Fully rewiring a boat is an expensive exercise, as I am discovering during my rebuild project. Even a modest amount of new cable can cost a fair amount, since marine-grade cable is not cheap.

And there is no point trying to cut costs by fitting plain copper domestic wiring – as soon as the surface of the copper oxidises slightly it will no longer be possible to make reliable connections.

Marine-grade cable on the other hand is fully tinned, so each strand of the copper core is covered in a protective layer of metallic tin. This protects it from oxidising and ensures more reliable electrical connections.

That is only part of the equation, though. The second part is the insulation, and there are two factors here. The first is the mechanical problem of vibration and external damage to the cable. Boats bounce around a lot more than a road vehicle, so the conductive core needs to be protected by the outer layer. Survey-grade cable is considerably thicker (and more expensive) than non-survey, with that extra bulk made up of a substantially more robust insulating plastic sheath. Both types of cable are usually double-insulated, with two separate layers of plastic around each conductive core. Luckily, most recreational vessels do not require the expense of the survey-grade product, although there is no problem with using it.

It is best to run the wiring before installing the ceiling lining.

The other reason for having extra insulation is that salt water conducts electricity. Hence, should water come into contact with the core of the cable it could cause it to short out. At best, this would lead to failure of the device that the cable is serving, but in a worse case it could result in a fire onboard – altogether a disaster. So that thick double plastic layer helps keep water out, and all connections should be similarly protected by multiple levels of insulation.

When it comes to deciding what size (thickness) of cable is required, we all know intuitively that heavier loads require heavier cable. Hence an anchor winch needs a much thicker cable than an interior light fitting. Luckily, when you go and buy cable at a marine chandler, they usually have a handy little chart that lists the maximum current that a particular diameter cable can carry for a specific length.


Note that cables are limited by their current carrying ability, measured in amps, not by the voltage. So, for example a cable rated to carry 15 amps can carry no more than this amount of current, regardless of whether it is running at 12 volts or 220 volts. Using cable that is too thin will result in the cable heating up, and in extreme cases the insulation will melt and even catch fire.

This is one reason why electric cars often operate with insanely-high internal voltages, such as the Porsche Taycan which runs an 800-volt motor and battery. By bumping the voltage up they can carry more total power over the same sized cables, without exceeding the amperage rating of the cable. Power is measured in watts, which is calculated as volts multiplied by amps. So doubling the voltage means you halve the number of amps for the same total power. And half the amps means thinner cable can be used. This is one reason why some boats run on a 24-volt system rather than the more common 12 volts – better power distribution.

These are all simple enough concepts and make good common sense. But there is another lesser-known issue that most DIY-ers do not know much about. This is called Voltage Drop, and can come back to bite you if you don’t take it into account. In essence, it means that the longer the cable, the thicker it needs to be. This is not the same safety issue as the current capacity, rather a cable that is well within tolerances for the amount of current may simply not operate the accessory if the voltage drop is too high. And the more current required by the load, the worse the problem becomes.

This was brought home to me with one of the first electrical items I installed on my boat – a horn that I had purchased and then installed on the radar arch right on top of Divecat. This draws a very modest three amps, and was well within the capacity of the thinnest marine grade cable I had. The cable has a cross-section of 0.75mm2 and was rated at 7.5 amps. I needed over 10m of cable to get from the helm button up to where the horn was, but that should be fine, right?

When you are rewiring you need lots and lots of cable.

Except that when I had run the cable and tested it, I got nothing out of the horn. Well, almost nothing – a few clicks helped confirm the wire and switch were in fact connected correctly, but no actual sound came out. Yet, when I tested the voltage at the horn end with an electrical meter it showed it was getting 12.5 volts. What was happening? It was only when I left the meter connected at the same time as I connected the horn that I saw what was happening – the voltage dropped to around 9 volts when the horn was connected. Since its operating voltage range was between 10.5 and 14.4 volts, it couldn’t operate.

This seemed crazy, since the fully-charged battery was well capable of delivering the required three amps at 12.5 volts. The sneak thief in this case was the voltage drop, a phenomena that affects every type of electrical cable and is caused by their resistance. All cables have at least some resistance to current, measured in ohms, and a thicker cable has less resistance than a thinner one. It can be thought of like a straw – if you want to sip a small amount of a soft drink, it does not make much difference whether you use a narrow or a wide straw. On the other hand, try a thick milkshake – sucking this through a narrow straw is very hard, but switch to a wider straw and it comes up much easier. Similarly, transmitting electricity is more difficult through a resistive cable and easier through a thicker, less resistive one.

Non-survey wiring on top, survey grade at the bottom. For almost the same overall dimensions, the non-survey has nearly twice the core diameter.
The red wire is 16mm, the black is 25mm. So the outside dimension is not a good indication of the capacity of the wire.

This can all be calculated using a fundamental law of electricity called Ohm’s law: V=IR

Where V=volts, I=current in amps and R=resistance in ohms.

This simple equation can be used to see how much voltage will be lost by the cable. The cable I was using has a resistance factor of 0.05 ohms per metre. You need to account for both the positive and negative runs, hence a 10m length has 20m of cable. This gave me a total resistance of 1 ohm (20m multiplied by 0.05 ohms). I therefore experienced a loss of 3 volts (3 amps multiplied by 1 ohm) over that length, resulting in just over 9 volts remaining for the horn. And it did not work with 9 volts!

This can easily become confusing, since that same cable would have delivered adequate voltage for something that was drawing less current. For example an LED light, which typically draws just over 1 amp, would experience a voltage drop of only 1 volt (1 amp x 1 ohm). On the other hand something drawing close to the cable’s rated 7.5 amps would have lost a massive 7.5 volts, leaving just 4.5 volts to power whatever the device was.

Resistance table.

The solution is to run a thicker cable with a lower resistance. The next size up that I had on hand has a 1.5mm2 cross-section and was rated to 15 amps. Importantly, its resistance was less than half that of the thinner cable and hence the voltage drop for those 3 amps was much less. With this cable the horn blared forth.

The take-away from this experience is that there is no point running long lengths of cable that is too thin – it is virtually useless for anything other than a very light load. Fortunately LED lights have a very low power consumption and they also operate within a wide range of voltages, so this was not such an issue for my interior lighting. But for all other power requirements it makes sense to keep the cable runs from battery to load as short as possible. It may be elegant to run the cable through hidden ducts and keep them out of sight, but the resulting wiring may be either unusable or much more expensive due to the need to run heavier cable.

There is also a good case to be made for running a very substantial common negative cable around the boat and picking up one half of the circuit from that. I have installed a 25mm2 cable (a thick battery connector cable) from the house battery around the entire boat, with an insulated stud located in every compartment. From there I can pick up a good negative connection for all my accessories, with only a longer run of the positive cable needed back to the control switch.

Running the cables up through a hidden channel can result in much longer cable runs
Try to keep the cables neatly bundled.

Another solution would be digital switching. Since digital signals are extremely low current (typically a few milliamps), they can be run on very thin cable. Then each device can draw power from a single heavy power bus running around the boat, and this is the only part requiring thick, expensive cable. Then the digital signalling cable, at a fraction of the cost, can be run long (and hidden) distances to the helm or control points.

For those looking to calculate potential voltage drop over a span of cable, I found this online tool very helpful:

www.12voltplanet.co.uk/voltage-drop-calculator.html BNZ