junction box that has several 10A wires running from it, the entire circuit would have to be protected by a 10A breaker unless further CPDs (fuses or CBs) are added to each individual 10A cable.

There are two principal types of CB available – magnetically or thermally activated.

Magnetic types rely on an induced current in a coil to trip the disconnect mechanism, whereas a thermal CB incorporates a bi-metallic contact arm that lifts off the other contact when it overheats.

Magnetic CBs are slightly quicker to disconnect in the case of a sudden overload.

Thermal breakers, which tend to be used in high-current applications such as windlasses, bow thrusters etc, can sometimes be prone to premature activation in hot climates.

Only CBs that cannot be held in their ‘on’ position are suitable for marine use as they force you to rectify an overload situation before the power can be reapplied.

Finally, CPDs should always be inserted in the cable as close as possible to the device supplying the power.

For instance, a cable running from the main positive battery bus bar to a distribution panel would have a CB positioned as close to the bus bar as possible. Cable ID and schematics

For your own, and future owners’ sakes, do create a means of identifying each cable, its purpose, and its route.

Professionals will mark each cable, usually with heat shrink numbered collars and refer to each numbered cable on a circuit diagram.

Bits of insulating tape with biro markings and fag packet scrawls might be helpful during installation, but a more permanent method is essential when the job is finally finished.

Rewiring a boat: Tips in brief

• Start by buying a good quality, ratchet-style crimper and plenty of crimp terminals of various types and sizes.
• Only use marine-grade, multi-strand tinned cable for flexibility and to prevent copper oxide forming
• Wiring should be terminated using correctly sized crimp terminals, preferably the ring-type  that cannot slip off terminal screws.
• To reduce resistance, crimp terminals should ideally be galvanically compatible with the wire coating (i.e. tinned).
• To ensure a watertight joint use crimps that are impregnated with an adhesive that solidifies when heated gently.
• Ensure all cables are firmly anchored down, especially close to terminals.
• Put a drip loop in every cable between the cable fixing point and terminal to keep water out of the joint.
• In exposed or wet areas protect terminal strips in a watertight junction box.
• Do not solder crimps or terminals as in the event of a fault the cable can heat up sufficiently for the wire to fall out of the joint.
• Use bus bars where several wires are attached to the same source.
• If you must use plastic terminal strips check that the bar and screws are brass or stainless steel, or they will corrode rapidly.
• Keep wires and terminations out of the bilges, or if unavoidable, feed cable through a PVC conduit sealed at both ends.
• If just replacing a switch panel, label each wire from the old switches and fuses before disconnecting them to make it easier when reconnecting.
• Always leave enough slack on panel cables to allow it to be easily hinged down or laid out for easy inspection and troubleshooting.
• The supply cable to the main positive bus bar must be of a capacity to cope with the entire load of all the attached circuits.
• When choosing circuit protection devices, allowance must be made for ‘start up loads’ in inductive circuits such as pumps, fridges, motors etc.
• Finally, the fewer connections there are in your boat’s electrical system the less chance there is of a dodgy one causing problems!

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